DOT HS 811 496 October 2011

Quieter Cars and the 

Safety of Blind Pedestrians, 

Phase 2: Development of Potential 

Specifications for Vehicle 

Countermeasure Sounds

Final Report
This publication is distributed by the U.S. Department of 

Transportation, National Highway Traffic Safety Administration, 

in the interest of information exchange. The opinions, findings 

and conclusions expressed in this publication are those of 

the author(s) and not necessarily those of the Department 

of Transportation or the National Highway Traffic Safety 

Administration. The United States Government assumes no 

liability for its content or use thereof. If trade or manufacturers’ 

names or products are mentioned, it is because they are considered 

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does not endorse products or manufacturers. 

APA Format Reference:
 

Hastings, A., Pollard, J. K., Garay-Vega, L., Stearns, M. D., & Guthy, C. (2011, October). Quieter Cars and the 

Safety of Blind Pedestrians, Phase 2: Development of Potential Specifications for Vehicle Countermeasure 

Sounds. (Report No. DOT HS 811 496). Washington, DC: National Highway Traffic Safety Administration. 
 

 

 

 

 

 

REPORT DOCUMENTATION PAGE Form Approved

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the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, 

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Paperwork Reduction Project (0704-0188), Washington, DC 20503. 

1. AGENCY USE ONLY (Leave blank) 

DOT HS 811 496 

2. REPORT DATE 

October 2011 

3. REPORT TYPE AND DATES COVERED 

Final Report: April 2010 to April 

2011 

4. TITLE AND SUBTITLE 

Quieter Cars and the Safety of Blind Pedestrians, Phase 2: Development of Potential 

Specifications for Vehicle Countermeasure Sounds 

5. FUNDING NUMBERS 

HS59A3 

6. AUTHOR(S) 

Dr. Aaron Hastings, John K. Pollard, Dr. Lisandra Garay-Vega, Dr. Mary D. Stearns, and 

Catherine Guthy 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

 U.S. Department of Transportation 

Research and Innovative Technology Administration 

John A. Volpe National Transportation Systems Center 

Cambridge, MA 02142 

8. PERFORMING ORGANIZATION 

REPORT NUMBER 

DOT-VNTSC-NHTSA-11-04 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 

U.S. Department of Transportation 

National Highway Traffic Safety Administration 

Human Factors Engineering Integration Division 

Vehicle Safety Research Office, NVS-331 

Washington, DC 20590 

10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER 

11. SUPPLEMENTARY NOTES 

12a. DISTRIBUTION/AVAILABILITY STATEMENT 

This document is available to the public through the National Technical Information 

Service www.ntis.gov 

12b. DISTRIBUTION CODE 

13. ABSTRACT This project performed research to support the development of potential specifications for vehicle 

sounds, (i.e., audible countermeasures) to be used in vehicles while operating in electric mode in specific low speed 

conditions. The purpose of the synthetic vehicle sound is to alert pedestrians, including blind pedestrians, of vehicle 

presence and operation. The project developed various options and approaches to specify vehicle sounds that could be 

used to provide information at least equivalent to the cues provided by ICE vehicles, including speed change. Acoustic 

data from a sample of ICE vehicles was used to determine the sound levels at which synthetic vehicle sounds, 

developed as countermeasures, could be set. Psychoacoustic models and human-subject testing were used to explore 

issues of detectability, masking, and recognition of ICE-like and alternative sound countermeasures. Data were used to 

develop potential options that could be pursued to develop specifications for synthetic vehicle sounds. Project results 

indicate that vehicle detectability could potentially be met through various options including: recording(s) of actual 

ICE sounds; synthesized ICE-equivalent sounds; alternative, non-ICE-like sounds designed for detectability; and a 

hybrid of the options listed above. 

14. SUBJECT TERMS 

Electric vehicle, pedestrians, acoustic measurement, auditory detectability 

15. NUMBER OF PAGES 

156 

16. PRICE CODE 

17. SECURITY CLASSIFICATION 

OF REPORT 

Unclassified 

18. SECURITY CLASSIFICATION 

 OF THIS PAGE 

Unclassified 

19. SECURITY CLASSIFICATION 

OF ABSTRACT 

Unclassified 

20. LIMITATION OF 

ABSTRACT 

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89) Prescribed by ANSI Std. 239-18298-102 

i 
 

 

Acknowledgments 

ACKNOWLEDGMENTS 

The authors of this report are, Dr. Aaron Hastings, John K. Pollard, Dr. Lisandra Garay-Vega, 

Dr. Mary D. Stearns, and Catherine Guthy of the John A. Volpe National Transportation 

Systems Center (Volpe Center), Research and Innovative Technology Administration, United 

States Department of Transportation. 

Special appreciation is due to the National Highway Traffic Safety Administration, including 

Tim Johnson, director of the Office of Human-Vehicle Performance Research and Stephen 

Beretzky, project lead in the Human Factors/Engineering Integration Division. 

Special thanks to Steven Rothstein, president of the Perkins School for the Blind, Watertown, 

Massachusetts, for the recommendations provided in terms of recruitment, transportation, and 

logistics for the human-subject experiments. We acknowledge Kim Charlson, Director of the 

Perkins Braille & Talking Book Library, who helped us distribute recruitment information. 

We thank Arthur O’Neill, vice president of the Carroll Center for the Blind, Newton, 

Massachusetts, for his willingness to collaborate in this research. We would like to thank Rabih 

Dow, director of the Carroll Center’s Residential Rehabilitation Training Program. We 

acknowledge the work of Joe Kolb, Heather Platt, and Naomi Zabot. These orientation and 

mobility instructors provided valuable information while demonstrating the various walking and 

listening situations typical for pedestrians who are blind. 

Megan Robertson, director of orientation and mobility department, Massachusetts Commission 

for the Blind, and Bob Hachey, president, Bay State Council of the Blind for distributing 

recruitment information. 

The authors appreciate the feedback and information exchange with researchers and 

representatives of several organizations including: Alliance of Automobile Manufacturers; 

American Council of the Blind; American Foundation for the Blind; Association of International 

Automobile Manufacturers; Environmental Protection Agency; International Organization for 

Standardization; Japan Automobile Manufacturers Association; National Federation of the Blind; 

Society of Automotive Engineers; and Unites Nations Economic Commission for Europe, Word 

Forum for Harmonization of Vehicle Regulation. 

The authors acknowledge Volpe Center team members who made valuable contributions to this 

study. 

Finally, the authors extend special appreciation to the volunteers who participated in the humansubject experiments. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

ii 
 

 

List of Abbreviations and Acronyms 

LIST OF ABBREVIATIONS AND ACRONYMS 

ANOVA Analysis of variance 

AASHTO American Association of State Highway and Transportation Officials 

AVAS Approaching Vehicle Audible System 

CANbus Controller Area Network bus 

dB Decibel 

dB(A) A-weighted decibel level 

DSP Digital signal processer 

EPA Environmental Protection Agency 

ERB Equivalent Rectangular Bandwidths 

EVs Electric vehicles 

GPS Global positioning system 

HEVs Hybrid-electric vehicles 

ICE Internal combustion engine 

IRB Institutional Review Board 

LAeq 1/2 sec A-weighted 1⁄2-second equivalent sound pressure level 

LAF(max) Max A-weighted SPL fast time weighting over each 1⁄2-second interval 

LAF(min) Min A-weighted SPL fast time weighting over each 1⁄2-second interval 

LZeq, 1⁄2 sec Un-weighted 1⁄2-second equivalent sound pressure level 

LD824 Larson Davis Model 824 

MLIT Ministry of Land, Infrastructure, Transport and Tourism 

mph Miles per hour 

NCSA National Center for Statistics and Analysis 

NHTSA National Highway Traffic Safety Administration 

OEM Original equipment manufacturer 

RITA Research and Innovative Technology Administration 

SAE Society of Automotive Engineers 

SLM Sound level meter 

SPL Sound pressure level 

SUV Sport Utility Vehicle 

USDOT United States Department of Transportation 

Volpe Center Volpe National Transportation Systems Center 

VRTC Vehicle Research Test Center, NHTSA 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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Acoustic Definitions 

ACOUSTIC DEFINITIONS 

Acoustics Science of sounds. Includes sound production, transmission and effects. 

Ambient Noise Composite of sound from many sources near and far at a given place. 

Audiometer Device used to measure hearing sensitivity as a function of frequency. 

Detection In acoustics, determination of the presence of an acoustic signal. 

Frequency For a sine wave, the number of periods occurring in 1 second. 

Loudness Attribute of auditory sensation in terms of which sounds may be ordered 

on a scale extending from soft to loud. 

Pitch Attribute of auditory sensation in terms of which sounds may be ordered 

on a scale extending from low to high. 

Pure Tone Line spectrum that consists of a signal at a single frequency. 

Sound 

Pressure Level 

Level of a sound in decibels relative to an internationally defined 

reference level. 

Spectrum Description of the resolution of a signal into components, each of 

different frequency. 

Timbre 

Catch-all term for every other attribute of a sound except its steady-state 

amplitude and pitch. These include the relative proportions of tonal and 

random components, the shape of the spectral plot, the rise and decay 

times, amplitude and frequency modulations, etc. Enables listeners to 

judge that two non-identical sounds (similarly presented and having the 

same loudness and pitch) are dissimilar, e.g. two different musical 

instruments playing the same note at the same loudness. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

iv 
 

Table of Contents 

TABLE OF CONTENTS 

Executive Summary ................................................................................................... xiv
 

1. INTRODUCTION ...................................................................................................... 1
 

1.1 BACKGROUND ............................................................................................. 1
 

1.2 RESEARCH PURPOSE................................................................................... 2
 

1.3 RESEARCH OBJECTIVES............................................................................... 2
 

1.4 TECHNICAL APPROACH ................................................................................ 2
 

1.5 OUTREACH.................................................................................................. 4
 

1.6 REPORT ORGANIZATION............................................................................... 5
 

2. BACKGROUND....................................................................................................... 6
 

2.1 AUDIBLE CUES FOR PEDESTRIANS ................................................................ 6
 

2.2 DETECTION DISTANCE.................................................................................. 6
 

2.3 MASKING EFFECT ........................................................................................ 7
 

2.4 OVERVIEW OF INTERNATIONAL ACTIVITIES ..................................................... 7
 

2.4.1 Japan ....................................................................................................... 7
 

2.4.2 United Nations.......................................................................................... 8
 

2.4.3 US: Pedestrian Safety Enhancement Act 2010 ....................................... 8
 

3. VEHICLE OPERATING MODES ............................................................................. 9
 

3.1 VEHICLE APPROACHING AT LOW SPEED (FORWARD)....................................... 9
 

3.2 VEHICLE APPROACHING AT LOW SPEED (REVERSE)........................................ 9
 

3.3 VEHICLE TRAVELING IN PARALLEL AND SLOWING (DECELERATION) .................. 9
 

3.4 VEHICLE ACCELERATING FROM A STOP ......................................................... 9
 

3.5 VEHICLE STATIONARY: IDLE........................................................................ 10
 

3.6 VEHICLE STARTING-UP .............................................................................. 10
 

3.7 PHASE 2 DATA COLLECTION BY VEHICLE OPERATION ................................... 10
 

4. ACOUSTIC MEASUREMENT OF VEHICLES AND AMBIENT SOUNDS ............ 11
 

4.1 PURPOSE.................................................................................................. 11
 

4.2 ACOUSTIC MEASUREMENTS OF ICE VEHICLES ............................................. 11
 

4.2.1 Vehicle Measurement Site ..................................................................... 11
 

4.2.2 Vehicles Measured................................................................................. 12
 

4.2.3 Operating Modes.................................................................................... 13
 

4.2.4 Measurement Procedure ........................................................................ 13
 

4.3 MEASUREMENT OF COUNTERMEASURE SOUNDS .......................................... 16
 

4.3.1 Electronic Measurements....................................................................... 16
 

4.3.2 Standard Acoustic Measurement ........................................................... 17
 

4.4 ACOUSTIC MEASUREMENTS OF AMBIENT LEVELS ......................................... 17
 

4.4.1 Ambient for ICE Vehicle Measurements ................................................ 17
 

4.4.2 Ambient for Countermeasure Measurements ........................................ 17
 

4.4.3 Ambient for Countermeasure Subjective Testing................................... 17
 

5. ACOUSTIC ANALYSIS ......................................................................................... 19
 

5.1 ACOUSTIC ANALYSIS OF ICE VEHICLES ....................................................... 19
 

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5.1.1 Overall A-weighted Sound Pressure Level Measurements .................... 20
 

5.1.2 Determination of Frequency Range of Analysis..................................... 23
 

5.1.3 LAeq, 1⁄2 sec Sound Pressure Levels Corrected for Ambient Level.............. 24
 

5.1.4 Preliminary Specification for Detectability .............................................. 26
 

5.2	 ACOUSTIC ANALYSIS OF COUNTERMEASURE SOUNDS .................................. 31
 

5.2.1 LAeq, 1⁄2 sec Sound Pressure Levels Corrected for Ambient Level – 

Electrical Measurement ...................................................................................... 31
 

5.2.2 LAeq, 1⁄2 sec Sound Pressure Level Corrected for Ambient 

Level-Standard Measurement ............................................................................ 34
 

5.2.3 Measurement of Vehicle Directivity ........................................................ 36
 

5.3	 ACOUSTIC ANALYSIS OF AMBIENT SOUNDS .................................................. 38
 

5.3.1 Ambient Levels during ICE Measurements-‘Cottage Farm’ Ambient ..... 38
 

5.3.2 Ambient Levels Measured along Suburban Streets-

‘Suburban’ Ambients........................................................................................... 39
 

5.3.3 Ambient Level During Countermeasure Subjective Testing­

‘Countermeasure Test’ Ambient ......................................................................... 40
 

6. PSYCHOACOUSTIC ANALYSIS .......................................................................... 44
 

6.1	 COMPARISON OF VARIOUS PSYCHOACOUSTIC MODELS FOR TWO SOUNDS .... 44
 

6.2	 MOORE’S LOUDNESS WITH AMBIENT ASSOCIATED WITH THE EVENT 

CORRESPONDING TO THE MAXIMUM FOR ICE VEHICLES ............................... 46
 

6.2.1 Cottage Farm Ambient ........................................................................... 46
 

6.2.2 Countermeasure Test Ambient .............................................................. 53
 

6.3	 MOORE’S LOUDNESS WITH AMBIENT TIME HISTORY ASSOCIATED WITH THE
 

EVENT CORRESPONDING TO THE MAXIMUM ICE VEHICLES ........................... 57
 

6.4	 MOORE’S LOUDNESS WITH AMBIENT ASSOCIATED WITH THE EVENT 

CORRESPONDING TO THE MAXIMUM FOR COUNTERMEASURE SOUNDS........... 60
 

6.5	 SUMMARY: ACOUSTIC ANALYSIS ................................................................. 61
 

7. EVALUATION OF COUNTERMEASURE SOUNDS ............................................. 62
 

7.1	 PURPOSE.................................................................................................. 62
 

7.2	 SOUND CHARACTERISTICS TO IMPROVE DETECTABILITY ............................... 62
 

7.3	 COUNTERMEASURE EVALUATION TEST SITE ................................................ 62
 

7.4	 COUNTERMEASURE SOUNDS EVALUATED .................................................... 63
 

7.4.1 Countermeasure Sound Pressure Levels and Calibration ..................... 65
 

7.5	 STUDY DESIGN.......................................................................................... 65
 

7.5.1 Dependent Variables.............................................................................. 65
 

7.5.2 Independent Variables ........................................................................... 66
 

7.5.3 Presentation of Trials ............................................................................. 66
 

7.6	 STUDY PROCEDURE................................................................................... 67
 

7.6.1 Research Staff Training ......................................................................... 67
 

7.6.2 Recruitment of Study Participants .......................................................... 67
 

7.6.3 Informed Consent Form ......................................................................... 68
 

7.6.4 Hearing Screening Procedure................................................................ 68
 

7.7	 CHARACTERISTICS OF STUDY SAMPLE ........................................................ 68
 

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Table of Contents 

7.8	 EXPERIMENTAL METHODS .......................................................................... 70
 

7.8.1 Apparatus............................................................................................... 70
 

7.9	 ANALYSIS OF COUNTERMEASURE EVALUATION............................................. 74
 

7.9.1 Detection Distance and Proportion of Detections .................................. 74
 

7.9.2 Detectability Relative to Reference Vehicle ........................................... 74
 

7.9.3 Ranking of Countermeasure Sounds ..................................................... 77
 

7.10 VISION EFFECTS........................................................................................ 80
 

7.11 IDLE, ACCELERATION, AND START-UP SOUNDS ............................................ 80
 

7.12 SUMMARY: COUNTERMEASURE EVALUATION................................................ 81
 

8. DISCUSSION: POTENTIAL SPECIFICATIONS FOR VEHICLE SOUNDS .......... 83
 

8.1	 SCOPE...................................................................................................... 84
 

8.2	 PITCH-SHIFTING SPECIFICATIONS ............................................................... 86
 

8.3	 MINIMUM SOUND PRESSURE LEVEL AND SPECTRAL CONTENT 

SPECIFICATIONS BASED ON ICE VEHICLE ACOUSTIC CHARACTERISTICS ........ 87
 

8.3.1 Performance Metrics .............................................................................. 87
 

8.4	 OPTION A: RECORDINGS OF ACTUAL ICE SOUNDS....................................... 89
 

8.4.1 Sound..................................................................................................... 89
 

8.4.2 System ................................................................................................... 89
 

8.4.3 Test Methods ......................................................................................... 89
 

8.4.4 Assumptions ........................................................................................... 89
 

8.4.5 Observations .......................................................................................... 90
 

8.4.6 Suggestions for Further Research ......................................................... 90
 

8.5	 OPTION B: SYNTHESIZED ICE-EQUIVALENT SOUNDS.................................... 90
 

8.5.1 Sound..................................................................................................... 91
 

8.5.2 System ................................................................................................... 91
 

8.5.3 Test Methods ......................................................................................... 91
 

8.5.4 Assumptions ........................................................................................... 92
 

8.5.5 Observations .......................................................................................... 92
 

8.5.6 Suggestions for Further Research ......................................................... 92
 

8.6	 OPTION C: ALTERNATIVE, NON-ICE-LIKE SOUNDS DESIGNED FOR 

DETECTABILITY.......................................................................................... 92
 

8.6.1 Sound..................................................................................................... 93
 

8.6.2 System ................................................................................................... 94
 

8.6.3 Test Methods ......................................................................................... 94
 

8.6.4 Assumptions ........................................................................................... 95
 

8.6.5 Observations .......................................................................................... 95
 

8.6.6 Suggestions for Further Research ......................................................... 95
 

8.7	 OPTION D: HYBRID OF OPTIONS LISTED ABOVE ........................................... 95
 

8.7.1 Sound..................................................................................................... 95
 

8.7.2 System ................................................................................................... 96
 

8.7.3 Test Methods ......................................................................................... 96
 

8.7.4 Assumptions ........................................................................................... 97
 

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Table of Contents 

8.7.5 Observations .......................................................................................... 97
 

8.7.6 Suggestions for Further Research ......................................................... 97
 

9.	 SUMMARY OF FINDINGS AND CONSIDERATIONS FOR 

FURTHER RESEARCH ......................................................................................... 98
 

9.1 SUMMARY OF FINDINGS.............................................................................. 98
 

9.2 CONSIDERATIONS FOR FURTHER RESEARCH.............................................. 101
 

10. BIBLIOGRAPHY.................................................................................................. 102
 

APPENDIX A. SUPPLEMENTAL DATA AND INFORMATION ............................ 105
 

A.1 Intersections Used for Ambient Measurements for 

Countermeasure Testing ................................................................................... 106
 

A.2 Acoustic Measurement Results-Tabulated Data ............................................. 107
 

A.3 Sample Experimental Script.............................................................................. 134
 

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Table of Contents 

LIST OF TABLES 

TABLE 1. STOPPING SIGHT DISTANCE AS A FUNCTION OF VEHICLE SPEED AND 

DRIVER REACTION TIME..........................................................................................................7
 

TABLE 2. DATA COLLECTION BY VEHICLE OPERATION ....................................................................10
 

TABLE 3. INTERNAL COMBUSTION ENGINE VEHICLES MEASURED ...................................................12
 

TABLE 4. SUMMARY OF OVERALL A-WEIGHTED LEVELS AS MEASURED (NOT CORRECTED FOR 

AMBIENT LEVEL AT MICROPHONE LINE, LAF ,MIN) ......................................................................20
 

TABLE 5. SUMMARY OF OVERALL A-WEIGHTED LEVELS AS MEASURED (NOT CORRECTED FOR 

AMBIENT LEVEL) AT MICROPHONE LINE, LAEQ ,1/2 SEC .................................................................20
 

TABLE 6. SUMMARY OF OVERALL A-WEIGHTED LEVELS AS MEASURED (NOT CORRECTED FOR 

AMBIENT LEVEL) AT MICROPHONE LINE, LAF, MAX .....................................................................21
 

TABLE 7. SUMMARY OF ICE MEASUREMENTS USING LAEQ, 1⁄2 SEC ADJUSTED FOR AMBIENT 

LEVEL AND RESTRICTED TO FREQUENCY RANGE FROM 100 HZ TO 20 KHZ. ............................25
 

TABLE 8. DIFFERENCE BETWEEN CORRECTED (AND RANGE LIMITED) AND UNCORRECTED TIME 

AVERAGED LEVELS, LAEQ, 1⁄2 SEC ................................................................................................25
 

TABLE 9. PRELIMINARY SPECIFICATIONS FOR MINIMUM OVERALL A-WEIGHTED LEVEL (LAEQ, 1⁄2 SEC)
 

BY VEHICLE OPERATION........................................................................................................26
 

TABLE 10. LAEQ, 1⁄2 SEC SOUND PRESSURE LEVELS CORRECTED FOR AMBIENT LEVEL FOR 

COUNTERMEASURE SOUNDS USING DIRECT ELECTRICAL MEASUREMENT FOR 

LOW PLAYBACK SETTING.......................................................................................................31
 

TABLE 11. LAEQ, 1⁄2 SEC SOUND PRESSURE LEVELS CORRECTED FOR AMBIENT LEVEL FOR 

COUNTERMEASURE SOUNDS USING DIRECT ELECTRICAL MEASUREMENT FOR 

HIGH PLAYBACK SETTING......................................................................................................32
 

TABLE 12. LAEQ, 1⁄2 SEC SOUND PRESSURE LEVELS CORRECTED FOR AMBIENT LEVEL FOR 

COUNTERMEASURE VEHICLES USING TRADITIONAL MEASUREMENT .......................................35
 

TABLE 13. ATTENUATION, IN DB, AS A FUNCTION OF ANGLE RELATIVE TO VEHICLE CENTER 

LINE FOR THREE DISTANCES FROM SPEAKER (6, 12, AND 25 FT)............................................37
 

TABLE 14. SUMMARY OF AMBIENT LEVELS DURING ICE MEASUREMENTS, A-WEIGHTED 

LEVEL, DB(A)........................................................................................................................38
 

TABLE 15. AMBIENT LEVEL DURING COUNTERMEASURE TESTING USED FOR COMPUTATION OF 

PSYCHOACOUSTIC METRICS..................................................................................................41
 

TABLE 16. COMPARISON OF PSYCHOACOUSTIC MODELS................................................................45
 

TABLE 17. NUMBER OF ERBS AND CORRESPONDING FREQUENCY, HZ...........................................48
 

TABLE 18. QUALITATIVE DESCRIPTION OF SOUND COUNTERMEASURES .........................................64
 

TABLE 19. DISTRIBUTION OF PARTICIPANTS BY SEX AND VISION CATEGORY ...................................69
 

TABLE 20. SUMMARY STATISTICS FOR AGE BY SESSION ................................................................69
 

TABLE 21. DISTRIBUTION OF PARTICIPANTS BY HEARING LOSS CATEGORY.....................................69
 

TABLE 22. COMPARISON OF COUNTERMEASURE SOUNDS TO REFERENCE SOUND ACROSS 

CONDITIONS AT P = 0.0311 LEVEL.........................................................................................77
 

TABLE 23. COMPARISON OF ARTIFICIAL SOUNDS TO EACH OTHER ACROSS CONDITIONS AT 

P = 0.0178 LEVEL. ................................................................................................................79
 

TABLE 24. COMPARISON OF ARTIFICIAL SOUNDS TO EACH OTHER ACROSS CONDITIONS AT 

P = 0.0178 LEVEL (CONTINUATION). ......................................................................................79
 

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Table of Contents 

TABLE 25. MISSED DETECTIONS FOR VARIOUS START-UP, IDLE, AND ACCELERATION SOUNDS .......81
 

TABLE 26. MINIMUM OVERALL A-WEIGHTED LEVEL (LAEQ, 1⁄2 SEC) BY VEHICLE OPERATION...................87
 

TABLE 27. A-WEIGHTED ONE-THIRD-OCTAVE-BAND SPECTRA AT MICROPHONE LINE LAEQ,1/2SEC ......88
 

TABLE 28. INTERSECTIONS USED FOR AMBIENT MEASUREMENTS.................................................106
 

TABLE 29. OVERALL A-WEIGHTED UNCORRECTED SOUND PRESSURE LEVELS: 6 MPH, REVERSE ..107
 

TABLE 30. OVERALL A-WEIGHTED UNCORRECTED SOUND PRESSURE LEVELS: 6 MPH, FORWARD .109
 

TABLE 31. OVERALL A-WEIGHTED UNCORRECTED SOUND PRESSURE LEVELS: 10 MPH,
 

FORWARD...........................................................................................................................111
 

TABLE 32. OVERALL A-WEIGHTED UNCORRECTED SOUND PRESSURE LEVELS: 15 MPH,
 

FORWARD...........................................................................................................................113
 

TABLE 33. OVERALL A-WEIGHTED UNCORRECTED SOUND PRESSURE LEVELS: 20 MPH,
 

FORWARD...........................................................................................................................114
 

TABLE 34. OVERALL A-WEIGHTED UNCORRECTED SOUND PRESSURE LEVELS: ACCELERATION ....116
 

TABLE 35. OVERALL A-WEIGHTED UNCORRECTED SOUND PRESSURE LEVELS: STARTUP..............118
 

TABLE 36. OVERALL A-WEIGHTED UNCORRECTED SOUND PRESSURE LEVELS: IDLE .....................119
 

TABLE 37. OVERALL A-WEIGHTED UNCORRECTED SOUND PRESSURE LEVELS: AMBIENT ..............119
 

TABLE 38. AMBIENT LEVELS AT COTTAGE FARM MEASUREMENT SITE DURING 

ICE MEASUREMENTS, A-WEIGHTED LEVELS, DB(A).............................................................121
 

TABLE 39. IDLE A-WEIGHTED LEVELS, DB(A) FOR ICE VEHICLES MEASURED (UNCORRECTED)
 

AT COTTAGE FARM .............................................................................................................123
 

TABLE 40. 20 MPH PASS-BY LEVELS OF ICE VEHICLES MEASURED (UNCORRECTED) AT
 

COTTAGE FARM ..................................................................................................................125
 

TABLE 41. 20 MPH PASS-BY LEVELS OF ICE VEHICLES MEASURED (UNCORRECTED) AT
 

COTTAGE FARM ..................................................................................................................127
 

TABLE 42. COTTAGE FARM ICE VEHICLES AT MICROPHONE LINE - EVENT WITH MINIMUM LAEQ1/2SEC129
 

TABLE 44. COTTAGE FARM ICE VEHICLES AT MICROPHONE LINE - EVENT WITH
 

TABLE 43. COTTAGE FARM ICE VEHICLES AT MICROPHONE LINE – AVERAGE* LAEQ1/2SEC ...............130
 

MAXIMUM LAEQ1/2SEC..............................................................................................................131
 

TABLE 45. SAMPLE AMBIENT LEVEL DURING COUNTERMEASURE TESTING ON AUGUST 14TH,
 

2010, A-WEIGHTED ONE-THIRD OCTAVE BAND LEVELS, DB(A)............................................132
 

TABLE 46 COMPARISON OF AMBIENT LEVEL FOR LEFT AND RIGHT SIDES OF BINAURAL HEAD 

DURING COUNTERMEASURE TESTING ..................................................................................133
 

TABLE 47. SAMPLE EXPERIMENTAL SCRIPT: BLOCK A FOR AUGUST 7TH SESSION..........................134
 

LIST OF FIGURES 

FIGURE 1. TECHNICAL APPROACH TO DEVELOP POTENTIAL SPECIFICATIONS FOR VEHICLE SOUNDS .4
 

FIGURE 2. VEHICLE MEASUREMENT SITE FOR ICE VEHICLES – MASON STREET BROOKLINE,
 

MASSACHUSETTS..................................................................................................................12
 

FIGURE 3. MICROPHONE LINE (PP’) RELATIVE TO VEHICLE ASSUMING ENGINE MOUNTED IN 

FRONT OF VEHICLE. VEHICLE CENTERLINE IS LABELED CC’ ...................................................14
 

FIGURE 4. CORRELATION BETWEEN (NOT CORRECTED FOR AMBIENT LEVEL) LAEQ, 1/2 SEC AND LAF, MAX 22
 

FIGURE 5. CORRELATION BETWEEN (NOT CORRECTED FOR AMBIENT LEVEL) LAEQ, 1/2 SEC AND LAF, MIN.22
 

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FIGURE 6. CORRELATION BETWEEN (NOT CORRECTED FOR AMBIENT LEVEL) LAEQ, 1,2 SEC AND 

LAF, MIN (START-UP REMOVED) ................................................................................................23
 

FIGURE 7. COMPARISON OF MEASURED LEVELS OF ICE VEHICLES FOR TWO OPERATING 

CONDITIONS (20 MPH AND IDLE) WITH THE MINIMUM ESTIMATE OF THE AMBIENT LEVEL. 

THESE DATA FOR ALL OPERATING CONDITIONS ARE GIVEN IN APPENDIX A-2. .........................24
 

FIGURE 8. MINIMUM LEVEL FOR EACH ONE-THIRD OCTAVE BAND: 6 MPH, REVERSE.......................27
 

FIGURE 9. MINIMUM LEVEL FOR EACH ONE-THIRD OCTAVE BAND: 6 MPH, FORWARD......................27
 

FIGURE 10. MINIMUM LEVEL FOR EACH ONE-THIRD OCTAVE BAND: 10 MPH, FORWARD..................28
 

FIGURE 11. MINIMUM LEVEL FOR EACH ONE-THIRD OCTAVE BAND: 15 MPH, FORWARD..................28
 

FIGURE 12. MINIMUM LEVEL FOR EACH ONE-THIRD OCTAVE BAND: 20 MPH, FORWARD..................29
 

FIGURE 13. MINIMUM LEVEL FOR EACH ONE-THIRD OCTAVE BAND: ACCELERATION .......................29
 

FIGURE 14. MINIMUM LEVEL FOR EACH ONE-THIRD OCTAVE BAND: STARTUP ................................30
 

FIGURE 15. MINIMUM LEVEL FOR EACH ONE-THIRD OCTAVE BAND: IDLE........................................30
 

FIGURE 16. COUNTERMEASURE SOUNDS AT LOW LEVEL, NOMINAL 59.5 DB(A).
 

SEE TABLE 10 FOR DATA PLOTTED........................................................................................34
 

FIGURE 17. COUNTERMEASURE SOUNDS MEASURED AT ICE VEHICLE MEASUREMENT SITE.
 

SEE TABLE 12 FOR DATA PLOTTED........................................................................................36
 

FIGURE 18. MEASUREMENT DIAGRAM FOR DIRECTIVITY MEASUREMENTS. REFERENCE 

LINE IS AT 0O

 .........................................................................................................................37
 

FIGURE 19. FIVE MEASURES OF THE AMBIENT AT THE ICE VEHICLE MEASUREMENT SITE.
 

*THE MINIMUM OVERALL LEVEL WAS USED FOR PSYCHOACOUSTIC COMPUTATIONS. 

**THE MINIMUM IN ONE-THIRD OCTAVE BAND WAS USED FOR SOUND PRESSURE LEVEL 

CORRECTIONS. DATA ARE REPEATED IN TABLE 14................................................................39
 

FIGURE 20. AMBIENT LEVEL DISTRIBUTION IN SUBURBAN SETTINGS INCLUDING BOTH LIGHT AND 

HEAVY TRAFFIC ....................................................................................................................40
 

FIGURE 21. SAMPLE AMBIENT LEVEL DURING COUNTERMEASURE TESTING ON AUGUST 14TH,
 

2010. AMBIENT LEVELS RANGED FROM 58 TO 61 DB(A) DURING TESTING .............................41
 

FIGURE 22. COMPARISON OF AMBIENT LEVEL FOR LEFT AND RIGHT SIDES OF BINAURAL HEAD 

DURING COUNTERMEASURE TESTING....................................................................................43
 

FIGURE 23. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COTTAGE FARM AMBIENT: 

6 MPH, REVERSE, DRIVER’S SIDE ..........................................................................................47
 

FIGURE 24. RELATIONSHIP BETWEEN NUMBER OF ERBS AND FREQUENCY. SEE 

TABLE 17 FOR DATA..............................................................................................................47
 

FIGURE 25. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COTTAGE FARM AMBIENT: 

6 MPH, FORWARD, DRIVER’S SIDE (DRIVER SEAT ON THE RIGHT) ...........................................49
 

FIGURE 26. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COTTAGE FARM AMBIENT: 

10 MPH, FORWARD, DRIVER’S SIDE .......................................................................................50
 

FIGURE 27. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COTTAGE FARM AMBIENT: 

15 MPH, FORWARD, DRIVER’S SIDE .......................................................................................50
 

FIGURE 28. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COTTAGE FARM AMBIENT: 

20 MPH, FORWARD, DRIVER’S SIDE .......................................................................................51
 

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Table of Contents 

FIGURE 29. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COTTAGE FARM AMBIENT: 

ACCELERATION, DRIVER’S SIDE.............................................................................................51
 

FIGURE 30. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COTTAGE FARM AMBIENT: 

STARTUP, DRIVER’S SIDE......................................................................................................52
 

FIGURE 31. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COTTAGE FARM 

AMBIENT: IDLE, DRIVER’S SIDE..............................................................................................52
 

FIGURE 32. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COUNTERMEASURE 

AMBIENT: 6 MPH, REVERSE, DRIVER’S SIDE ...........................................................................53
 

FIGURE 33. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COUNTERMEASURE 

AMBIENT: 6 MPH, FORWARD, DRIVER’S SIDE..........................................................................54
 

FIGURE 34. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COUNTERMEASURE 

AMBIENT: 10 MPH, FORWARD, DRIVER’S SIDE........................................................................54
 

FIGURE 35. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COUNTERMEASURE 

AMBIENT: 15 MPH, FORWARD, DRIVER’S SIDE........................................................................55
 

FIGURE 36. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COUNTERMEASURE 

AMBIENT: 20 MPH, FORWARD, DRIVER’S SIDE........................................................................55
 

FIGURE 37. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COUNTERMEASURE 

AMBIENT: ACCELERATION, DRIVER’S SIDE .............................................................................56
 

FIGURE 38. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COUNTERMEASURE 

AMBIENT: STARTUP, DRIVER’S SIDE ......................................................................................56
 

FIGURE 39. SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE ACCORDING TO 

MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE COUNTERMEASURE 

AMBIENT: IDLE, DRIVER’S SIDE..............................................................................................57
 

FIGURE 40. GROWTH OF SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE 

ACCORDING TO MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE 

COTTAGE FARM AMBIENT: 6 MPH, FORWARD, DRIVER’S SIDE – VEHICLE 7 .............................58
 

FIGURE 41. GROWTH OF SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE 

ACCORDING TO MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE 

COUNTERMEASURE AMBIENT: 6 MPH, FORWARD, DRIVER’S SIDE – VEHICLE 7 ........................58
 

FIGURE 42. GROWTH OF SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE 

ACCORDING TO MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE 

COTTAGE FARM AMBIENT: 6 MPH, FORWARD, DRIVER’S SIDE – VEHICLE 1 .............................59
 

FIGURE 43. GROWTH OF SPECIFIC LOUDNESS OF ICE VEHICLE AT MICROPHONE LINE 

ACCORDING TO MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE 

COUNTERMEASURE AMBIENT: 6 MPH, FORWARD, DRIVER’S SIDE – VEHICLE 1 ........................59
 

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Table of Contents 

FIGURE 44. SPECIFIC LOUDNESS OF COUNTERMEASURE SOUNDS AT MICROPHONE LINE 

ACCORDING TO MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE 

COUNTERMEASURE AMBIENT: LOW LEVEL, 59.5 DB(A) ..........................................................60
 

FIGURE 45. SPECIFIC LOUDNESS OF COUNTERMEASURE SOUNDS AT MICROPHONE LINE 

ACCORDING TO MOORE’S LOUDNESS WITH AMBIENT IN THE PRESENCE OF THE 

COUNTERMEASURE AMBIENT: HIGH LEVEL, 63.5 DB(A) .........................................................61
 

FIGURE 46. TEST SITE AT THE VOLPE CENTER, CAMBRIDGE, MASSACHUSETTS..............................63
 

FIGURE 47. HEV FITTED WITH A LOUDSPEAKER TEMPORARILY MOUNTED ON THE HOOD ...............70
 

FIGURE 48. A LINE OF SIX TRIPOD-MOUNTED LOUDSPEAKERS WAS POSITIONED 25 FEET IN 

FRONT OF THE LINE OF PARTICIPANTS TO PRESENT THE IDLE, START-UP AND 

ACCELERATION-FROM-STOP SOUNDS ...................................................................................71
 

FIGURE 49. RECORDING PARTICIPANT RESPONSES BY VIDEO ........................................................73
 

FIGURE 50. MEAN DETECTABILITY AT LOW AMPLITUDE FOR ALL SOUNDS BY SESSION AND 

DIRECTION. THE HORIZONTAL LINE INDICATES THE STOPPING DISTANCE (25.5 FT) FOR A 

VEHICLE TRAVELING AT 6 MPH, ASSUMING A 2.5-SEC DRIVERS REACTION TIME......................76
 

FIGURE 51. MEAN DETECTABILITY OF ARTIFICIAL SOUNDS BY SESSION, DIRECTION, AND 

AMPLITUDE (SESSION 2) .......................................................................................................78
 

FIGURE 52. MEAN DETECTABILITY OF ARTIFICIAL SOUNDS BY SESSION, DIRECTION, AND 

AMPLITUDE (SESSION 3) .......................................................................................................78
 

FIGURE 53. SPECTRA OF SOUND A5 AT 0 AND 20 MPH ...................................................................86
 

FIGURE 54. AVERAGE DETECTION DISTANCE (FOR 6 MPH FORWARD PASS-BY & SOUND 

LEVEL = 59.5 DB(A)..............................................................................................................99
 

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xiii 
 

Executive Summary 

Executive Summary 

PURPOSE 

This report documents research conducted to identify potential methods of developing 

specification for vehicle sounds (audible countermeasures), for use in electric vehicles (EV), plugin hybrid electric vehicles (PHEV) or hybrid electric vehicles (HEV) operating in electric mode. 

The research focused on developing objective specifications for detectability using sound pressure 

levels (SPLs) and spectral profile characteristics. The feasibility of objectively specifying other 

aspects of sound quality for the purpose of predicting recognizability was explored. 

This research was funded by the National Highway and Traffic Safety Administration’s (NHTSA) 

Office of Human-Vehicle Performance Research and is a follow-on to NHTSA’s Phase 1 research, 

which initially studied the issue of Quieter Cars. The Volpe National Transportation Systems 

Center (Volpe Center) of the U.S. Department of Transportation’s Research and Innovative 

Technology Administration (RITA) completed this research under an Inter-Agency Agreement 

(IAA) with NHTSA. 

Acoustic countermeasures should alert pedestrians, including blind pedestrians, of vehicle presence 

and operation, and provide alerting information at least equivalent to the cues provided by internal 

combustion engine (ICE) vehicles. Based on this premise, acoustic data acquired from a sample of 

ICE vehicles were used to determine the sound levels at which ICE-like synthetic vehicle sounds, 

developed as countermeasures, could be set. Acoustic measurement included overall A-weighted 

sound levels and one-third octave band data. Draft specifications for ICE-equivalent sounds are 

based on the mean sound level of all trials for a given operating mode. 

Acoustic measurements were collected for the following vehicle operating modes: 

 Low speed pass-by forward at 6, 10, 15 and 20 mph; 

 Low speed pass-by reverse at 6 mph; 

 Acceleration (from 0 to 10 mph); 

 Idle; and 

 Start-up. 

Ten vehicles from model years 2000 to 2009 were measured in a quiet residential environment for 

these operating modes. 

Psychoacoustic models and human-subject testing were used to explore issues of detectability, 

masking, and recognition of ICE-like and alternative sound countermeasures. Human-subject 

testing was conducted to compare countermeasure sounds in a representative urban-ambient noise 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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Executive Summary 

condition; a semi-controlled environment with an overall A-weighted sound level of approximately 

58 to 61 dB(A). The metric of performance for the human-subject testing was the distance at 

which these sound sources were detected and recognized as a vehicle. Countermeasure sounds 

were evaluated for 6 mph forward pass-by at two sound pressure levels within the range of typical 

ICE vehicles (i.e., 59.5 and 63.5 dB(A)). A qualitative and quantitative description of these sounds 

is provided in this report. 

The countermeasure systems evaluated included: 

 Sounds produced by vehicles with integrated countermeasures rented from 

manufacturers; and 

 Sounds produced by prototype countermeasure systems rented from 

manufacturers, and played back by loudspeakers temporarily mounted on 

HEVs rented separately. 

A total of nine sounds were evaluated (labeled A1, A2, A5, B, C, D, E1, E3, and E4). The sounds 

included ICE-like sounds, alternative sounds designed according to psychoacoustic principles, and 

sounds that combine alternative sounds with some ICE-like components. In addition to the 

countermeasures sounds, an ICE vehicle, that produced 60 dB(A) in the 6 mph pass-by test, was 

included in the study as a baseline for comparison purposes. The ICE vehicle was labeled system 

‘R’. 

RESULTS 

A summary for the acoustic measurements of the ten ICE vehicles is given in Table ES-1, along 

with the ambient level for reference. Here the average overall A-weighted level (LAeq, 1⁄2 sec) is given 

for each operating mode. These levels are interpreted as the minimum acceptable level for 

countermeasure sounds for the specified operating mode. One-third octave band levels for ICE 

vehicles are also documented in this report. 

Table ES-1. Minimum Overall A-weighted Level (LAeq, 1⁄2 sec) by Vehicle Operation 

Vehicle Operation LAeq, 1/2 sec, dB(A) 

6 mph, Reverse 58.4 

6 mph, Forward 61.1 

10 mph, Forward 63.6 

15 mph, Forward 68.1 

20 mph, Forward 70.2 

Acceleration from 0 to 10 mph 66.7 

Start-up 70.7 

Idle 55.2 

Ambient 58.4 

Moore’s Loudness model was used to explore whether ICE vehicle sounds would be perceived in 

the presence of a given ambient sound level. According to the model, all ICE vehicles included in 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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Executive Summary 

the acoustic measurements would be detectable at the point of closest approach. Frequency 

components between 1600 and 5000 Hz one-third octave bands would be more detectable due to 

strong signal strength and relatively low ambient levels in this range. Frequency components less 

than 500 Hz would have been less (or not at all) detectable due, in part, to ambient levels typically 

being high below 500 Hz. During testing tonal components emitted by two ICE vehicles would be 

particularly detectable, according to the model. 

Human-subject testing was conducted to examine various sounds at two playback levels in an 

urban ambient condition. Table ES-2 shows the mean detection distances for the sounds evaluated 

in the human-subject studies; sounds at the top of the list can be described as alternative sounds 

designed according to psychoacoustic principles and sounds at the end of the list can be described 

as ICE-like sounds with only the fundamental combustion noise or otherwise lacking in the 

qualities that support detectability. The lower and upper sound levels evaluated in the humansubject test were 59.5 and 63.5 dB(A) respectively. The two sound pressure levels of the playback 

were selected represent most of the range of SPL values for the 10 ICE vehicles recorded in the 6­

mph-pass-by tests, and to approximately bracket the 61 dB(A) mean of those tests. An ICE vehicle 

that produced 60 dB(A) in the 6 mph pass-by test was used as a reference in the evaluation. 

Table ES-2. Mean Detection Distance (in Feet) for all Countermeasure Sounds at two 

Amplitudes and for the Reference ICE Vehicle 

Sound Number Average Detection Distance (feet) for 

amplitude equal 59.5 dB(A) 

Average Detection Distance (feet) for 

amplitude equal 63.5 dB(A) 

E4 72 85 

A2 57 77 

E3 52 70 

A5 50 47 

ICE vehicle, 60 dB(A) 41 NA 

A1 35 44 

C 32 41 

E1 30 32 

B 20 25 

D 19 NA 

In general, synthetic sounds that resemble those of an ICE vehicle produce similar detection 

distances as actual ICE vehicles. In some instances, synthetic sounds designed according to 

psychoacoustic principles can produce double the detection distances at slightly lower overall 

sound levels. Synthetic sounds that contain only the fundamental component of combustion noise, 

but lack the harmonics and other high-frequency noises (fans, gears, etc.) of an actual ICE were 

relatively ineffective, i.e., their detection distances were only about half those of an ICE. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

xvi 
 

Executive Summary 

DISCUSSION 

The most important components of a sound to alert pedestrians of an approaching vehicle under 

conditions of urban ambient noise generally lie in the frequency range of 300 to 5,000 Hz. Much 

of the acoustic power emitted by ICEs lies in the range below 300 Hz, but it is not necessary to 

reproduce these sounds, since they are usually masked by the ambient, and because they do not 

provide good directional cues. Sounds above 5,000 Hz are generally at much lower levels, so that 

they are not as important for detection, and maybe inaudible to persons with high-frequency 

hearing loss. 

The literature of auditory warnings and the psychoacoustic models show that for a warning sound 

to be detected, it is necessary that it contain some components (one-third-octave bands), preferably 

4 or more, that exceed the corresponding bands in the ambient noise. Detectability of 

countermeasure sounds can be specified through various options, such as: 

 Recording(s) of Actual ICE Sounds (Option A). 

1. Defined by overall A-weighted levels. 

2. Defined by one-third octave band average spectra. 

3. Defined by one-third octave band minima spectra. 

4. Combinations of the above. 

 Synthesized ICE-Equivalent Sounds (Option B). 

1. Defined by overall A-weighted levels. 

2. Defined by one-third octave band average spectra. 

3. Defined by one-third octave band minima spectra. 

4. Combinations of the above. 

 Alternative, non ICE-like Sounds Designed for Detectability (Option C). 

 Hybrid of options listed above (Option D). 

Countermeasure sounds could be based on recordings of actual ICE vehicles made at various 

speeds and under various operating conditions and selected by the processor to correspond to a 

given vehicle’s operation at each moment while operating below 20 mph (Option A). 

A second alternative is to allow the sounds to be generated by a digital signal processor chip 

programmed to emulate the sounds of an ICE, at least in the 300 to 5000 Hz range (Option B). 

This alternative would permit a wider range of sounds and would still ensure that countermeasure 

sounds resemble ICE sounds. 

Sounds designed for maximum detectability at a given sound-pressure level follow certain wellknown principles described in this report (Option C). Finally, as a fourth option, digital signal 

processors can simultaneously create both sounds that resemble ICE noise and sounds that embody 

special characteristics to enhance detection (Option D). The two types of sounds can be blended in 

any proportion, with both components being pitch shifted according to vehicle speed. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

xvii 
 

Executive Summary 

USE OF OBJECTIVE VERSUS SUBJECTIVE CRITERIA FOR DETECTION AND 

RECOGNITION 

For the reasons given in the Phase 1 report [DOT HS 811 304] on this research, the Phase 2 work 

focused initially on the following idea. The lack of detectability of quieter cars could be remediated 

if they are fitted with synthetic sound generators that emulate the sound of typical ICEs and that 

future specifications for the vehicle sounds can be specified in terms of objectively measured 

parameters– namely, overall sound output as measured by the SAE J2889-1 procedure together 

with spectral distribution specifications for the minimum amount of sound level in one-third-octave 

bands. However, recognizability is a different matter. Most sounds, and sounds as complex as 

those emitted by an ICE, have numerous properties in addition to loudness and spectral distribution 

that affect human perception. Among these properties are rise time, decay time, repetition rates, 

variations in pitch and loudness, and phase relations among various components of the sound. It 

can be demonstrated, for example, that changes in these properties can render a sound 

unrecognizable even though loudness and spectral distribution are unchanged by playing a 

recording of the sound backwards. There are no established quantitative metrics for many qualities 

of a sound that a person might use for recognition. 

Some of the prototype sounds that were tested in this research were synthesized to resemble ICE 

sounds, while others were not. Some sounds, were comprised of entirely synthetic, non-ICE 

sounds generated by a digital-signal-processor (DSP), and demonstrated that, in some instances, it 

is possible to achieve significantly better detection distances, at the same sound level (as measured 

by the SAE 2889-1 draft procedure), compared with ICE-like sounds. Sounds constructed 

according to established principles for auditory warnings were recognized on first hearing in the 

human-subject tests when they were played at a level approximately equal to or higher than the 

ambient noise level. These sounds lack the low-frequency, fundamental combustion noise of an 

ICE, which allows them to be played through small, well-sealed speakers similar to those used in 

back-up warning devices. Given these advantages, there is likely to be an industry preference for 

use of entirely synthetic sounds. However, an objective specification for non-ICE-like sounds is 

more difficult to develop than one for synthetic sound generators that emulate the sound of typical 

ICEs. This former approach could result in a wider variety of sounds, some of which might be not 

recognized as a vehicle or which might be perceived as annoying. Most of the sounds offered for 

testing by manufacturers were chosen through a jurying process. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

xviii 
 

Introduction 

1. INTRODUCTION 

1.1 Background 

HEVs are two times more likely than ICE vehicles to be in a pedestrian crash where the vehicle is 

backing out, slowing/stopping, starting in traffic, and, entering or leaving a parking 

space/driveway. The vehicles involved in such crashes are likely to be moving at low speeds where 

the difference between the sounds emitted by HEVs and ICEs is substantial. The crash incidence 

rate for the combined set of maneuvers is 1.2 percent and 0.6 percent for HEVs and ICE vehicles 

respectively and the difference is statistically significant. The analysis, conducted by NCSA, uses a 

small sample (8,387 HEVs and 559,703 ICE vehicles) and does not intend to provide national 

estimates (Hanna, 2009). 

A reduction in the sound emitted by vehicles operating at low speeds and in electric mode may 

have implications for all pedestrians; however pedestrians who are blind may be particularly 

affected because they depend almost entirely on auditory cues to navigate. Groups representing 

people who are blind have expressed concern about the lack of sound emitted by HEVs (Maurer, 

2008). 

Previous research by NHTSA (Phase 1) documented the acoustic characteristics of ICE and HEVs 

and evaluated the auditory detectability of these vehicles. Acoustic measurements were obtained 

for the following operating conditions: backing out at 5 mph; slowing from 20 to 10 mph; 

approaching at a constant low speed; accelerating from a stop; and idle. Acoustic measurements for 

vehicles approaching at low speeds (6 mph and 10 mph) and moderate speeds (20 mph, 30 mph, 40 

mph) were used to document how the overall sound level for ICE vehicles and their HEV twins 

differ as a function of vehicle speed. Human-subject studies examined the auditory detectability of 

ICE vehicles and HEVs (operated in electric mode) for two ambient sound levels. Three operating 

conditions used for the human-subject studies were: vehicle backing out at 5 mph; vehicle slowing 

from 20 to 10 mph; and vehicle approaching at a constant speed (6 mph). The study also identified 

countermeasure concepts from literature reviews. (Garay-Vega, Hastings, Pollard, Zuschlag, and 

Stearns, 2010). 

The results of NHTSA (Phase 1) shows that the overall sound levels for the HEVs tested are 

noticeably lower at low speeds than for the ICE vehicles tested (Garay-Vega, et al., 2010). 

Considering the results of acoustic measurements and human subject data, countermeasures are 

only needed when vehicles are operated at speeds generally less than 10 to 20 mph where tire noise 

is not dominant (Garay-Vega, et al., 2010; Japanese Automobile Standard Internationalization 

Centre, 2009). On average, participants took significantly longer to detect vehicles operating in 

electric mode than the ICE counterparts. These results are consistent with studies at the University 

of California Irvine (Rosenblum, 2008) and Western Michigan University (Wall Emerson, 

unpublished) as well as studies conducted by the Japanese Automobile Standards 

Internationalization Centre (JASIC). Response time to detect a target vehicle varies by vehicle 

operating condition, ambient sound level, and vehicle type (i.e., ICE versus HEV in EV mode) 

(Garay-Vega, et al., 2010). 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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Introduction 

At the present time, synthetic vehicle sounds appear to be the only countermeasure that is useful in 

providing relevant sound cues needed by pedestrians. Relevant sound cues provide information 

about vehicle presence, speed, and rate of change in speed. Considering the results of previous 

studies, such sounds are only needed when vehicles are operated at low speeds (generally less than 

10-20 mph) (Garay-Vega, et. al, 2010; Japanese Automobile Standard Internationalization Centre, 

2009) and possibly when stationary. The characteristic sound of an ICE vehicle being started is 

also desired as it is often the first cue of the presence of a potential threat, for example, in a parking 

lot. The sound of a vehicle accelerating from stop, for example at a traffic signal, provides relevant 

cues about the state of the traffic flow and is also considered important. 

Groups representing people who are blind have expressed a preference for sound(s) that will be 

recognized as that of an approaching vehicle so that it will be intuitive for all pedestrians (Goodes, 

Bai, & Meyer, 2009; Maurer, 2008). 

1.2 Research Purpose 

The purpose of this research was to develop potential specifications for vehicle sounds (i.e., 

audible countermeasures) to be used in vehicles while operating in electric mode in specific low 

speed conditions. The specifications include quantitative specifications for sound levels and 

spectral profiles for detectability. The feasibility of objectively specifying other aspects of sound 

quality for the purpose of predicting recognizability of the synthetic sounds was explored. 

1.3 Research Objectives 

The following objectives were established to meet the goal of the research: 

 Determine the distribution of the sound levels of typical ICE vehicles in the 

operating modes of concern; 

 Develop detectability specifications for vehicle sounds; 

 Conduct human-subject testing to measure the detectability and recognizability of 

synthetic vehicle sounds; and 

 Determine feasibility of an objective specification test (or develop alternative 

evaluation procedure). 

1.4 Technical Approach 

Acoustic data acquired from a sample of ICE vehicles were used to determine the sound levels at 

which ICE-like synthetic vehicle sounds, developed as countermeasures, would be set. 

Measurements include overall sound levels and one-third octave band data. Draft specifications 

were based on the mean sound level of all trials for a given operating mode. 

Vehicle operations were identified from various sources including: NHTSA crash data analysis; 

previous research by NHTSA (Phase 1 of this research program); concerns expressed by 

representatives of the blind community; and feedback from subject matter experts. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

2 
 

Introduction 

Acoustic measurements are collected for the following vehicle operating modes: 

 Low speed pass-by forward at 6, 10, 15 and 20 mph; 

 Low speed pass-by reverse at 6 mph; 

 Acceleration (from 0 to 10 mph); 

 Idle; and 

 Start-up. 

One of the operating modes, deceleration from 20 to 10 mph (as if to turn right), that was tested in 

Phase 1 of this program could not be included in the Phase 2 study. Deceleration from 20 to 10 

mph (as if to turn right) was not included in Phase 2 because a location of sufficient length to test 

vehicle deceleration was not readily available. In addition, to conduct a valid test of the 

detectability of decelerations with real vehicles, it would be necessary to limit the number of 

subjects being tested to only one or two in a single experimental session, so that they would be 

exposed to essentially the same auditory content. Conducting numerous additional sessions, or 

extending session durations to approximately six hours so that deceleration could be included was 

inadvisable for reasons of cost, timeliness of project completion, and the health and comfort of the 

subjects. Start-up operation was not tested in Phase I but such sounds were measured in Phase 2. 

The ability of human subjects to detect and recognize startup, idle, and 0-to-10-mph acceleration 

sounds was recorded during the first experimental session. 

Draft specifications were then refined using information from psychoacoustic model(s) and 

human-subject testing. Psychoacoustic model(s) and human-subject testing were used to explore 

issues of detectability, masking, and recognition of alternative sound countermeasures. Figure 1 

shows a flow diagram of the technical approach used to develop potential specifications for 

synthetic vehicle sounds. A summary of findings, discussion of alternative approaches to specify 

vehicle sounds for quieter cars, and recommendations for future work in the area are included in 

this report. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

3 
 

 

Introduction 

Summary of Findings 

/Discussion 

Volpe 

Center 

Draft Specification 

Review Scenarios 

(Vehicle 

Operations) 

Literature 

Review 

NHTSA 

Crash 

Data 

Sound Level & 

Spectra for 

Sample ICE 

Vehicles 

Sound Level & 

Spectra for 

Sample Ambient 

Noise 

Gather 

Countermeasures 

Sounds & Vehicles 

Automobile 

Manufacturers 

Phase1 

Data 

Possible other 

sources 

Cognitive 

Walkthroughs Acoustic Data 

Psychoacoustic 

Modeling: 

Refine specification 

Human Subject 

Testing: Refine 

specifications 

Electronics 

& Sound 

Developers SME & 

Representative 

for the Blind 

Human Factors 

Literature 

Figure 1. Technical Approach to Develop Potential Specifications for Vehicle Sounds 

1.5 Outreach 

NHTSA and the Volpe Center have taken an active role in connecting with various public and 

private entities and seeking their input concerning this research. These entities include: 

 Alliance of Automobile Manufacturers; 

 American Council of the Blind; 

 American Foundation for the Blind; 

 Association of International Automobile Manufacturers; 

 International Organization for Standardization; 

 International Organization of Motor Vehicle Manufacturers; 

 Japan Automobile Manufacturers Association; 

 Massachusetts Commission for the Blind; 

 National Federation of the Blind; 

 Perkins School for the Blind; 

 Society of Automotive Engineers; 

 The Carroll Center for the Blind; 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

4 
 

Introduction 

 United Nations Economic Commission for Europe, World Forum for 

Harmonization of Vehicle Regulation; and 

 U.S. Environmental Protection Agency. 

NHTSA also has a docket to facilitate information exchange with the public. 

1.6 Report Organization 

 Chapter 2 includes background information. 

 Chapter 3 discusses the pedestrian-vehicle scenarios and vehicle operating 

considered in the evaluation. 

 Chapter 4 describes acoustic measurements of ICE vehicles, countermeasure 

sounds, and ambient sounds. 

 Chapter 5 discusses the acoustic analysis of ICE vehicles, including 

preliminary specifications, as well as acoustic analysis of countermeasures 

and ambient sounds. 

 Chapter 5 discusses the acoustic characteristics of ICE vehicles and ambient 

sounds. 

 Chapter 6 focuses on psychoacoustic models used to explore issues of 

detectability, masking of ICE vehicles and alternative sound 

countermeasures. 

 Chapter 7 describes human-subject testing used to explore issues of 

detectability and recognition of alternative sound countermeasures. 

 Chapter 8 presents a discussion of potential specifications for sounds for 

quieter cars. 

 Chapter 9 includes a summary of findings and considerations for future 

research. 

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Chapter 2: Background 

2. BACKGROUND 

2.1 Audible Cues for Pedestrians 

The acoustic countermeasure should alert pedestrians, including blind pedestrians, of vehicle 

presence and operation. The audible countermeasure sounds should provide information 

equivalent to the information provided by ICE vehicles, including speed change. This report 

documents research conducted to identify potential methods to specify levels and spectral 

content for vehicle sounds to be used as countermeasures on quieter cars. For the purpose of this 

report, countermeasure sounds would apply to vehicles operating in electric only mode while 

operating in the specified conditions. 

2.2 Detection Distance 

Detection distance is the distance between the pedestrian and the vehicle at the time the vehicle 

was detected. Detection distance is the performance metric used in this report to evaluate the 

relative effectiveness of potential countermeasure sounds. The needed detection distance is 

computed from the vehicle speed and the pedestrian response time. For a vehicle approaching at a 

given speed, the larger the average detection distance the greater the likelihood that pedestrians 

will avoid conflict with that vehicle. 

Stopping sight distance is the distance that enables a vehicle traveling at or near the design speed 

to stop before reaching a stationary object in its path. The stopping sight distance has two 

components: brake reaction and braking distance. Brake reaction is the distance traveled from the 

time the driver detects an object to the instant the driver applies the brakes. The recommended 

design criterion for brake reaction time (American Association of State Highway and 

Transportation Officials, AASHTO) is 2.5 seconds. A 2.5-second brake reaction time for 

stopping situations considers the capabilities of most drivers, including older drivers. Braking 

distance refers to the distance needed to stop the vehicle once the drivers applied the brakes, and 

depends on vehicle speed, deceleration rate, and roadway grade (AASHTO, 2004). For example, 

the stopping sight distance for a vehicle approaching at 6 mph is 25.5 ft (assuming brake reaction 

time of 2.5 s and a constant deceleration rate of 11.2 ft/s2

). The vehicle would travel 

approximately 22.5 ft while the driver reacts, plus need another 3.5 ft to decelerate the vehicle. In 

this example, the pedestrian must detect the vehicle (and respond) when the vehicle is at least 

25.5 ft away in order to avoid a potential collision. Table 1 shows stopping sight distance as a 

function of vehicle speed and driver reaction time. Stopping sight distance can be compared with 

detection distances to assess the relative effectiveness of countermeasure sounds. 

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Chapter 2: Background 

Table 1. Stopping Sight Distance as a Function of Vehicle Speed and Driver Reaction Time 

Vehicle Speed Driver Reaction 

Time = 1 sec 

Driver Reaction 

Time = 1.5 sec 

Driver Reaction 

Time = 2 sec 

Driver Reaction 

Time = 2.5 sec 

6 mph 12.3 ft 16.7 ft 21.1 ft 25.5 ft 

10 mph 24.3 ft 31.6 ft 39.0 ft 46.3 ft 

15 mph 43.6 ft 54.6 ft 65.7 ft 76.7 ft 

20 mph 67.7 ft 82.4 ft 97.1 ft 111.8 ft 

2.3 Masking Effect 

Masking is the process by which the auditory threshold for one sound is raised by the presence of 

another (masking) sound. A sound signal is most easily masked by a sound having components 

close to those of the signal. In a very quiet rural or suburban environment, vehicles operating in 

pure electric mode at low speeds can often be detected by their tire noise, for example, the clicks 

produced by pebbles lodged in the tread. However at the levels of ambient noise that are typically 

present in commercial areas of suburbs, and certainly of cities, ambient noise masks low-speed tire 

noise. Pedestrians derive audible cues of vehicle approach primarily from ICE noise, once the 

ambient noise level rises above 45 dB(A). ICE noise provides useful cues at ambient levels up to 

roughly 75 dB(A), although there is no sharp transition from safe detectability conditions to unsafe 

ones, but rather a gradual one marked by increasing risk for the blind pedestrian. Blind pedestrians 

try to avoid crossing streets in the presence of very high levels of ambient noise without assistance 

from a sighted person, service animal, or a traffic control signal with audible indications. 

The issue of masking was explored in this study using psychoacoustic models (Section 6). 

2.4 Overview of International Activities 

2.4.1 Japan 

In 2009, the MLIT of Japan assembled a committee to study the issue of the quietness of HEVs. 

The committee concluded that an Approaching Vehicle Audible System (AVAS) was a realistic 

alternative for pedestrians who are blind or visually impaired. Several studies were conducted on 

the issue (MLIT and JASIC, 2010). 

In 2010, MLIT announced guidelines for AVAS based on the recommendations of the study 

committee. Although several vehicles were considered in the initial scope, MLIT concluded that 

AVAS shall be installed only on HEVs that can run on electric motors, EVs and fuel-cell vehicles. 

In terms of activation condition, the MLIT recommends that AVAS automatically generate sound 

at least in a speed range from the start of a vehicle until reaching 20 km/h and when moving in 

reverse. The AVAS would not be required when a vehicle is stopped. The system may include a 

switch to temporarily halt the operation of the AVAS. The reason for including a pause switch is 

because they believe that the system is not needed in expressways where there are no pedestrians 

and to reduce other issues such drivers deliberately increasing vehicle speed in order to stop the 

AVAS (MLIT and JASIC, 2010). 

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Chapter 2: Background 

The MLIT includes the following guidelines for the type and volume for the sound generator 

system (MLIT and JASIC, 2010): 

 “The sound shall be continuous sound associating motor vehicles running 

condition.” 

 “Siren, chime, bells, melody, horns sounds, animals, insects, and sound of 

natural phenomenon such as wave, wind, river current, etc., are not allowed.” 

 “The sounds generated shall be automatically altered in volume or tone 

depending on the vehicle speed for easier recognition of the move of the 

vehicle.” 

 “Sound volume shall not exceed a level of the sound generated when vehicles 

driven by internal combustion only run at speed of 20 km/h.” 

The use of add-on devices, generating sound continuously for 5 seconds or longer, have been 

approved in order to increase AVAS penetration. MLIT will look into social acceptability and 

verification of technology implementation issues before moving from a voluntary process to a 

mandate (MLIT and JASIC, 2010). 

2.4.2 United Nations 

UNECE World Forum for Harmonization of Vehicle Regulation has an informal group on Quiet 

Road Transport Vehicles (QRTV). The objective if the QRTV is to...“Determine the viability of 

‘quiet vehicle’ audible acoustic signaling techniques and the potential need for their global 

harmonization.” The QRTV’s program plan includes: review the available research; determine 

human factors needed for pedestrians; develop technical performance parameters for vehicles 

based on human factors needs; determine audible sound characteristics and ways to convey desired 

vehicle performance information to pedestrians; and determine technical and economical feasibility 

of potential audible warning techniques (GRB Quiet Road Transport Vehicles, 2010). 

2.4.3 US: Pedestrian Safety Enhancement Act 2010 

The Pedestrian Safety Enhancement Act of 2010 (Public Law 111-373) was signed into law on 

January 4th, 2011. The Act directs the Secretary of Transportation to study and establish a motor 

vehicle safety standard that provides for a means of alerting blind and other pedestrians of motor 

vehicle operation. The goal is to promulgate a motor vehicle safety standard by establishing 

performance requirements for an alert sound that allows blind and other pedestrians to 

reasonably detect a nearby electric or hybrid electric vehicle. The law will apply to new electric 

and hybrid electric vehicles. The alert sound shall not require activation by the driver or the 

pedestrian, and shall allow pedestrians to reasonably detect an EV or HEV in critical operating 

scenarios such as constant speed, accelerating, or decelerating (Public Law 111-373, 2011). 

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Chapter 3: Vehicle Operating Modes 

3. VEHICLE OPERATING MODES 

Vehicle operations were identified from various sources including: NHTSA crash data analysis; 

previous research by NHTSA (Phase 1 of this research program); concerns expressed by 

representatives of the blind community; and feedback from subject matter experts in orientation 

and mobility for individuals who are blind. Vehicle operations are described in this section. 

3.1 Vehicle Approaching at Low Speed (forward) 

One of the strategies used by pedestrians who are blind is to cross when the road is quiet. The 

technique assumes that a vehicle is loud enough to be heard far enough away to determine that it is 

safe to proceed when masking sounds are not excessive and no other vehicles are detected. A 

quieter vehicle approaching at low speed may not be detected until it is too close to the pedestrian. 

The scenario includes a pedestrian standing on the curb waiting to cross a street or pathway where 

there may be vehicles approaching at low speed from the right or left. The task is to detect a 

vehicle that would affect the decision about when to start across the street. Low speed operation 

refers to speeds below 20 mph. 

3.2 Vehicle Approaching at Low Speed (reverse) 

Some HEVs can use the electric motor as the sole source of propulsion for low speeds operation 

and this is particularly true when the batteries are fully charged. There is a concern that HEVs, in 

electric mode, may not be detectable when backing out. The scenario includes a pedestrian walking 

along a sidewalk with driveways on the left or right. The task is to detect a vehicle backing out of a 

driveway at 6 mph. 

3.3 Vehicle Traveling in Parallel and Slowing (deceleration) 

The sound characteristics of a vehicle slowing in the parallel street helps pedestrians, blind 

pedestrians in particular, to identify vehicles that may turn into their crossing path. The scenario 

includes a pedestrian trying to decide when to start crossing the street, there is a surge of parallel 

traffic on the immediate left. The task is to detect the presence of a vehicle in the parallel street and 

then distinguish between a vehicle traveling straight through the intersection at 20 mph and a 

vehicle decelerating from 20 mph to 10 mph as if to turn right into the pedestrian path. 

3.4 Vehicle Accelerating from a Stop 

A vehicle operating in electric mode may not be heard during initial acceleration. The sound 

characteristics of a vehicle accelerating from a stop provide important cues for pedestrians. For 

example, a vehicle accelerating from stop in the parallel street indicates that the perpendicular 

traffic does not have the right of way and thus a crossing opportunity may be available. 

Pedestrians may initiate their crossing as soon as they detect the surge of parallel traffic or may 

delay the decision to make sure traffic is moving straight through the intersection and not turning 

into their path. A significant delay in detecting the surge of parallel traffic may reduce the chance 

to complete a crossing within the designated walking interval. Another example includes a vehicle 

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Chapter 3: Vehicle Operating Modes 

accelerating from a stopped position while coming out of a side street, parking lane, or driveway. 

In actual ICE vehicles, sounds of acceleration from a stationary position are much louder than 6­

mph constant-speed pass-by. They were immediately detected and recognized by all participants 

in the human-subject test when played at their original volume, or even several dB lower. 

3.5 Vehicle Stationary: Idle 

Pedestrians who are blind use the sound characteristics of vehicles idling to establish alignment 

and to avoid veering into traffic while crossing a street. The sound characteristics of vehicle idling 

in the far lane gives cues about the width of the road (number of lanes), conveying information 

about the distance to walk, and the time required to cross the road. There is a concern that a vehicle 

operating in electric mode may not be detected when it is stationary and idling. There is a potential 

safety issue at intersections or parking lots when a vehicle starts moving at the same time the 

pedestrian enters the conflicting path. Idle sounds in current ICEs are often difficult to perceive in 

urban ambient. 

3.6 Vehicle Starting-Up 

The characteristic sound of a vehicle being started is often the first indication of a potential threat 

for blind pedestrians, for example, in a parking lot, parking lane, or driveway. In actual ICE 

vehicles, startup sounds are relatively loud and there is no doubt that such sounds would be 

immediately detected. 

3.7 Phase 2 Data Collection by Vehicle Operation 

Table 2 shows each vehicle operation along with an indication of whether we collected acoustic 

measurements and/or human subject data as part of this research. 

Table 2. Data Collection by Vehicle Operation 

Vehicle Operation Acoustic Measurements Subjective Response 

Low speed forward Yes (6, 10, 15, and 20 mph) Yes (6 mph) 

Low speed reverse Yes (6 mph) None 

Slowing from 20 mph to 10 mph None None 

Accelerating from 0 to 10 mph Yes Some 

Idle Yes Some 

Start up Yes Some 

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Chapter 4: Acoustic Measurement of Vehicles and Ambient Sounds 

4. ACOUSTIC MEASUREMENT OF VEHICLES AND 

AMBIENT SOUNDS 

4.1 Purpose 

Acoustic measurements were conducted for ICE vehicles, ambient conditions, and prototype 

countermeasure sounds. Sound pressure levels and spectra of the ICE vehicles were measured in 

order to establish preliminary specifications for minimum sound level and spectral characteristics 

for operating conditions simulating the operating modes described in Section 3. Ambient 

conditions were measured in order to correct for measured sound levels of both ICE and 

countermeasure data, as inputs to psychoacoustic models described in Section 6, and in order to 

determine a suitable level for the ambient conditions during human-subject testing described in 

Section 7. 

4.2 Acoustic Measurements of ICE Vehicles 

Ten vehicles from model years 2000 to 2009 were measured in a quiet residential environment for 

operating modes described in detail in Section 3. 

4.2.1 Vehicle Measurement Site 

The ICE vehicle measurement site was located in a quiet residential area, on Mason Street in 

Brookline, Massachusetts (Latitude / Longitude = 42.348165, -71.11248). A photo of the site and 

instrumentation setup is shown in Figure 2. This site was chosen for its low traffic volume; 

moderate ambient levels; low wind speeds; flat, clean asphalt pavement; and the presence of a 

large flat and open region for equipment setup. Except for the hour from 8:00 to 9:00 AM when 

students arrived at a local private school, the traffic in the area was sparse (typically 3 to 4 vehicles 

per hour). This was important in order to minimize noise contamination from other nearby roads. 

Mason Street itself was closed to through traffic during the measurements. Although not 

completely isolated, the ambient level in the area was low and representative of a quiet suburban 

area which may be encountered by pedestrians. There were typically more acoustic intrusions due 

to pedestrians during the measurements than vehicles. During measurements, the majority of the 

ambient noise was due to birds, insects, wind-induced noise in vegetation, and distant vehicles. 

The road surface consisted of bituminous asphalt in generally good shape (Figure 2). There were a 

few minor cracks in the road surface, which were easily avoided during measurements. The road 

surface was clean except for vegetative detritus from over-hanging tree limbs that was swept prior 

to measurements. The road was between a small, flat residential park on one side and three homes 

on the other side approximately 35 feet from the vehicle path. The close proximity of the homes to 

the measurement location was considered, however, because the microphones were typically 

within two feet of the vehicles at pass-by, it was expected that the direct sound would dominate the 

measurements. When analyzing the data, no evidence of sound reflected from the homes was 

found. 

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Chapter 4: Acoustic Measurement of Vehicles and Ambient Sounds 

Figure 2. Vehicle Measurement Site for ICE Vehicles – Mason Street Brookline, 

Massachusetts 

4.2.2 Vehicles Measured 

Ten ICE vehicles with model years ranging from 2000 to 2009 were measured. Vehicles included 

seven mid-size vehicles, a pickup truck, a minivan, and a small SUV. Vehicles included American, 

Japanese, and European manufacturers. The vehicles are listed in Table 3. A full data set for the 

Solara was not obtainable due to a combination of low signal levels and high ambient levels during 

its scheduled measurement period. Vehicles were in good operating condition and did not generate 

sounds attributable to any type of defect. Tires had a tread depth considered sufficient for safe 

operation, even wear, and were representative of standard OEM tires. Volpe Center employees 

volunteered the vehicles for these measurements and thus represent a random sample of vehicles 

available in the public fleet. 

Table 3. Internal Combustion Engine Vehicles Measured 

Vehicle Year Vehicle Make Vehicle Model 

2000 Nissan Maxima 

2003 Toyota Tacoma 

2003 SAAB 95 

2004 Honda Odyssey 

2004 Mercury Sable 

2005 SAAB 93 

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Chapter 4: Acoustic Measurement of Vehicles and Ambient Sounds 

Vehicle Year Vehicle Make Vehicle Model 

2006 Toyota Solara 

2007 Mercury Milan 

2009 Nissan Altima 

2009 Toyota RAV-4 

4.2.3 Operating Modes 

Acoustic measurements of ICE vehicles were collected for operating conditions that simulated the 

operating modes described in Section 3 with the exception of the deceleration mode. Deceleration 

was not measured because such measurements require a test location of sufficient length to allow a 

vehicle to accelerate to 20 mph, stabilize, and then slow to 10 mph. Such tests can be conducted 

only at a large facility (e.g., VRTC). The operating conditions tested were: 

 Low speed pass-by forward at 6, 10, 15 and 20 mph; 

 Low speed pass-by reverse at 6 mph; 

 Acceleration (from 0 to 10 mph); 

 Idle; and 

 Start-up. 

Acoustic metrics included minimum, average, and maximum A-weighted sound pressure levels 

and un-weighted one-third octave band levels between 12.5 Hz to 20,000 Hz. Each measurement 

was over a time interval of 1⁄2-second. Details of these metrics are given in Section 5. 

4.2.4 Measurement Procedure 

The preliminary specifications for these measurements were not to examine pedestrian perception, 

nor to make recordings for future analysis, but to make measurements in a format consistent with 

what would be expected for compliance testing. To this end, SAE-J2889-1 draft test procedure 

(Measurement of Minimum Noise Emitted by Road Vehicles, Revision 2009) provides an initial 

framework for assuring that the best signal-to-noise ratios are obtainable in a compliance testing 

format. SAE-J2889-1 Rev 2009 specifies an engineering method for measuring the minimum 

noise emission of road vehicles and considers two operating modes: stationary and 6.2 mph (10 

km/h). This procedure does not include measurement of all the operating modes of concern nor 

does it include measurement of all metrics of interest for the current study, therefore, SAE-J2889-1 

Rev 2009 was used as basis for the measurement procedure, but was modified as follows: 

 Additional operating modes were measured: reverse at 6 mph; forward at 10, 

15, and 20 mph; acceleration from stop; and start-up. 

 Minimum, average, and maximum overall A-weighted sound pressure levels 

for each event were reported for the 1⁄2-second interval for which the vehicle 

was closest to the microphone line (for pass-by events) or for which the level 

was greatest (for start-up and idle). 

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Chapter 4: Acoustic Measurement of Vehicles and Ambient Sounds 

 For pass-by and start-up measurements, un-weighted average one-third 

octave band sound levels for each event were reported for the 1⁄2-second 

interval for which the vehicle was closest to the microphone line (for pass-by 

events) or for which the level was greatest (for start-up events). 

 SAE J2889-1 section 7.2.1 indicates that, “Before the measurements are 

started, the vehicle shall be brought to its normal operating conditions.” This 

is the case for all operating modes except start-up, in which case, the vehicle 

will be positioned at PP’ (see Figure 3) with the engine off prior to 

measurement. 

 For the purposes of this measurement procedure, two measurements were 

sufficient rather than four. (SAE-J2889-1 calls for 4 repetitions of each 

measurement, however, the purpose of the SAE measurements is to obtain an 

estimate of a single vehicle’s minimum level, while the purpose of 

measurements here is to obtain a distribution of levels. In this case, raw data 

is more useful and the estimate of the mean for individual vehicles is not 

critical.) 

 When reporting original measurements, data were not averaged except for 

idle and ambient events. (Data were averaged during analysis when 

appropriate). 

Figure 3. Microphone Line (PP’) Relative to Vehicle Assuming Engine Mounted in Front of 

Vehicle. Vehicle Centerline is Labeled CC’ 

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Chapter 4: Acoustic Measurement of Vehicles and Ambient Sounds 

4.2.4.1 Measurement Protocol 

Periodic ambient measurements were conducted in accordance with SAE J2889-1 section 6.3. The 

following steps were followed during an event: 

 Initiated acoustic system; 

 Continuously collected data every 1/2-second; 

 Noted the maximum sound level displayed by the acoustic system; 

 Noted any sounds heard from the measured vehicle, e.g. electric motor, 

engine, compressor, fan, tire noise, etc. Noted if the acoustic system 

indicated a minimum 3-dB rise and fall; 

 Noted any potentially contaminating sounds; 

 When possible, observed that wind speeds did not exceed the predetermined 

limit of 10 mph; 

 Reset the “current” memory of the acoustic system in preparation for the next 

event; and 

 Prepared the log sheets for the next event. 

4.2.4.2 Low Speed Pass-by Reverse Measurement Procedure 

 Vehicle accelerated to a constant specified speed. 

 Target speed was 6 mph in reverse. 

 Target speed was attained at least 1.5 seconds prior to passing PP’, the 

microphone line. 

 Target speed was maintained within a tolerance of +/- 1 mph 

 A minimum of 2 repetitions for this operating condition were measured for 

each vehicle. Any data that was clearly not representative of a typical 

vehicle, for example, dogs barking, leaf blowers, etc., was rejected. 

4.2.4.3 Low Speed Pass-by Forward Measurement Procedure 

 Vehicle accelerated to a constant specified speed. 

 Target speeds included 6, 10, 15, and 20 mph in the forward direction 

 Target speed was attained at least 1.5 seconds prior to passing PP’, the 

microphone line. 

 Target speed was maintained within a tolerance of +/- 1 mph. 

 A minimum of 2 repetitions for this operating condition were measured for 

each vehicle. Any data that was clearly not representative of a typical 

vehicle, for example, dogs barking, leaf blowers, etc., was rejected. 

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Chapter 4: Acoustic Measurement of Vehicles and Ambient Sounds 

4.2.4.4 Acceleration from Stop Pass-by Measurement Procedure 

 Vehicle started at rest at a distance of 30 meters from the microphone (as 

measured along the road) and then accelerated at a constant rate of 1 m/s2 to 

a speed of 10 mph. 

 The driver(s) attempted to accelerate at the same rate for each repetition. 

 A minimum of 2 repetitions for this operating condition were measured for 

each vehicle. Any data that was clearly not representative of a typical 

vehicle, for example, dogs barking, leaf blowers, etc., were rejected. 

4.2.4.5 Idle / Stationary Measurement Procedure 

 Vehicle was started and remained at rest adjacent to the microphones. 

 The measurement began after the vehicle came to steady state idle. 

 The engine was running at idle, but all unnecessary accessory devices were 

off. 

 A 1- minute measurement was conducted for each vehicle. 

4.2.4.6 Start-up Measurement Procedure 

 Starting with the vehicle off, the vehicle was turned on with the drive train in 

Park. 

 A minimum of 2 repetitions for this operating condition were measured for 

each vehicle. Any data that was clearly not representative of a typical 

vehicle, for example, dogs barking, leaf blowers, etc., was rejected. 

4.3 Measurement of Countermeasure Sounds 

Prototype countermeasure sounds were rented/leased from various automotive manufacturers and 

vendors. The speaker systems for these countermeasure sounds were either installed inside the 

front grill or mounted on the hood, just above the front grill. The broadband sound levels during 

the human-subject testing of countermeasures were determined in a manner similar to that 

described in Section 4.2.4 for forward pass-by operations at 6 mph except that the measurement 

site was located on Potter Street, Cambridge, MA (Latitude / Longitude = 42.3647829, ­

71.085711). Because the ambient level here was about 58 to 60 dB(A), acoustic measurements of 

the one-third octave band spectra were not practical. Thus, alternative methods were used to 

determine the one-third octave band spectra for these sounds. Two methods were used: 1) for 

vehicles that could not be driven off of the Volpe Center campus, a direct electronic measurement 

was conducted, 2) for vehicles that could be driven off of the Volpe Center campus, the ICE 

vehicle measurement site was used. 

4.3.1 Electronic Measurements 

Since several of the vehicles were not road ready, they could not be driven to the ICE vehicle 

measurement site. Since the ambient at the Volpe Center site was too high to measure the 

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Chapter 4: Acoustic Measurement of Vehicles and Ambient Sounds 

countermeasure sounds without significant contamination by the ambient, the one-third octave 

band spectra of these sounds were determined by measuring the input voltage to the vehicle 

speaker due to the countermeasure sound by using an adapter to the sound level meter’s preamplifier so that it could accept an electronic signal rather than an acoustic signal. Under these 

conditions measurements were made while the vehicles traveled at 6 mph. 

4.3.2 Standard Acoustic Measurement 

Two vehicles were road ready and could be driven to the ICE vehicle measurement site. For these 

two vehicles, 6 mph forward pass-by measurements were conducted as described in Section 4.2.4. 

Note, that both of these vehicles had the speaker systems internally mounted at inaccessible 

locations such that it was not practical to measure the countermeasure signals electronically. 

Because these were prototype systems provided by third parties, little modification could be done 

to the playback systems. 

4.4 Acoustic Measurements of Ambient Levels 

Ambient measurements were made for three purposes: 1) to determine if measured data were due 

to the vehicle or countermeasure sound, the noise (ambient), or both; 2) to determine appropriate 

levels for the ambient during countermeasure subject testing; and 3) to model the human 

perception of the signal, which uses both signal and ambient for inputs. 

4.4.1 Ambient for ICE Vehicle Measurements 

The ambient levels during ICE measurements were primarily of interest in order to make sure that 

the signal’s acoustic characteristics were correctly estimated. However, the ambient levels during 

the ICE measurements were also used to predict human perception of the vehicle sounds. 

Psychoacoustic models are described in detail in Section 6. Ambient levels for the ICE 

measurement site were collected intermittently between vehicle operation events as described in 

Section 4.2.4. 

4.4.2 Ambient for Countermeasure Measurements 

The ambient levels during ICE measurements were only of interest in order to make sure that the 

best estimate of the signal’s acoustic characteristics were correctly estimated. For standard 

acoustic measurements, ambient levels were determined the same as for the ICE measurement 

ambient. For direct electrical measurements, the ambient level was characterized by the instrument 

noise floor as determined by use of a “dummy” microphone, which simulates a microphone’s load 

without otherwise contributing to the measurement. 

4.4.3 Ambient for Countermeasure Subjective Testing 

Prior to conducting the human subject countermeasure tests, typical ambient levels were 

determined in urban and suburban areas, along Centre Street in Newton, MA, which is typically 

used as the “final exam” route for orientation and mobility students at the Carroll Center for the 

Blind. These measurements were conducted at 17 locations near intersections with relatively high 

traffic volume and intersections removed from the main road. (A list of the intersections is 

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Chapter 4: Acoustic Measurement of Vehicles and Ambient Sounds 

included in Appendix A.1). The locations examined included signalized and stop-controlled 

intersections, one-way streets, and side streets or driveways. Walkthroughs were conducted with 

different orientation and mobility instructors; data were collected on different days of the week and 

time of day. The purpose of these measurements was to determine what a reasonable ambient level 

would be for the countermeasure human-subject testing. 

Additionally, during the human-subject testing, the ambient sound was measured intermittently 

between test events as described in Section 4.2.4. These measurements were collected to support 

the analysis of human-subject testing of countermeasure sounds and for modeling human 

perception of both ICE vehicle sounds and prototype countermeasure sounds. 

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Chapter 5: Acoustic Analysis 

5. ACOUSTIC ANALYSIS 

For the acoustic measurements presented in this report, 1⁄2-second intervals were measured 

continuously for several seconds for each event. Four metrics of the acoustic character of the event 

where considered during the acoustic analysis: 1) the un-weighted 1⁄2-second equivalent sound 

pressure level (LZeq, 1⁄2 sec), 2) the A-weighted 1⁄2-second equivalent sound pressure level (LAeq, 1⁄2 sec), 

3) the maximum A-weighted sound pressure level using fast time weighting (LAF, max) over each 1⁄2-

second interval, and 4) the minimum A-weighted sound pressure level using fast time weighting 

(LAF, min) over each 1⁄2-second interval. These metrics were determined for 1) the sum over all 

frequencies, 2) the sum over a band-limited spectrum, and 3) over each one-third octave band, for 

LAeq, 1⁄2 sec only. 

The un-weighted 1⁄2-second equivalent sound pressure level is defined as (IEC 61672-1, 2002): 

૚,ࢗࢋࢆࡸ

૚/૛ 

૙࢖

࢚

 ࢀି࢚ ૛ ࢉࢋ࢙ ࢖ ׬ ሻࢀ/૚ቂሺ ૚૙ ൜ࢍ࢕࢒ ૙૛ ൌ ૛ dB ൠ ൗ ቃࣀࢊሻࣀሺࢆ

ሺߞሻ is ଶ ݌ ௓where ߞ is a dummy variable for the integration over time, ܶ is the period of integration, 

the squared, un-weighted instantaneous sound pressure, and ݌ ଴is the reference sound pressure. 

Similarly, the A-weighted 1⁄2-second equivalent sound pressure level is defined as: 

࡭࢖ ૚,ࢗࢋ࡭ࡸ

૛ሺࣀሻࣀࢊቃ

 ࢀି࢚ ૛ ࢉࢋ࢙ ׬ ሻࢀ/૚ቂሺ ૚૙ ൜ࢍ࢕࢒ ૙૛ ൌ ࢚ (A(dB૙ ࢖ ൗൠ૛ /૚

஺ሺߞሻ is the squared, A-weighted instantaneous sound pressure. The maximum A-weighted ଶ where ݌

sound pressure level using fast time weighting is determined by taking the maximum of the timeweighted sound pressure level given by: 

࢖ ૛ ࢋሻࣀሺ࡭ ࣎/ሻࣀି࢚ሺି ቃࣀࢊ

ࢀ

૙ ࡸ ࣎࡭ሺ࢚ሻ ൌ ૛૙ ࢍ࢕࢒൜ ૙૚ ቂሺ૚/࣎ሻ ׬૚ / ૛ൗൠ ࢖ ૙dB(A) 

Here ߬ is the time constant associated with the fast time weighting, 125 milliseconds, and t varies 

from 0 to T = 1⁄2-second. The minimum A-weighted sound pressure level using fast time weighting 

is determined by taking the minimum of the time-weighted sound pressure level of the above 

equation. Results are presented for the 1⁄2-second interval(s) that correspond to the vehicle’s closest 

proximity to the microphone line. For pass-by, start-up, and acceleration events, this is a single 1⁄2-

second interval. For idle events the average of multiple 1⁄2-second intervals are reported. 

5.1 Acoustic Analysis of ICE Vehicles 

This section provides an acoustic analysis of the ICE vehicles measured for the purposes of 

developing a preliminary specification for the minimum level of countermeasure sounds to be used 

in vehicles operating in electric only mode. 

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Chapter 5: Acoustic Analysis 

5.1.1 Overall A-weighted Sound Pressure Level Measurements 

The original measured A-weighted levels, LAF,min, LAeq, 1⁄2 sec, and LAF,max at the microphone line are 

summarized for all trials, vehicle side (left/right), and operating modes for each vehicle in Table 4 

to Table 6. At this point no corrections for ambient levels have been made. (Overall A-weighted 

uncorrected data are given for each event in Appendix A.2, Table 29 to Table 36). Table 4 

summarizes the measured minimum level for each vehicle operation as well as the ambient 

measurements. The “Min(LAF, min)” column indicates the minimum of all LAF, min measurements 

(associated with the vehicle’s closest proximity to the microphone line) for the specified operation. 

For example, the minimum LAF, min for the 6 mph, forward operation for all trials was 57.1 dB(A). 

Similarly the mean, maximum, and range for all trials are given in their respective columns. Table 

5 summarizes the LAeq, 1⁄2-sec metric in a similar fashion and Table 6 summarizes the LAF, max metric. 

Table 4. Summary of Overall A-weighted Levels as Measured (not Corrected for Ambient 

Level at Microphone Line, LAF ,min) 

Vehicle Operation Minimum 

(LAF, min) 

Mean 

(LAF, min) 

Maximum 

(LAF, min) 

Range (LAF, min) 

6 mph, Reverse 54.6 58.2 63.6 9.0 

6 mph, Forward 57.1 60.7 63.8 6.7 

10 mph, Forward 59.7 63.1 66.1 6.4 

15 mph, Forward 62.2 67.0 69.9 7.7 

20 mph, Forward 62.3 69.0 73.2 10.9 

Acceleration 60.5 66.0 71.3 10.8 

Start-up 46.1 65.0 75.6 29.5 

Idle 52.2 55.5 59.9 7.7 

Ambient 45.4 49.5 53.0 7.7 

Table 5. Summary of Overall A-Weighted Levels as Measured (not Corrected for Ambient 

Level) at Microphone Line, LAeq ,1/2 sec
 

Vehicle Operation Minimum 

(LAeq ,1/2 sec) 

Mean 

(LAeq ,1/2 sec) 

Maximum 

(LAeq ,1/2 sec) 

Range (LAeq ,1/2 

sec) 

6 mph, Reverse 54.9 58.7 64.8 9.9 

6 mph, Forward 57.3 61.2 63.9 6.6 

10 mph, Forward 60.0 64.1 66.4 6.4 

15 mph, Forward 62.7 68.1 71.2 8.5 

20 mph, Forward 63.6 70.2 73.9 10.3 

Acceleration 60.8 66.7 72.9 12.1 

Start-up 65.5 70.7 77.0 11.5 

Idle 52.5 55.7 60.1 7.6 

Ambient 46.2 50.0 53.4 7.2 

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Table 6. Summary of Overall A-Weighted Levels as Measured (not Corrected for Ambient 

Level) at Microphone Line, LAF, max
 

Vehicle Operation Minimum 

(LAF, max) 

Mean 

(LAF, max) 

Maximum 

(LAF, max) 

Range (LAF, max) 

6 mph, Reverse 55.2 59.0 65.5 10.3 

6 mph, Forward 58.2 61.7 64.1 5.9 

10 mph, Forward 60.9 64.1 66.7 5.8 

15 mph, Forward 65.2 68.8 71.6 6.4 

20 mph, Forward 64.8 71.0 74.3 9.5 

Acceleration 61.2 67.2 73.6 12.4 

Start-up 67.4 72.9 78.4 11.0 

Idle 53.2 56.1 60.4 7.1 

Ambient 47.1 50.8 54.0 6.9 

The first question of interest is whether each metric provides unique information. In order to 

understand the uniqueness of each metric in characterizing ICE vehicles under the operating 

conditions specified, LAeq, 1⁄2-sec data were compared with LAF, max in Figure 4, and with LAF, min in 

Figure 5. It can be seen that LAeq, 1⁄2-sec and LAF, max are highly correlated, having coefficient of 

determination of 0.99. However considering Figure 5, it appears that LAeq, 1⁄2-sec and LAF, min are not 

as highly correlated with a coefficient of determination of 0.80. Further examination revealed that 

the outliers (in the lower right corner) are all associated with the low values for LAF, min for start-up 

sounds. Since start-up events were very short, this is likely due to the LAF, min capturing ambient 

levels rather than the actual event. This indicates that LAF, min is not a desirable metric for 

quantifying the minimum sound level of start-ups. Removing these data from the comparison 

between LAeq, 1⁄2-sec and LAF, min significantly improves the correlation, as can be seen in Figure 6, 

where the coefficient of determination increases to 0.99. The average differences between LAeq, 1⁄2­

sec and LAF, max were about 0.2 dB and the average differences between LAeq, 1⁄2-sec and LAF, min 

(excluding start-up) were about 0.1 dB. Thus it was concluded that LAeq, 1⁄2-sec provided a reasonable 

characterization of the acoustic character of all operations. Therefore, LAeq, 1⁄2-sec is used for the 

remainder of the analysis. 

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Chapter 5: Acoustic Analysis 

Figure 4. Correlation Between (not Corrected for Ambient Level) LAeq, 1/2 sec and LAF, max 

Figure 5. Correlation Between (not Corrected for Ambient Level) LAeq, 1/2 sec and LAF, min 

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Chapter 5: Acoustic Analysis 

Figure 6. Correlation Between (not Corrected for Ambient Level) LAeq, 1,2 sec and LAF, min 

(Start-up Removed) 

5.1.2 Determination of Frequency Range of Analysis 

In this phase of the study, it was desired to measure ICE vehicles under ambient conditions that 

could be expected to be encountered by pedestrians. (In contrast to phase 1 of this study where it 

was desired to measure vehicles in the lowest ambient conditions possible.) These ambient 

conditions have sound levels which can effect the ability to measure the sound level of the ICE 

vehicles accurately. Therefore it was necessary to determine what portion of the measured sound 

was due predominantly to the measured vehicle and which portion was due predominantly to the 

ambient. It was expected that a frequency range that was due predominantly to the measured 

vehicle could be identified and that range would form the basis for further analysis. The frequency 

range was determined by comparing measurements of ICE vehicles with measurements of the 

ambient level during the measurements. Details of the measurement procedure are given in Section 

4.2.4. In order to get an estimate that did not exclude any potential useful data, the ambient level 

for this comparison was set to the minimum in each third octave band for all ambient 

measurements during the ICE vehicle measurements. In this way, an estimate of the vehicle sound 

levels is conservatively high, that is, there is not a risk of underestimating the mininum level of the 

ICE vehicles. 

In Figure 7, two operating modes (20 mph pass-by and Idle) are compared with the minimum 

ambient estimate. Here it can be seen that the ambient level is very close to the vehicle measured 

level below 100 Hz, making it impossible to accurately estimate the vehicle sound levels in this 

frequency range using the measured data. Therefore for further analysis of sound pressure levels, 

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Chapter 5: Acoustic Analysis 

the data are band limited to include one-third octave bands from 100 Hz to 20 kHz. Typically, band 

limiting the data results in difference in the overall A-weighted level no greater than 0.1 dB. 

Figure 7. Comparison of Measured Levels of ICE Vehicles for two Operating Conditions 

(20 mph and Idle) With the Minimum Estimate of The Ambient Level. These Data for all 

Operating Conditions are Given in Appendix A-2. 

5.1.3 LAeq, 1⁄2 sec Sound Pressure Levels Corrected for Ambient Level 

SAE-J2889-1 Rev 2009 provides a method for correcting measured levels to account for ambient 

levels present during the measurement. This method operates on the overall level. Because thirdoctave band data is being analyzed for this report, corrections based on the overall level are not 

sufficient, therefore corrections were conducted in a manner similar to SAE-J2889-1 Rev 2009, but 

on a one-third octave band basis as follows: If the sound pressure level of the measured event in a 

specified one-third octave band is greater than or equal to 3 dB more than the ambient level for the 

specified one-third octave band, then the reported vehicle test condition sound pressure level shall 

be corrected according to: 

܎܍ܚቅ dB ૛

૛ ۾/ቁܜܖ܍ܑ܊ܕ܉

 ࡸ ૙૚ ܏ܗܔ૙૚ ൌ ࢊࢋ࢚ࢉࢋ࢘࢘࢕ࢉ ۾ቄቀ ૛ ۾െ ܚܝܛ܉܍ܕ܌܍

where, Pref is 20 PA. 

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Chapter 5: Acoustic Analysis 

If this criterion is not met, then the specified one-third octave band for the measured event is 

removed from further analysis. It is necessary to remove this data because “zeroing it out” or 

setting its level to the ambient will produce incorrect average results. 

Once the individual one-third octave bands were corrected for the ambient level present during 

measurements, the broad band level was recomputed by summing the A-weighted one-third octave 

bands from 100 Hz to 20,000 Hz. The minimum, average, maximum, and range of LAeq, 1/2sec are 

shown for the vehicles measured for each operating condition in Table 7. The difference between 

the corrected (and range limited) broad band levels and the original measured levels are shown in 

Table 8. Note that these differences are generally small, with an average difference of only -0.2 dB, 

that is, the corrected, band limited levels were on average about 0.2 dB lower than the original 

measured levels. This difference is small because most of the energy discounted was at low levels 

and low frequencies and therefore did not contribute significantly to the broad band level. 

However, these corrections were important so that low frequency one-third octave bands are 

correctly represented in the spectral specification in Section 5.1.4 as well as the psychoacoustic 

analysis in Section 6. 

Table 7. Summary of ICE Measurements Using LAeq, 1⁄2 sec Adjusted for Ambient Level and 

Restricted to Frequency Range from 100 Hz to 20 kHz. 

Vehicle Operation Minimum 

(LAeq) 

Mean 

(LAeq) 

Maximum 

(LAeq) 

Range (LAeq) 

6 mph, Reverse 54.4 58.4 64.8 10.4 

6 mph, Forward 57.2 61.1 63.8 6.6 

10 mph, Forward 60.0 63.6 66.3 6.3 

15 mph, Forward 62.8 68.1 71.1 8.2 

20 mph, Forward 63.4 70.2 73.8 10.5 

Acceleration 60.7 66.7 72.9 12.2 

Start-up 65.3 70.7 77.0 11.7 

Idle 51.6 55.2 59.9 8.4 

Ambient 46.1 50.0 53.4 7.2 

Table 8. Difference Between Corrected (and Range Limited) and Uncorrected Time 

Averaged Levels, LAeq, 1⁄2 sec
 

Vehicle Operation Delta Minimum Delta Mean Delta Maximum Delta Range 

6 mph, Reverse -0.5 -0.3 0.0 0.5 

6 mph, Forward -0.1 -0.1 -0.1 0.0 

10 mph, Forward 0.0 -0.5 -0.1 -0.1 

15 mph, Forward 0.1 0.0 -0.1 -0.3 

20 mph, Forward -0.2 0.0 -0.1 0.2 

Acceleration -0.1 -0.1 0.0 0.1 

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Chapter 5: Acoustic Analysis 

Vehicle Operation Delta Minimum Delta Mean Delta Maximum Delta Range 

Start-up -0.2 0.0 0.0 0.2 

Idle -1.0 -0.5 -0.1 0.8 

Ambient -0.1 -0.1 0.0 0.1 

5.1.4 Preliminary Specification for Detectability 

The preliminary specification for countermeasure sounds is based on the supposition that typical 

ICE vehicles presently emit an acceptable amount of noise during low speed operations. These 

preliminary specifications represent the minimum overall A-weighted sound pressure level, and the 

minimum level in each one-third octave band for a countermeasure sound that simulates an ICE 

vehicle. Table 9 gives the preliminary specification for the broad band A-weighted sound pressure 

level for a vehicle generating a countermeasure sound. Figure 8 to Figure 15 show the 

corresponding A-weighted one-third octave band spectra. Based on the sounds we studied, each of 

these minima must be met in order to assure that the countermeasure sound emits sound at a level 

at least as great as the estimated typical ICE vehicle. Note that the logarithmic sum of the one-third 

octave bands do not equal the overall A-weighted sound pressure level requirement for a given 

operating mode. This is because the overall A-weighted sound pressure level is the average of the 

logarithmic sums while the sum of the one-third octave bands is a logarithmic sum of linear 

averages. That is, the order of the non-linear operator is not the same. 

Table 9. Preliminary Specifications for Minimum Overall A-weighted Level (LAeq, 1⁄2 sec) by 

Vehicle Operation 

Vehicle Operation LAeq, 1/2 sec, dB(A) 

6 mph, Reverse 58.4 

6 mph, Forward 61.1 

10 mph, Forward 63.6 

15 mph, Forward 68.1 

20 mph, Forward 70.2 

Acceleration 66.7 

Start-up 70.7 

Idle 55.2 

Ambient 58.4 

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Figure 8. Minimum Level for Each One-Third Octave Band: 6 mph, Reverse. 

Figure 9. Minimum Level for Each One-Third Octave Band: 6 mph, Forward 

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Chapter 5: Acoustic Analysis 

Figure 10. Minimum Level for Each One-Third Octave Band: 10 mph, Forward. 

Figure 11. Minimum Level for Each One-Third Octave Band: 15 mph, Forward 

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Chapter 5: Acoustic Analysis 

Figure 12. Minimum Level for Each One-Third Octave Band: 20 mph, Forward 

Figure 13. Minimum Level for Each One-Third Octave Band: Acceleration 

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Chapter 5: Acoustic Analysis 

Figure 14. Minimum Level for Each One-Third Octave Band: Startup 

Figure 15. Minimum Level for Each One-Third Octave Band: Idle 

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Chapter 5: Acoustic Analysis 

5.2 Acoustic Analysis of Countermeasure Sounds 

In addition to the ICE vehicles measured in order to develop a preliminary specification for 

detectability, a total of five countermeasure systems, capable of producing one or multiple sounds, 

were also measured. The systems are labeled A, B, C, D, and E and a total of nine sounds were 

evaluated: A1, A2, A5, B, C, D, E1, E3, and E4. These sounds were all intended to simulate 6 

mph forward pass-by operations. As discussed in Section 4.3, two methods were used to measure 

these countermeasure sounds, 1) direct electrical measurements of the speaker input where these 

were accessible (A, B and E sounds), as well as 2) standard acoustical measurements, which were 

the only type available when access to the speaker was not possible (sounds C and D). Direct 

electrical recording permits an accurate spectral analysis of the synthetic sound generator output. 

However, if that is not available, any spectral analysis would necessarily be contaminated with tire 

noise and ambient noise. Because the post-processing of these two data sets was different from the 

sounds measured for ICE vehicles, results are reported separately. The acoustic analysis follows. 

(The human-subject evaluation of countermeasures is discussed in Section 7) 

5.2.1 LAeq, 1⁄2 sec Sound Pressure Levels Corrected for Ambient Level – 

Electrical Measurement 

For countermeasure sounds measured electrically, a single measurement was made for each sound, 

however, during countermeasure subjective testing, these sounds were adjusted to be played such 

that the broadband A-weighted level was one of two levels, 59.5 and 63.5 dB(A). Therefore, 

during this acoustic analysis, the electrically measured signals were adjusted to produce these same 

results. Because of the countermeasure designs themselves as well as the physical limitations of the 

speakers used, countermeasure sounds had very little low frequency content. Even though the 

electrical noise of the measurement system was very low, ranging between 3 and 24 dB(A) per 

octave band, the countermeasure signals were still typically too low to be measured, therefore, only 

the levels from 50 Hz to 20,000 Hz are presented here. Note that sound levels in lower frequency 

bands were still much lower than the existing ambient during the human subject testing, as will be 

discussed in greater detail later. Table 10 and Table 11 show the one-third octave band levels for 

each of the countermeasure sounds measured electrically with corrections for the instrument noise 

floor for this frequency range. For comparison with the ICE vehicle measurements, the band sums 

from 100 Hz to 20,000 Hz are also given. 

Table 10. LAeq, 1⁄2 sec Sound Pressure Levels Corrected for Ambient Level for 

Countermeasure Sounds Using Direct Electrical Measurement for low Playback Setting 

1/3 Octave Band 

Center Frequency 

(Hz) 

Sound 

A1 

Sound 

A2 

Sound 

A5 

Sound 

B 

Sound 

E1 

Sound 

E3 

Sound 

E4 

100 to 20,000 59.6 59.6 59.6 58.7 59.1 59.6 59.5 

50 3.3 -21.9 

Below 

Ambient 48.3 22.8 28.6 0.8 

63 11.9 -16.9 

Below 

Ambient 37.7 32.1 21.5 6.0 

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Chapter 5: Acoustic Analysis 

80 28.5 -3.6 

Below 

Ambient 16.8 51.0 31.5 11.6 

100 40.4 11.9 

Below 

Ambient 57.8 53.3 43.1 21.4 

125 41.3 26.1 

Below 

Ambient 51.1 56.0 36.4 33.8 

160 45.1 40.7 -2.5 31.5 43.0 39.7 30.9 

200 46.2 43.0 14.9 41.2 50.8 47.6 38.9 

250 56.7 32.6 26.9 26.4 43.7 47.0 36.8 

315 46.0 43.8 37.0 11.4 42.4 46.1 38.7 

400 47.8 48.0 55.2 6.9 31.9 50.9 53.9 

500 45.7 46.0 56.5 12.3 40.6 51.1 46.4 

630 41.7 50.3 45.4 10.7 38.2 49.2 35.7 

800 44.6 49.2 43.9 12.7 24.4 50.3 43.5 

1k 42.4 47.6 41.7 0.0 34.5 48.7 40.1 

1.25k 50.7 48.1 41.7 13.8 18.6 47.9 43.9 

1.6k 39.3 50.9 43.4 15.2 29.7 47.6 41.7 

2k 44.3 50.1 38.0 1.2 1.2 44.6 45.6 

2.5k 24.9 50.0 29.6 1.3 1.3 42.7 56.7 

3.15k 1.2 48.1 35.3 1.2 1.2 41.3 35.8 

4k 1.0 42.8 27.8 1.0 1.0 39.3 21.6 

5k 0.5 42.8 22.9 0.5 0.5 38.8 16.9 

6.3k -0.1 29.7 15.8 -0.1 -0.1 36.4 -0.1 

8k -1.1 -1.1 -1.1 -1.1 -1.1 32.0 -1.1 

10k -2.5 -2.5 -2.5 -2.5 -2.5 24.8 -2.5 

12.5k -4.3 -4.3 -4.3 -4.3 -4.3 17.9 -4.3 

16k -6.6 -6.6 -6.6 -6.6 -6.6 -6.6 -6.6 

20k -9.3 -9.3 -9.3 -9.3 -9.3 -9.3 -9.3 

Table 11. LAeq, 1⁄2 sec Sound Pressure Levels Corrected for Ambient Level for 

Countermeasure Sounds Using Direct Electrical Measurement for High Playback Setting 

1/3 Octave Band Center 

Frequency (Hz) 

Sound 

A1 

Sound 

A2 

Sound 

A5 

Sound 

B 

Sound 

E1 

Sound 

E3 

Sound 

E4 

100 to 20,000 63.6 63.6 63.6 62.7 63.1 63.6 63.5 

50 7.3 -16.9 

Below 

Ambient 52.3 26.8 32.6 4.8 

63 15.9 -11.2 

Below 

Ambient 41.7 36.1 25.5 10.0 

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Chapter 5: Acoustic Analysis 

80 32.5 0.5 

Below 

Ambient 20.8 55.0 35.5 15.6 

100 44.4 15.9 -12.2 61.8 57.3 47.1 25.4 

125 45.3 30.1 -5.4 55.1 60.0 40.4 37.8 

160 49.1 44.7 2.7 35.5 47.0 43.7 34.9 

200 50.2 47.0 19.0 45.2 54.8 51.6 42.9 

250 60.7 36.6 30.9 30.4 47.7 51.0 40.8 

315 50.0 47.8 41.0 16.3 46.4 50.1 42.7 

400 51.8 52.0 59.2 12.6 35.9 54.9 57.9 

500 49.7 50.0 60.5 16.8 44.6 55.1 50.4 

630 45.7 54.3 49.4 15.7 42.2 53.2 39.7 

800 48.6 53.2 47.9 17.7 28.5 54.3 47.5 

1k 46.4 51.6 45.7 17.0 38.5 52.7 44.1 

1.25k 54.7 52.1 45.7 18.9 23.0 51.9 47.9 

1.6k 43.3 54.9 47.4 20.2 33.8 51.6 45.7 

2k 48.3 54.1 42.0 21.0 1.2 48.6 49.6 

2.5k 29.1 54.0 33.7 1.3 1.3 46.7 60.7 

3.15k 1.2 52.1 39.3 1.2 1.2 45.3 39.9 

4k 1.0 46.8 31.9 1.0 1.0 43.3 26.2 

5k 0.5 46.8 27.2 0.5 0.5 42.8 22.0 

6.3k -0.1 33.8 21.2 -0.1 -0.1 40.4 19.3 

8k -1.1 15.2 -1.1 -1.1 -1.1 36.0 15.5 

10k -2.5 -2.5 -2.5 -2.5 -2.5 29.0 -2.5 

12.5k -4.3 -4.3 -4.3 -4.3 -4.3 22.5 -4.3 

16k -6.6 -6.6 -6.6 -6.6 -6.6 -6.6 -6.6 

20k -9.3 -9.3 -9.3 -9.3 -9.3 -9.3 -9.3 

The data in Table 10 is also given in Figure 16, where it can easily be seen that only 

countermeasures ‘B’ and ‘E1’ had any significant low frequency content, and that the rest typically 

had significant content only above 100 to 200 Hz (here a component is considered significant if it 

is within 10 dB of neighboring one-third octave bands). This supports the further consideration of 

the frequency range from 100 to 20,000 Hz for analysis. Further, countermeasure sounds tended to 

have distinct peaks in their frequency spectra. Most had narrow-band components that were at 

least 10 dB higher than the neighboring content for a given countermeasure sound. For 

countermeasures ‘A2’ and ‘A5’, these narrow band components straddled two one-third octave 

bands, causing them to appear broader, however, this is an artifact of one-third octave band 

analysis. Countermeasures ‘B’ and ‘E1’ had the most relative low frequency content, 

countermeasures ‘A5’ and ‘E4’ had content that was centered in the mid-frequencies from 315 to 

3150 Hz, and countermeasures ‘A2’ and ‘E3’ had the most broad band content. 

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Chapter 5: Acoustic Analysis 

0 

10 

20 

30 

40 

50 

60 

70 

5080

125200315500800

12.5k

2k

3.15k

5k8k

12.5k

20k 

A-weighted Sound Pressure Level, dB(A) 

One-Third Octave Band Center Frequency, Hz 

Sound A1 

Sound A2 

Sound A5 

Sound B 

Sound E1 

Sound E3 

Sound E4 

Figure 16. Countermeasure Sounds at Low Level, Nominal 59.5 dB(A). See Table 10 for 

Data Plotted 

5.2.2 LAeq, 1⁄2 sec Sound Pressure Level Corrected for Ambient LevelStandard Measurement 

The spectra and 100 to 20,000 Hz band sums are given for the two countermeasure systems 

measured at the ICE measurement site in Table 12. For both of these systems, the output level was 

not easily controllable. For system ‘D’, the maximum level was used, which could only achieve 

the lower level desired for the human-subject testing. For system ‘C’, the level was adjustable, 

with a small degree of uncertainty, and could be set to achieve the lower and higher levels desired 

for the human-subjects testing. A single level was used for this measurement, which targeted the 

higher level for human-subject testing. The measured level presented in Table 12 represents a 

single measurement, for each operation, and falls within the expected experimental error for the 

target level of 63.5 dB(A). Note, that neither reverse measurements (‘D’ or ‘C’) were used in the 

human-subject testing. These were measured simply to document the countermeasure sounds for 

both of these vehicles in reverse. Figure 17 shows these spectra. Note that below 200 Hz, the 

effect of the ambient at the measurement site made the measurement of the vehicle sounds 

uncertain and much of the data were rejected below this frequency, therefore this range is not 

shown in the figure. On the other hand the tonal rise around 3150 Hz is a reliable measurement. It 

appears that both countermeasure designers chose to emphasize this frequency range due to human 

hearing sensitivity at this frequency. The human auditory system is most sensitive at the threshold 

of hearing to sound between 1000 and 5000 Hz (Fletcher, H. and Munson, W. A. (1933). The 

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Chapter 5: Acoustic Analysis 

developer of system ‘C’ indicated that part of their design for reverse was to increase the level 

coming from the front speaker so that the sound would be sufficiently loud while backing up. 

However, the measurement procedure used here was insensitive to the shielding effects of the 

vehicle since it focused on measuring the front of the vehicle. The measurement procedure could 

be adjusted so that the measurement references the rear of the vehicle for reverse operations. 

Table 12. LAeq, 1⁄2 sec Sound Pressure Levels Corrected for Ambient Level for 

Countermeasure Vehicles Using Traditional Measurement 

1/3 Octave Band 

Center Frequency 

(Hz) Sound C Forward Sound C Reverse Sound D Forward Sound D Reverse 

100 to 20 k 62.4 77.7 59.8 59.4 

50 Below Ambient Below Ambient Below Ambient Below Ambient 

63 Below Ambient 30.4 Below Ambient 0.5 

80 Below Ambient 4.5 Below Ambient Below Ambient 

100 Below Ambient Below Ambient 7.7 7.2 

125 Below Ambient Below Ambient Below Ambient 37.9 

160 Below Ambient 10.9 Below Ambient Below Ambient 

200 39.4 35.6 36.6 40.1 

250 44.4 38.0 42.6 44.9 

315 46.3 39.4 41.4 46.8 

400 48.6 41.9 39.0 42.1 

500 54.4 50.7 40.4 42.0 

630 57.5 51.5 44.3 44.1 

800 53.3 46.5 46.6 44.9 

1k 48.7 40.7 43.4 45.8 

1.25k 44.4 46.7 43.3 47.0 

1.6k 42.6 46.2 42.7 46.1 

2k 44.9 76.2 56.9 54.0 

2.5k 55.1 72.2 53.7 54.6 

3.15k 42.5 52.2 36.6 39.0 

4k 40.6 38.0 33.2 33.1 

5k 37.1 31.5 43.1 32.4 

6.3k 35.9 29.4 32.5 27.9 

8k 38.1 13.1 30.0 25.7 

10k 32.9 25.5 26.4 22.7 

12.5k 24.1 19.0 24.1 20.7 

16k 23.7 17.7 19.9 3.7 

20k 17.8 15.1 -9.3 -9.3 

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Chapter 5: Acoustic Analysis 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

200315500800

1.25k

2k

3.15k

5k8k

12.5k

20k 

A-weighted Sound Pressure Level, dB(A) 

One-Third Octave band Center Frequency, Hz 

Sound C Forward 

Sound C Reverse 

Sound D Forward 

Sound D Reverse 

Figure 17. Countermeasure Sounds Measured at ICE Vehicle Measurement Site. See 

Table 12 for Data Plotted
 

5.2.3 Measurement of Vehicle Directivity 

When evaluating detectability of sounds according to the levels measured with the orientation 

shown in Figure 3, it is important to consider that the level changes not only with distance but with 

the orientation between the vehicle and the subject. For example, if a sound radiates most 

effectively to the right of the vehicle, then the level will be greater to the right compared to, for 

example, the front of the vehicle. Therefore, detectability of a sound will depend not only on the 

measured level and the change due to distance, but will also depend on the relative change in level 

due to the orientation between the vehicle and the subject at the time of detection. 

Directivity patterns describe the change in sound pressure level as a function of angular position 

relative to a reference line. Directivity measurements were made according to the layout shown in 

Figure 18. At each angle the sound pressure level of pink noise was measured at 6, 12, and 25 foot 

distances from the speaker. The results for each distance relative to the 0o

 value for the distance 

are given in Table 13. This method requires a stationary vehicle and therefore could only be 

conducted on systems A, B, and E. Systems C and D could not be measured because their sounds 

could not be evaluated while the vehicles were stationary. These directivities represent the 

condition where a single external speaker was mounted on the front bumper. No directivity data is 

available for comparison with ICE vehicles at this time. These results are for context only and are 

not being used to suggest a preferred directivity pattern. 

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Chapter 5: Acoustic Analysis 

Figure 18. Measurement Diagram for Directivity Measurements. Reference Line is at 0o 

Table 13. Attenuation, in dB, as a Function of Angle Relative to Vehicle Center Line for 

Three Distances From Speaker (6, 12, and 25 ft) 

Angle Relative to Vehicle 

Centerline (degrees) 

Attenuation 6 ft from 

speaker 

Attenuation 12 ft from 

speaker 

Attenuation 25 ft from 

speaker Average Attenuation 

-90 -7.9 -8.3 -8.3 -8.2 

-45 -3.9 -2.9 -3.7 -3.5 

0 0.0 0.0 0.0 0.0 

45 -3.3 -3.6 -4.6 -3.8 

90 -9.0 -9.2 -9.4 -9.2 

The results indicate that for the systems evaluated, the difference in level for a subject directly in 

front of the speaker and one at a ninety degree angle to the speaker can be as high as 9 dB for the 

same distance. This result makes it unclear as to what sound pressure level the subject actually 

detects the vehicle, because the actual sound pressure level is a function of both distance and angle. 

Note however, that for any pass-by situation it is reasonable to expect a vehicle to be detected in 

the range between -45 to 45 degrees. In such cases the attenuation is significantly less, on the 

order of 3 dB. 

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37 
 

 

 

Chapter 5: Acoustic Analysis 

5.3 Acoustic Analysis of Ambient Sounds 

5.3.1 Ambient Levels during ICE Measurements-‘Cottage Farm’ 

Ambient 

The primary purpose for these measurements was two-fold: 1) to determine corrections for 

measurements and 2) as an input to Psychoacoustic models. The corrections to measurements 

were discussed in Section 5.1.3. For these corrections, the minimum level for each one-third octave 

band was used. The ambient estimate used in the psychoacoustic models was derived from the 

minimum overall A-weighted level. Ambient levels for ICE measurements are summarized in 

Table 14 and Figure 19. 

Table 14. Summary of Ambient Levels During ICE Measurements, A-weighted Level, 

dB(A) 

1/3 Octave Band 

Center Frequency, 

Hz 

Linear Average 

(1/3 Octave Band) 

Min (Overall Aweighted) Max (Overall Aweighted) Min (1/3 Octave 

Band) 

Max (1/3 Octave 

Band) 

100 to 20k 49.6 46.1 53.4 45.3 54.7 

100 34.6 30.7 34.1 30.7 38.4 

125 35.5 32.4 36.8 32.4 42.1 

160 36.1 32.1 37.9 32.0 41.5 

200 36.9 32.7 37.9 32.7 41.2 

250 36.5 33.9 38.1 33.1 40.7 

315 36.5 32.5 37.6 32.1 41.5 

400 36.0 31.9 38.1 31.8 39.7 

500 36.7 33.6 39.8 33.1 41.1 

630 38.2 34.4 41.7 34.0 42.2 

800 40.2 36.0 46.1 35.8 46.1 

1k 41.1 36.4 46.4 36.4 46.4 

1.25k 40.0 35.3 45.1 35.3 45.1 

1.6k 37.6 32.9 43.1 32.9 43.1 

2k 34.7 30.3 37.8 30.3 37.8 

2.5k 34.5 32.8 35.4 30.8 42.1 

3.15k 35.5 36.9 37.1 30.0 39.6 

4k 34.0 33.0 34.3 28.3 40.2 

5k 29.0 25.0 29.8 24.3 32.8 

6.3k 25.7 22.3 26.9 19.7 31.7 

8k 20.2 16.6 22.4 14.1 24.2 

10k 14.4 10.3 17.3 7.6 18.3 

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Chapter 5: Acoustic Analysis 

1/3 Octave Band 

Center Frequency, 

Hz 

Linear Average 

(1/3 Octave Band) 

Min (Overall Aweighted) Max (Overall Aweighted) Min (1/3 Octave 

Band) 

Max (1/3 Octave 

Band) 

12.5k 8.9 5.0 11.7 3.2 13.0 

16k 3.1 0.7 5.6 -0.8 8.7 

20k -1.9 -3.1 -0.4 -3.5 2.0 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

100125160200250315400500630800

1k

1.25k

1.6k

2k

2.5k

3.15k

4k5k

6.3k

8k

10k

12.5k

16k20k

A-weighted Sound Pressure Level, dB(A)

One-Third Octave Band Center Frequency, Hz 

Ambient ICE Vehicle Measurement Site 

Max (1/3 OB) Max (Overall) Linear Avg (1/3OB) 

Min (Overall) * Min (1/3 OB) ** 

Figure 19. Five Measures of the Ambient at the ICE Vehicle Measurement Site. *The 

Minimum Overall Level Was Used for Psychoacoustic Computations. **The Minimum in 

One-Third Octave Band Was Used for Sound Pressure Level Corrections. Data are 

Repeated in Table 14 

5.3.2 Ambient Levels Measured along Suburban Streets-‘Suburban’ 

Ambients 

In order to determine what ambient levels would be reasonable for the human-subject testing, the 

ambient levels at several suburban intersections were measured and analyzed. The measurement 

locations were grouped into two categories: 1) busy intersections and 2) quiet back streets. A total 

of 17 sites over the course of three days were measured. The distribution of the one-half second 

equivalent A-weighted sound pressure levels are show in Figure 20. Here it can be seen that the 

average level of the quiet back street intersections was around 49 dB(A), the average level of the 

busy intersections was around 69 dB(A), and the average of the two was around 59 dB(A). 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

39 
 

Chapter 5: Acoustic Analysis 

Figure 20. Ambient Level Distribution in Suburban Settings Including Both Light and 

Heavy Traffic 

5.3.3 Ambient Level During Countermeasure Subjective Testing­

‘Countermeasure Test’ Ambient 

Given the 20 dB range for typical sub-urban streets, it was necessary to establish at what ambient 

level the human-subject tests would be conducted. Because the highest ambient levels would 

likely mask all but the loudest countermeasure sounds, and since the lowest ambient levels would 

likely fail to mask all but the quietest countermeasure sounds, it was decided to target an ambient 

in the middle, that is around 59 dB(A). The site chosen was on the Volpe Center campus and had 

an ambient level that ranged from approximately 58 to 61 dB(A). At some points ambient levels 

were higher, but in general testing was halted to work around these times. The minimum, average, 

and maximum sound pressure levels for a sample period of the measured ambient are given in 

Figure 21 for August 14th. Similar results were obtained for the other days tested. An abridged 

version of the presented ambient spectrum was also used in psychoacoustic modeling. The 

abridged version is given in Table 15. 

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Chapter 5: Acoustic Analysis 

Figure 21. Sample Ambient Level During Countermeasure Testing on August 14th, 2010. 

Ambient Levels Ranged from 58 to 61 dB(A) During Testing 

Table 15. Ambient Level During Countermeasure Testing Used for Computation of 

Psychoacoustic Metrics 

0 

10 

20 

30 

40 

50 

60 

12.5

20

31.5

5080

125200315500800

1.25K

2K

3.15K

5k8k

12.5K

20k 

A-weighted SPL, dB(A) 

One-Third Octave Band Center Frequency, Hz 

Min 

Max 

Mean 

1/3 Octave Band 

Center Frequency, 

Hz SPL, dB(A) 

50 30.5 

63 34.1 

80 37.0 

100 44.4 

125 46.0 

160 44.1 

200 45.5 

250 49.0 

315 45.5 

400 47.0 

500 47.8 

630 49.5 

800 48.7 

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Chapter 5: Acoustic Analysis 

1/3 Octave Band 

Center Frequency, 

Hz SPL, dB(A) 

1000 47.8 

1250 45.7 

1600 44.2 

2000 43.6 

2500 37.1 

3150 33.5 

4000 29.4 

5000 24.2 

6300 20.3 

8000 18.5 

10000 16.9 

12500 15.4 

16000 14.4 

20000 13.0 

The predominant sources of this ambient noise were the air-conditioning fans on the roofs of 

buildings about 400 feet west of the subject seating area. These were of essentially constant 

amplitude. Among the variable sources were direct wind noises, the rustling of tree leaves, and 

distant vehicles. 

While the ambient noise produced by the air-conditioning fans varied by less than 1 dB (as 

measured by a monaural microphone) along the line of 28 seated subjects, it was clearly louder 

from the west than the east, when listened to subjectively. To confirm this, measurements were 

made using a binaural measurement system at the east end of the subject seating area. The ambient 

levels measured by the left (west) microphone were about 1.4 dB greater than the ambient levels 

measured by the right (east) microphone. The difference was even greater for frequencies from 

800 to 20,000 Hz, where the average difference between the two sides was about 3.1 dB (see 

Figure 22). Thus, it is expected, because of the occlusion caused by the subject’s head, that the 

effective ambient will be greater for subjects when they attempt to detect vehicles approaching 

from the west. 

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Chapter 5: Acoustic Analysis 

0 

10 

20 

30 

40 

50 

60 

12.5

20

31.5

5080

125200315500800

12502000315050008000

1250020000 

Sound Pressure Level, dB(A) 

One-Third Octave Band Center Frequency, Hz 

Left Channel Right Channel 

Figure 22. Comparison of Ambient Level for Left and Right Sides of Binaural Head During 

Countermeasure Testing
 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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Chapter 6: Psychoacoustic Analysis 

6. PSYCHOACOUSTIC ANALYSIS 

Using the ICE and countermeasure signal data, several psychoacoustic models were applied in an 

effort to refine minimum specifications. 

6.1 Comparison of Various Psychoacoustic Models for Two 

Sounds 

Sound pressure level based metrics such as the A-weighted level provide a first estimate of the 

perceived loudness of a sound, however, these metrics fail to account for several factors that affect 

the perceived loudness including: 1) the level dependence on the frequency sensitivity, 2) level 

dependence on frequency selectivity, 3) frequency based masking effects, 4) pre- and postmasking, 5) co-modulation masking release, etc. 

The level dependence on the frequency sensitivity refers to the fact that for the same change in 

sound pressure level for a low frequency sound and a high frequency sound, the low frequency 

sound will be perceived as increasing in loudness more than the high frequency sound. Sound 

pressure level frequency weightings such as A-, C-, and D-weighting account for this very roughly, 

however, these weighting functions are most appropriately chosen based on the overall level, and 

thus implicitly assume uniform spectral content. 

The typical method of approximating frequency selectivity using sound pressure level metrics is to 

analyze the sound using one-third octave bands. These bands offer a very rough approximation of 

the human auditory system’s ability to analyze different frequencies of sounds separately. The 

actual shape and bandwidth of the auditory system’s frequency selectivity are more accurately 

related to the auditory tuning curves which are narrower for low levels and become broader with 

increased level (as well as with increased center frequency). 

The human auditory system cannot completely separate frequency components. That is, the 

energy associated with a given frequency excites one region of the basilar membrane most 

effectively, but also excites neighboring regions to a lesser extent. (The basilar membrane is a 

membrane that attaches to the organ of Corti and together they are largely responsible for place 

based frequency selectivity.) Thus, a high energy component can prevent or reduce the perception 

of a lower energy component at a different frequency. That is, for example, an ambient with a 

high level of low frequency sound can mask a signal with components in a higher frequency range. 

Other factors can also affect the perceived loudness of a sound to a lesser extent. For example, 

although not as strong, masking effects can be measured for sounds that happen either directly 

before or directly after the signal sound. Additionally, masking effects can be reduced by various 

processes such as, for example, co-modulation masking release. The key point here is that sound 

pressure level metrics offer a good first approximation to the perceived loudness of a sound, 

however, when it is desired to understand how quiet sounds are perceived in the presence of an 

ambient level, more refined models may provide additional guidance. 

Several psychoacoustic models exist that relate sound pressure level data to either the perceived 

loudness of the signal or detectability/audibility. Loudness models include various versions of 

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Chapter 6: Psychoacoustic Analysis 

models proposed by Zwicker (Zwicker & Fastl, 1999; ISO 532-B; DIN 45 631) and Moore 

(Moore, 1993; ANSI S3.4-2007). Detectability models include Audibility (Fleming et al., 2005), 

Detect Sound (Zheng et al., 2007); Giguère et al., 2008) , as well as Zwicker’s loudness model 

with ambient (Kerber, S. & Fastl , 2008) and Moore’s loudness model with ambient (Moore & 

Glasberg, 1997; Moore, Glasberg, and Bear, 1997). Both Zwicker’s and Moore’s loudness models 

address the first three factors mentioned above, and addressing these factors provides significant 

improvement on the modeling of perceived loudness. For detectability models, two approaches are 

taken. The Audibility model is more closely related to sound pressure level metrics, but provides a 

formal method to account for frequency selectivity using one-third octave bands, accounts for 

masking effects due to the ambient and the auditory system’s internal noise, and accounts for a 

threshold which may need to be exceeded in order for a signal to be considered audible. Detect 

Sound and a version of Zwicker’s loudness which accounts for the ambient both use loudness 

models (Detect Sound uses Moore’s model) to compute the excitation due to both the ambient and 

the signal of interest, and then determine when the ambient no longer completely masks the signal. 

Note, Moore has also published a version of his model that takes into account the ambient, 

however, it is not known by the authors of this report, whether or not there is a minimum threshold. 

It is assumed that there is none, in which case, provided that a signal produced non-zero loudness 

in the presence of an ambient, it would be considered audible. A summary of the models discussed 

here is given in Table 16. 

Table 16. Comparison of Psychoacoustic Models 

Model Type Pros Cons 

Loudness (Zwicker) Loudness International Standard, 

Mature, Comprehensive 

Output departs from standard 

concept of frequency 

Loudness (Moore) Loudness International Standard, 

Mature, Comprehensive 

Output departs from standard 

concept of frequency 

Audibility Detection Simple to Compute, Simple 

to Understand and Explain 

Does not include masking 

across one-third octave bands 

Detect Sound Detection Simple Output Does not give insight into why 

a signal may or may not be 

detected. 

Loudness with Ambient 

(Zwicker) 

Loudness / 

Detection 

Most comprehensive model 

for detection, can be used to 

evaluate the growth of the 

loudness as the vehicle 

approaches 

Not currently available in the 

public domain 

Loudness with Ambient 

(Moore) 

Loudness / 

Detection 

Comprehensive, well 

documented model 

Needs small additions in order 

to evaluate the growth of the 

loudness as the vehicle 

approaches 

Of the available models Moore’s loudness, with the ambient included, provides the most 

information for the perceived loudness and detectability of the signal. Therefore, Moore’s loudness 

model with ambient included was used for the remainder of the psychoacoustic analysis. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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Chapter 6: Psychoacoustic Analysis 

6.2 Moore’s Loudness with Ambient Associated with the 

Event Corresponding to the Maximum for ICE Vehicles 

For each vehicle, the event with the maximum LAeq, 1⁄2 sec at pass-by for each event was used to 

compute a specific loudness spectrum by using Moore’s Loudness with Ambient for both the 

driver’s side and the passenger’s side. The driver side results are shown in Section 6.2.1 for the 

measured ambient condition, Cottage Farm Ambient. The driver side results are shown in Section 

6.2.2 for the ambient condition present during countermeasure subject testing, Countermeasure 

Test Ambient. The purpose of considering both ambient conditions was to show the loudness of 

the measured vehicles in the presence of a realistic quiet and noisy ambient to understand how the 

ambient affects the perception of the sound to be detected. Results for the passenger side are given 

in Appendix A-2. 

6.2.1 Cottage Farm Ambient 

Figure 23 shows the perceived loudness of 10 ICE vehicles in the presence of the Cottage Farm 

Ambient. The abscissa is analogous to frequency, however the unit is Number of Equivalent 

Rectangular Bandwidths (ERBs). ERBs are a single value representation of the auditory filter 

bandwidth. The relationship between number of ERBs and frequency is given by: 

݂൅1൰ 1000

4.37

൬଴ ଵ݃݋݈ 21.4ൌ ݎܾ݉݁ݑ݂ܰ݋ ݏܤܴܧ

At high number of ERBs, a small change in the number of ERBs corresponds to a large change in 

frequency. This is illustrated in Figure 24. To facilitate interpretation of these graphs, the number 

of ERBs and corresponding frequency are given in Table 17. 

The ordinate is analogous to sound pressure level density. By integrating the specific loudness 

over the entire curve, the total loudness can be obtained. Because any non-zero specific loudness 

indicates a perceived loudness, any signal present in these graphs (with minima set to zero) is 

considered to be audible. In general both the ambient and the signal have non-zero specific 

loudness over at least part of the equivalent frequency range. This indicates that both the ambient 

and the signal are audible. In this case both the ambient and the signal vie for the subject’s 

attention. 

Tonal components can be seen as narrow peaks in Loudness spectra, such as those in Figure 25 for 

vehicle 1 at approximately 4 ERBs (123 Hz) and vehicle 3 at approximately 10 ERBs (442 Hz). 

These tonal components are most likely due directly to the combustion process, rather than the 

more broadband contributions from tire, gear, and other mechanical noise also present in the 

signal. With the exception of vehicle 7, all ICE vehicles were audible in the low ambient condition 

present at Cottage Farm. Although vehicle 7 was an ICE, it was extremely quiet and often did not 

exceed the ambient sufficiently to provide an accurate measurement. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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Chapter 6: Psychoacoustic Analysis 

Figure 23. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Cottage Farm Ambient: 6 mph, Reverse, 

Driver’s Side 

0 

2000 

4000 

6000 

8000 

10000 

12000 

14000 

16000 

18000 

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 

Center Frequency, Hz 

ERBs 

Figure 24. Relationship Between Number of ERBs and Frequency. See Table 17 for Data 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

47 
 

5 

10

15

20

25

30

Chapter 6: Psychoacoustic Analysis 

Table 17. Number of ERBs and Corresponding Frequency, Hz 

Number of ERBs Frequency, Hz 

1 26 

2 55 

3 87 

4 123 

163 

6 208 

7 257 

8 312 

9 374 

 442 

11 519 

12 603 

13 698 

14 803

 921 

16 1051 

17 1196 

18 1358 

19 1539 

 1739 

21 1739 

22 1963 

23 2212 

24 2489 

 2798 

26 3142 

27 3525 

28 3951 

29 4426 

 4955 

31 5544 

32 6200 

33 6930 

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Chapter 6: Psychoacoustic Analysis 

Number of ERBs Frequency, Hz 

34 7743 

35 8649 

36 9657 

37 10781 

38 12031 

39 13424 

40 14975 

Figure 25. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Cottage Farm Ambient: 6 mph, Forward, 

Driver’s Side (Driver Seat on the Right) 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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Chapter 6: Psychoacoustic Analysis 

Figure 26. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Cottage Farm Ambient: 10 mph, Forward, 

Driver’s Side 

Figure 27. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Cottage Farm Ambient: 15 mph, Forward, 

Driver’s Side 

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Chapter 6: Psychoacoustic Analysis 

Figure 28. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Cottage Farm Ambient: 20 mph, Forward, 

Driver’s Side 

Figure 29. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Cottage Farm Ambient: Acceleration, 

Driver’s Side 

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Chapter 6: Psychoacoustic Analysis 

Figure 30. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Cottage Farm Ambient: Startup, Driver’s 

Side 

Figure 31. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Cottage Farm Ambient: Idle, Driver’s Side 

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Chapter 6: Psychoacoustic Analysis 

6.2.2 Countermeasure Test Ambient 

The Cottage Farm ambient represents a condition where it is expected that typical ICE vehicles 

should be easily audible. The loudness model corroborates this and additionally shows low, mid, 

and high frequency components are audible. However, it is expected that pedestrians will need to 

detect vehicles in higher ambient levels. The Countermeasure Test Ambient was determined to 

have an overall level that was equivalent to the average of two typical ambient conditions 

encountered by pedestrians as discussed in Section 5.3.2. The masking effects of this ambient are 

much more pronounced. Even so, the loudness model predicts that the ICE vehicle will still be 

audible for all operating modes measured. However, low frequencies are much less detectable and 

in some cases not at all. See for example Figure 33 where only vehicle 1 has components (in this 

case a tonal component) audible below about 257 Hz. For idle, Figure 39, vehicles are generally 

not audible below 10 ERBs (442 Hz) and only become the dominant source somewhere between 

20 to 25 ERBs (1739 to 2798 Hz). 

These data show that for both ambient conditions the ICE vehicles are detectable when the vehicle 

is at its closest point to the microphone. These data do not answer the question, at what distance 

can the vehicles first be detected? This question is of interest for pass-by events. To answer this 

question, Loudness spectra for consecutive measurements as the vehicle approaches the 

microphone would need to be computed. This is beyond the scope of this study, but a sample 

illustration of the process for vehicles 1 and 7 is given in Section 6.3. 

Figure 32. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Countermeasure Ambient: 6 mph, Reverse, 

Driver’s Side 

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Chapter 6: Psychoacoustic Analysis 

Figure 33. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Countermeasure Ambient: 6 mph, Forward, 

Driver’s Side 

Figure 34. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Countermeasure Ambient: 10 mph, 

Forward, Driver’s Side 

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Chapter 6: Psychoacoustic Analysis 

Figure 35. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Countermeasure Ambient: 15 mph, 

Forward, Driver’s Side 

Figure 36. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Countermeasure Ambient: 20 mph, 

Forward, Driver’s Side 

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Chapter 6: Psychoacoustic Analysis 

Figure 37. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Countermeasure Ambient: Acceleration, 

Driver’s Side 

Figure 38. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Countermeasure Ambient: Startup, Driver’s 

Side 

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Chapter 6: Psychoacoustic Analysis 

Figure 39. Specific Loudness of ICE Vehicle at Microphone Line According to Moore’s 

Loudness with Ambient in the Presence of the Countermeasure Ambient: Idle, Driver’s 

Side 

6.3 Moore’s Loudness with Ambient Time History Associated 

with the Event Corresponding to the Maximum ICE 

Vehicles 

Typically, as a vehicle approaches a subject it gets louder, and thus is easier to detect, provided that 

the sound emitted by the vehicle and the ambient sound level do not change. Examining how 

Loudness increases as a vehicle approaches can give insight as to when a vehicle can be detected 

and what the key components contributing to detection may or may not be. In order to evaluate the 

increase in the Loudness spectra as a vehicle approaches, Loudness spectra were computed for the 

quietest ICE, vehicle 7, and the loudest ICE, vehicle 1 (see Figure 40 to Figure 43). Loudness 

spectra were computed for both the Cottage Farm and Countermeasure Ambients for the 6 mph, 

forward operating condition. Due to data quality restrictions, the analysis was only conducted for 

the 5 seconds prior to pass-by. In all cases the loudness model indicates that at least some portion 

of the vehicles sound was audible during this entire period. Increase in Loudness was greater as 

the vehicle got closer to the microphone line, most likely due to the increased number of distance 

halving as the distance to the microphone got small. Also of interest, was the fact that the low 

frequency tonal component associated with vehicle 1 only becomes perceptible close to the 

microphone, even though the rest of vehicle 1’s Loudness spectra indicates a more gradual growth. 

This may indicate that the tonal component is highly directional; for example it may be due to 

sound radiating out through the wheel well. This may indicate that directivity may need to be 

closely controlled if a tonal component is to be considered part of a countermeasure signal. 

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Figure 40. Growth of Specific Loudness of ICE Vehicle at Microphone Line According to 

Moore’s Loudness with Ambient in the Presence of the Cottage Farm Ambient: 6 mph, 

Forward, Driver’s Side – Vehicle 7
 

Figure 41. Growth of Specific Loudness of ICE Vehicle at Microphone Line According to 

Moore’s Loudness with Ambient in the Presence of the Countermeasure Ambient: 6 mph, 

Forward, Driver’s Side – Vehicle 7
 

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Figure 42. Growth of Specific Loudness of ICE Vehicle at Microphone Line According to 

Moore’s Loudness with Ambient in the Presence of the Cottage Farm Ambient: 6 mph, 

Forward, Driver’s Side – Vehicle 1 

Figure 43. Growth of Specific Loudness of ICE Vehicle at Microphone Line According to 

Moore’s Loudness with Ambient in the Presence of the Countermeasure Ambient: 6 mph, 

Forward, Driver’s Side – Vehicle 1 

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6.4 Moore’s Loudness with Ambient Associated with the 

Event Corresponding to the Maximum for 

Countermeasure Sounds 

The Loudness spectra of the countermeasure signals are given for low, 59.5 dB(A) output levels in 

Figure 44 and for high output levels, 63.5 dB(A), in Figure 45. These spectra were only computed 

for the Countermeasure Ambient, 58.3 dB(A), as the purpose here is to provide potential 

explanation of countermeasure performance during the human-subject testing. One point to make 

here is that most of the countermeasure signals had strong tonal components. About half of the 

countermeasure sounds have tonal components below 2000 Hz, where the ambient was loud, and 

about half have tonal or broad band components above 2000 Hz, where the ambient was not as 

loud. 

Figure 44. Specific Loudness of Countermeasure Sounds at Microphone Line According to 

Moore’s Loudness with Ambient in the Presence of the Countermeasure Ambient: Low
 

Level, 59.5 dB(A) 

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Figure 45. Specific Loudness of Countermeasure Sounds at Microphone Line According to 

Moore’s Loudness with Ambient in the Presence of the Countermeasure Ambient: High 

Level, 63.5 dB(A) 

6.5 Summary: Acoustic Analysis 

 ICE Vehicle sounds are, on average, broad band in nature and do not have 

strong tonal components. 

 According to Moore’s Loudness model all vehicles were detectable at the 

point of closest approach. 

 Frequency components less than 500 Hz were less (or not at all) detectable 

due, in part, to ambient levels typically being high below 500 Hz. 

 Frequency components between 1600 and 5000 Hz were more detectable due 

to strong signal strength and relatively low ambient levels in this range. 

 During testing tonal components emitted by two ICE vehicles were 

particularly detectable, according to the model. 

 None of the existing models were designed to deal with moving sound 

sources. 

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Chapter 7: Evaluation of Countermeasure Sounds 

7. EVALUATION OF COUNTERMEASURE SOUNDS 

7.1 Purpose 

Human-subject testing was conducted to compare alternative countermeasure sounds, in terms of 

the time and distance (when applicable) at which these sound sources were detected and 

recognized as a vehicle, in representative urban-ambient noise conditions. Countermeasure sounds 

were evaluated at two sound pressure levels typical of ICE vehicles at low speeds (i.e., 59.5 and 

63.5 dB(A)). An ICE vehicle that produced 60 dB(A) in the 6 mph pass-by test was used as a 

reference in this evaluation. 

7.2 Sound Characteristics to Improve Detectability 

The following sound characteristics can improve detectability of a sound source (Stanton & 

Edworthy (Eds.) 1999): 

 Pulsating quality with pulse widths of 100 to 200 msec. 

 Inter-pulse intervals of about 150 msec. 

 Fundamental tonal component in 150 to 1000 Hz range. 

 At least three prominent harmonics in the 1 to 4 kHz range. 

 Pitch shifting denoting vehicle speed change. 

Some of the countermeasures evaluated in the human-subject studies have sound characteristics 

that could improve detectability when compared to ICE-equivalent sounds. 

7.3 Countermeasure Evaluation Test Site 

The evaluation of countermeasures took place in a parking lot located on the Volpe Center campus 

in Cambridge, Massachusetts. The test site has the acoustic characteristic of an urban area with a 

typical ambient noise level of approximately 58-61 dB(A). Ambient sound measurements at the 

test site were described in Section 5.3.3. Most of the ambient noise at the site was generated by 

fans on the roofs of buildings about 400 feet west of the test site, which resulted in a greater 

masking effect for vehicles approaching from the west (the “a-to-b” direction) than for the reverse. 

Figure 46 shows a photo of the test site. The test site has several characteristics essential for the 

conduct of the study. Vehicles that were not part of the study did not pass through the test site; all 

vehicular traffic was controlled by Volpe Center security guards. There was adequate approach 

distance for vehicles to get up to 6 mph speed before they became audible. An alternate route for 

test vehicles to go to the designated starting points while recharging their batteries was available. 

A large number of participants were seated next to each other and shaded from the sun. The road 

surface consisted of bituminous asphalt and was swept and washed down prior to each test session. 

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Figure 46. Test Site at the Volpe Center, Cambridge, Massachusetts 

7.4 Countermeasure Sounds Evaluated 

Countermeasure systems included HEVs, in electric mode, with sound generator systems, each of 

which generated one or multiple versions of ‘artificial engine noise’ or other sounds. 

The countermeasure systems evaluated included: 

 Sounds produced by vehicles with integrated countermeasures rented from 

manufacturers. 

 Sounds produced by prototype countermeasure systems rented from 

manufacturers, and played back by loudspeakers temporarily mounted on 

HEVs rented separately. 

Five vendors, motor vehicle manufacturers or suppliers of automotive electronics, agreed to rent 

prototypes of synthetic sound generators for EVs or HEVs to the government. All five systems are 

intended to receive signal from the CANbus that will cause the pitch of their sound to vary in 

proportion to the vehicle speed up to the speed at which the sound is no longer needed. Some emit 

special sounds in reverse, or emit louder sounds in reverse, or increase in loudness briefly when the 

vehicle starts moving. The countermeasures systems were labeled “A” to “E”. A total of nine 

sounds were evaluated: A1, A2, A5, B, C, D, E1, E3, and E4. In addition to the countermeasures 

sounds, an ICE vehicle was included in the study as a baseline for comparison purposes. The ICE 

vehicle was labeled system ‘R’. Volpe Center staff prepared a qualitative description of the sound 

systems in Table 18. The acoustic characteristics of these countermeasure sounds were discussed 

in Section 5.2. 

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Table 18. Qualitative Description of Sound Countermeasures 

System Sound ID Qualitative Description of Countermeasure Sounds (all sound are synthesized sounds) 

A 1 Engine-or-pump-like sounds accompanied by turbine-like whine. The frequency of 

the most prominent peak in the spectral distribution varies from 150 to more than 300 

Hz over the 0 to 20 mph range. The engine noise and the turbo sound include several 

harmonics. Both the engine noise and the turbo sound increase in pitch with vehicle 

speed, but there is no amplitude change under any condition. 

A 2 Engine sound with enhanced valve noise. The fundamental varies from under 100 Hz 

to about 200 Hz over the 0 to 20 mph range. There are numerous harmonics too 

closely spaced to count with peak amplitude only a few dB below that of the 

fundamental. All components of the sound shift upwards in pitch with speed, but 

there are no amplitude modulations. 

A 5 Whirring sound with Diesel engine sound. The fundamentals of the whirring noises 

are centered around 400 Hz at idle and rise to more than 600 Hz at 20 mph, while the 

repetition rate of the ‘whirring’ effect is about 1.6 Hz, varying slightly with speed. 

The Diesel valve noises are about 15 dB below the amplitude of the ‘whirring’. 

B 1 Sound emulates the exhaust note (the fundamental of the combustion noise) of an 

engine. The sound does not contain appreciable components above 250 Hz. 

C Activated ‘Wavy, turbo-like’ sound when driven forward. Most of the energy in the sound is 

broadband noise in the 200 Hz to 5 kHz band, but there is a distinct whistle sound that 

is few dB louder than the peak of the broadband noise. The pitch of the whistling 

noise varies from about 2.3 kHz at minimum speed to about 3.1 kHz at the maximum 

speed before the sound is deactivated. There is no sound emission at idle when the 

vehicle is in ‘Drive’ and stationary, but as soon as the vehicle begins to move there 

was a rapid ramp up in pitch and volume, with volume subsiding slightly after a 

second or two. The pitch of the turbo sound varied over a wide range as vehicle speed 

varies over the 1 to 20 mph range. When the vehicle is shifted into ‘Reverse’, it emits 

a sound that is several dB louder than sound emitted in ‘Drive’, whether or not the 

vehicle is moving. 

D Activated Broad band designed to suggest that of an electric motor coupled to other rotating 

machinery, such as a blower. The initial amplitude of the sound when the vehicle is 

first shifted into gear (either forward or reverse) is 4 or 5 dBA higher than its normal 

running value. A real-time spectrum analyzer shows that individual peaks appear to 

move upward by several hundred Hertz as vehicle speed increases from 0 to the 

cutoff speed. 

E 1 A pure engine-like sound with most energy below 300 Hz, but prominent second, 

third, and fourth harmonics. The peak energy lies below 60 Hz at idle, but increases 

to about 150 Hz at 10 mph, approximating the characteristics of an ICE. 

E 3 A mostly engine-like sound, but with a ‘whirring’ character and a flatter spectral 

distribution than Sound E1. No prominent harmonics of the combustion note. 

E 4 This sound contains short bursts of predominantly high-frequency sound with a 

repetition rate of 14 per second at 6 mph, combined with an engine-or pump-like 

sound. The sixth harmonic of the fundamental was particularly prominent, having 

amplitude almost equal to the fundamental. The peak amplitude of the fundamental 

varies in frequency from about 450 Hz to 700 Hz over the 0 to 10 mph range. On 

acceleration from stopped position, the initial amplitude of the sound is about 9 dB 

higher than its normal value. Ramp down to normal volume takes about 3 seconds. 

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Chapter 7: Evaluation of Countermeasure Sounds 

7.4.1 Countermeasure Sound Pressure Levels and Calibration 

The prototype sound systems tested were not designed to play their sounds at user specified levels. 

Although they included volume controls, these controls were not calibrated to sound pressure level 

values. Different sounds from the same system usually had different sound pressure level values, 

even if the volume control settings were the same for the different sounds. The outputs of Systems 

“A”, “B”, and “E” were recorded during 6 mph forward driving for all the sounds used in the 

experiment to facilitate presentation of different sounds to participants at the same sound pressure 

levels. 

The sound pressure levels of the recorded ‘.wav files’ were normalized to 63.5 dB (A) (the higher 

level used in the human-subject study). Additional versions of each recording were normalized to 

59.5 dB(A) (the lower level used in the human-subject study) and to 70 dB(A) (the level used for 

playback level calibration). Prior to each experimental session, the volume controls of the 

amplifiers that reproduced these sounds were adjusted so that the 70 dB(A) version produced 70 

dB(A) as measured by the SAE J-2889 test procedure. The speaker systems for these 

countermeasure sounds were either installed inside the front grill or mounted on the hood, just 

above the front grill. 

Two of the systems (“C” and “D”) were fully integrated into vehicles and had no accessible 

electrical connection from which a direct recording could be made. For system “C”, the 

provided volume control was manually adjusted to the target values of 59.5 and 63.5 dB(A) used 

in the experiments. For system “D”, the maximum level was used, which could only achieve the 

lower level desired for the human-subject testing. 

7.5 Study Design 

Human-subject testing includes data for three independent sessions. In the first session we tested as 

many conditions as possible and assessed participants’ responses while examining the feasibility of 

the experimental approach and study logistics with a large number of participants. Lessons learned 

from the first session were implemented in the following two sessions. The first session included 

four (4) operating modes: idle, acceleration from stop, start-up and 6 mph forward pass-by. The 

following two sessions included the 6 mph forward pass-by only. 

7.5.1 Dependent Variables 

Performance measures considered in the evaluation include: 

 Proportion of Detection: The proportion of trials of a given condition in which the 

participant detected the sound anytime before the vehicle passed the participant. 

 Detection distance: Physical lateral space, in feet, between the vehicle and 

the pedestrian at the moment the participant indicates detection. 

 Time-to-vehicle-arrival: Time, in seconds, from detection of a target vehicle 

sound to the instant the vehicle passes the pedestrian location. 

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Chapter 7: Evaluation of Countermeasure Sounds 

7.5.2 Independent Variables 

The primary independent variables examined in the evaluation include: 

 Countermeasure sounds; 

 Sound level of the countermeasure (Low = 59.5 dB(A); High = 63.5 dB(A)); 

 Masking effect due to vehicle direction of travel (from left and from right); 

and 

 Study session (to examine any effect due to the difference in the overall 

conditions at the test site from session to session). 

Note: The ambient levels measured by the left microphone (binaural recording) were about 1.4 dB 

greater than the ambient levels measured by the right microphone, because most of the background 

noise was generated by air-conditioning fans on top of a building about 400 feet to the left of the 

subjects. The difference was greater for frequencies from 800 to 20,000 Hz, where the average 

difference between the two sides was about 3.1 dB. Thus, it is expected, because of the occlusion 

caused by the subject’s head, that the effective ambient will be greater for subjects when they 

attempt to detect vehicles approaching from the left. 

Other variables that were considered (and controlled for) in the evaluation include: 

 Countermeasure projection systems; 

 Hearing ability of participants; 

 Masking effect due to variable extraneous noise in the ambient (loud trucks, 

machinery, rustling tree leaves, etc.); 

 Vision effect: performance blind versus sighted participants; and 

 Participant location in the lineup. 

7.5.3 Presentation of Trials 

Each participant was presented with the same experimental conditions (within-subject design). 

Three independent sessions were conducted on July 31, August 7, and August 14. Trials were 

presented in four blocks. Each block contained 24 trials (except for session 1 which included 

29trials per block). There were four (4) trials without a vehicle or countermeasure sound per block. 

Participants experienced a total of four (4) repetitions of each experimental condition. Half of the 6 

mph forward pass-by trials were presented from the left side and half from the right side. Varying 

the direction in which the stimuli (i.e., vehicle approaching at constant speed) is presented reduces 

subjects predisposed to focus on one direction (e.g., left or right) more than the other thus 

providing a more realistic listening situation. The presentation order was randomized within a 

block. 

Each session began with a practice session containing examples of the trials included in the 

evaluation. The practice session allowed participants to experience the range of sounds before the 

testing began, familiarize themselves with the task and use of the response button, and ask 

questions to the investigator. The investigator described the instructions and traveling situation 

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Chapter 7: Evaluation of Countermeasure Sounds 

before each experimental block. The experimenter described the task (in Sessions 2 and 3) as 

follows: 

“Please imagine you are standing on the curb waiting to cross a street where there may be 

vehicles approaching from both your right and left. You will hear distant vehicles in the 

background in all trials. If and when you detect and recognize a vehicle that would affect your 

decision about when to start crossing the street, press and release your push button firmly.” 

The investigator provided general feedback during the practice trials about whether or not there 

was a target sound present that they may or may not associate with a vehicle. Feedback was not 

offered during the experimental trials. 

7.6 Study Procedure 

7.6.1 Research Staff Training 

The investigators completed or renewed the National Institute of Health (NIH) Protecting Human 

Research Participants training. The research staff received training on how to give sighted guide to 

a person who is blind or visually impaired. The training was provided by a certified orientation and 

mobility instructor from the Carroll Center for the Blind. The training focused on four (4) 

techniques: positioning the guidee and the guide; guiding on level surfaces; guiding on stairs; and 

guiding through doorways (or other narrow spaces). We cannot assume that those individuals who 

are blind want sighted guide, however, by providing the training described above, the research staff 

was better prepared when such assistance was requested. 

7.6.2 Recruitment of Study Participants 

The target sample included sighted and blind adults who are independent travelers. Individuals 

must meet the criteria listed below in order to be eligible to participate in the study: 

 18 years or older at the time of the study; 

 independent traveler (i.e., cross streets without assistance from another 

person); 

 travel regularly (i.e., cross streets at least 10 times per week, on a regular 

week); 

 blind participants: legally blind regardless of whether they are totally blind, 

blind with light perception or blind with partial vision; 

 sighted participants must be able to guide a blind participant to their seats if 

needed; 

 self report to have normal hearing on both ears without hearing aids; 

 willing to be screened for hearing threshold shift; 

 willing to wear blindfolds; and 

 must have normal manual dexterity in both hands (for prompt button 

pressing). 

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Volpe Center personnel sent a recruiting email to representatives of various organizations that 

provide services to the blind community such as Carroll Center for the Blind, Perkins School for 

the Blind, Bay State Council of the Blind, and Massachusetts Commission for the Blind. These 

organizations then shared the information with their members. Sighted spouses, friends, etc. of 

blind individuals were also encouraged to serve as participants. Individuals were also recruited by 

word of mouth. Volpe Center employees, blind and sighted, were also recruited and contacted via 

email following procedure to send messages to the Volpe Center community. 

Individuals interested in the study contacted the Volpe Center recruiting coordinator and completed 

an eligibility questionnaire. Potential participants were given a description of the experimental set 

up, the task to be performed, duration of the test, and the test protocol. 

7.6.3 Informed Consent Form 

All participants were briefed by a Volpe Center investigator following the New England IRB 

protocol for the protection of human subjects. Informed consent forms (ICF) were sent to eligible 

participants before the day of the study. On the day of the study, participants met with an 

investigator to review the ICF, discuss any questions, and obtain signatures. Volpe Center 

employees who participated in the study completed the consent process before the day of the study. 

Training on how to give sighted guide to a person who is blind was available to sighted 

participants in two forms: A certified orientation and mobility specialist provided a live 

presentation and demonstration to Volpe Center employees who volunteered to participate in the 

study. A handout was provided to those volunteers that were unable to attend the presentation. The 

handout titled ‘How to Give Sighted Guide to a Person Who is Blind or Visually Impaired’ was 

prepared (and made available to the Volpe Center) by a certified orientation and mobility specialist 

from the Carroll Center for the Blind. The handout was provided to potential participants at the 

same time as the consent form. 

7.6.4 Hearing Screening Procedure 

A Tremetrics RA300 audiometer was used to screen participants’ hearings for later use if 

discrepancies arose. The screening procedure was used to estimate the number of participants with 

estimated hearing loss (HL) of 20 or more dB above the normal range (0-20 dB HL) in the 

frequency range of concern. This information was used to identify outliers in the data in an 

attempt to reduce sources of between-subjects variability. 

7.7 Characteristics of Study Sample 

Eighty-three participants completed the study. Four participants were excluded from the analysis 

either because of indication of moderate to severe hearing loss or irregular responses. Therefore a 

total sample of seventy nine participants was included in the analysis: 26 in session 1; 27 in session 

2; and 26 in session 3. The number of males and females by vision category is shown in Table 19. 

Legally blind participants included individuals who are totally blind, blind with light perception, 

and blind with some usable vision. All participants were required to wear a blindfold during the 

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Chapter 7: Evaluation of Countermeasure Sounds 

study. The distribution of participants by age group for each of the three study session is shown in 

Table 20. Participant ages ranged from 18 to 72 years old (Mean age = 43). 

Table 19. Distribution of Participants by Sex and Vision Category 

Vision Category Number of Males Number of Females Total of all Participants 

Legally Blind 25 20 45 

Sighted 17 17 34 

TOTAL 42 37 79 

Table 20. Summary Statistics for Age by Session 

Age Category Session 1 Session 2 Session 3 Total 

< 30 6 8 12 26 

30-39 1 3 2 6 

40-49 4 3 4 11 

50-59 10 9 6 25 

>59 5 3 2 10 

Not reported 0 1 0 1 

TOTAL 26 27 26 79 

The distribution of participants by estimated HL category is shown in Table 21. Three frequency 

categories were identified as: low = 500 Hz; medium = 2000 Hz; and high = 4000 Hz. 

Table 21. Distribution of Participants by Hearing Loss Category 

Hearing Loss Category Session 1 Session 2 Session 3 Total Number of 

Participants 

Normal range (0-20 dB HL) 3 10 12 25 

Mild HL in Low Frequency 4 1 2 7 

Mild HL in Medium Frequency 0 0 1 1 

Mild HL in High Frequency 3 1 2 6 

Mild HL in Low & High Frequency 0 1 0 1 

Mild HL in Medium & High 

Frequency 

2 1 1 4 

Moderate HL in High Frequency 0 1 0 1 

No audiogram 14 12 8 34 

Total Number of Participants 26 27 26 79 

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Chapter 7: Evaluation of Countermeasure Sounds 

7.8 Experimental Methods 

7.8.1 Apparatus 

7.8.1.1 Sound Sources 

Three of the vendors of countermeasures were represented by HEVs fitted with a loudspeaker 

temporarily mounted on the hood (as shown in Figure 47). These vehicles were also equipped with 

notebook computers to play .wav files of various countermeasure sounds at predetermined levels 

and power amplifiers to drive the loudspeakers from the notebook computers. The other two 

vendors supplied vehicles with completely integrated countermeasure sound generators. 

For the tests of detectability of idling, start-up and acceleration from stop sounds (performed only 

during the first experimental session), a line of six tripod-mounted loudspeakers was positioned 25 

feet in front of the line of participants (illustrated in Figure 48). This arrangement allowed the 

sounds to be presented to all of the participants with less than 1 dB of variation among participants. 

Figure 47. HEV Fitted With a Loudspeaker Temporarily Mounted on the Hood 

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Chapter 7: Evaluation of Countermeasure Sounds 

Figure 48. A Line of six Tripod-Mounted Loudspeakers was Positioned 25 Feet in Front of 

the Line of Participants to Present the Idle, Start-Up and Acceleration-From-Stop Sounds 

The order and timing of presentation of the various sounds was controlled by a counterbalanced 

schedule developed in advance by the experimenters. Appendix A-3 shows an example of the 

schedule for one session. The start of each trial was announced to participants over a public 

address system and simultaneously relayed to the drivers and computer operators in each of the 

test vehicles via walkie-talkies. 

7.8.1.2 Recording of Subject Responses 

A data acquisition system was constructed by Volpe Center staff to capture various aspects of the 

independent variables and record the occurrence of each subject response to each event. The input 

devices consisted of a pair of photo-electric sensors, positioned at measured locations at each end 

of the line of participants to determine the precise moment that each test vehicle passed, and push 

buttons for each subject, used to indicate when that subject detected a nearby vehicle. 

The input devices were connected to a digital input card in a desktop computer running LabVIEW 

software (more information about LabVIEW available at http://www.ni.com/labview/whatis/). If a 

subject responded by pressing his/her button, the data acquisition system recorded the exact 

moment (plus/minus one millisecond) in relation to the time the trial started and the times that the 

vehicle passed through the beams of each photo-detector. 

The LabVIEW software was programmed to generate a spreadsheet for each block of trials 

containing the following data items for each subject response: 

 Trial number; 

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Chapter 7: Evaluation of Countermeasure Sounds 

 Direction of vehicle passage; 

 Time/date; 

 Vehicle speed between the photo detectors; 

 Number of milliseconds after the trial started until each subject responded; 

 Calculated distance to the target vehicle (feet) at the moment each subject 

responded; 

 Calculated time (milliseconds) between the moment each subject responded 

and the vehicle arrived at the subject’s position; and 

 For the sounds (idle, start-up, and accelerating from stop) played through the 

tripod-mounted loudspeakers, only the time that the sound began, and the 

times that each subject responded were recorded. 

The calculated values are based on the assumption that the vehicle’s speed is the same at all points 

as its average speed while traveling between detectors. 

For the trials involving sounds for idle, start-up, and accelerating from stop, the LabVIEW data 

simply showed whether or not each subject responded and how many milliseconds after the trials 

started. 

Subject responses by show of raised hands were also recorded in high-definition video during all 

practice trials and during Block 4 of Sessions 2 and 3. In Session 1, the data-acquisition system 

malfunctioned, and the video recordings provided the only data collected. Using the speed data 

from each trial, detection distances for each subject response could be estimated from the videos by 

noting the time the vehicle passed the “A” detector, the time that each subject responded, and the 

previously measured distance from the “A” detector to each subject. Although not as precise as the 

pushbutton system, this method yields estimates of detection distances that are closely correlated 

with those of the push-button method. However, it was not possible to judge the precise location of 

the test vehicle in many instances, and the camera’s view of the more distant participants was often 

blocked by the test vehicle. The data that was extracted from the video recordings tended to 

indicate somewhat higher detection distances for the same sounds as the LabVIEW data collected 

in Sessions 2 and 3. In the left photo in Figure 49, many participants have detected a vehicle 

approaching from the left before it comes within the camera’s view. A relatively ineffective sound 

emitted by the vehicle in the right photo was not detected by some participants even when it was 

immediately adjacent. However, there is too much uncertainty about this data to justify its 

presentation in this report. 

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Chapter 7: Evaluation of Countermeasure Sounds 

Figure 49. Recording Participant Responses by Video 

For the stationary-sound trials conducted during Session 1, the video recordings provided usable 

data, because there was no vehicle to block the camera’s view, and the instant that the test sound 

was initiated could be determined from the audio track. 

7.8.1.3 Monitoring of Experimental Conditions 

The near real-world conditions of these experiments expose the participants to many more possible 

unintended stimuli than a normal laboratory experiment. Aircraft, distant noisy road vehicles, lawn 

mowers, and leaf blowers introduced sounds that could be confused with approaching ICE vehicles 

on numerous occasions. Other trials were disrupted by a malfunction in the steam-pressure- relief 

system on an adjacent building. Furthermore, because the presentation of stimuli was not 

controlled directly by computer, but rather depended on human actuation in response to a radioed 

instruction, there was a possibility for human error. To monitor all of these possibilities to 

determine which trials were invalid, four systems were used: 

 High-definition video recordings of subject behavior and vehicle passage 

including a “shotgun” microphone recording of sounds from the area in the 

immediate vicinity of the participants; 

 Recordings from a binaural head placed just behind the participants to 

approximate what the participants heard; 

 Continuous recording of the noise level near the participants with the same 

acoustic measurement system used to characterize the vehicles; and 

 A-weighted, fast-response, SPL values at 100 msec intervals during each 

trial. 

Sessions 2 and 3 were planned to generate 96 trials each in total (4 blocks of 24 trials each). 

During the course of each block, it was immediately obvious that certain trials were spoiled by 

extraneous noises, such as, aircraft or malfunctioning steam valves. These trials were redone at the 

end of each block when time allowed. During data analysis, a few additional trials were 

discovered to have been invalidated by more subtle extraneous noises (e.g., intermittent, distant 

traffic on nearby street). The final data set contained 79 valid trials in Session 2 and 95 valid trials 

in Session 3. No response and detection after the vehicle passed the pedestrian location are 

considered missed detection. 

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Chapter 7: Evaluation of Countermeasure Sounds 

7.9 Analysis of Countermeasure Evaluation 

7.9.1 Detection Distance and Proportion of Detections 

Trials with no detection or detection after a vehicle passed the participant are considered missed 

detections instead of missing data. This is done by assigning 0 feet to all cases where a sound was 

not detected. This procedure allows analyses of detection distance to include more participants for 

greater statistical power. In contrast, if no-detection trials were treated as missing data, the listwise deletion of data required for repeated measures analysis would delete any participant who 

does not have valid detection distances for all two or four trials of a given sound-amplitudedirection condition. This initial analysis is described below. 

First, we considered two potential dependent variables for the 6 mph pass-by scenario: 

 Raw Detection Distance, being the number of feet the vehicle was from the participant 

when the participant indicated she or he heard the sound. A failure to detect the sound 

before the vehicle passes is treated as missing data. 

 Proportion of Detection, being the proportion of trials of a given condition in which the 

participant detected the sound anytime before the vehicle passed the participant. 

The analyses indicated relationships between raw detection distances and proportion of detection. 

Across all conditions, participants who detected relatively more sounds also tended to detect the 

sounds at greater distances than participant who detected relatively fewer sounds (r = 0.326, n = 

53, p = 0.0168). Furthermore, high amplitude sounds were detected more often and at greater 

distances than low amplitude sounds (for raw detection distance, Ms = 55 and 44 feet, t(52) = 

11.45, p < 0.0001; for proportion of detections, Ms = 0.95 and 0.92, t(52) = 3.51, p = 

0.0009).These positive relations between raw detection distance and proportion of detection 

suggest they represent the same underlying construct. Thus, they are combined into a single 

“detectability” variable by assigning 0 feet to all cases where a sound was not detected. 

Repeated trials for a given sound, amplitude and direction were averaged to create a single value 

per condition per participant for subsequent analyses. 

7.9.2 Detectability Relative to Reference Vehicle 

To compare the countermeasure sounds to the detectability of the reference sound from an ICEpowered vehicle (‘R’), a mixed design, analysis of variance (ANOVA) was performed on 

detectability with session and vision as between-subjects independent variables, and sound and 

direction as within-subject independent variables. Session refers to the date of the study; vision 

refers to whether the participant is blind or sighted; sound refers to countermeasure or ICE vehicle 

sound; and direction refers to vehicle direction of travel (‘a’ to ‘b’ = left to right; ‘b’ to ‘a’ = right 

to left). 

The analysis only included low-amplitude sounds since their amplitude, 59.5 dB(A), correspond 

closely to the sound pressure level measured for the reference ICE vehicle , 60.0 dB(A). Using 

only low-amplitude sounds also allowed the inclusion of sound ‘D’, which produced only a lowamplitude sound (in addition to no artificial sound). 

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Chapter 7: Evaluation of Countermeasure Sounds 

The analysis indicated significant main effects of sound (F(4.3, 185.8) = 75.5, p < 0.001, 

Greenhouse-Geisser corrected for sphericity), but, more importantly, a significant three-way 

interaction of session, sound, and direction (F(5.6, 241.3) = 4.3, p = 0.001, Greenhouse-Geisser 

corrected for sphericity). This interaction is depicted in Figure 50; a reference line is added to 

indicate the distance (25.5 ft) required to stop a vehicle traveling at 6 mph to stop, assuming 

drivers’ reaction time of 2.5 seconds (see Section 2.2 for more information of stopping distance). 

This implies that the relative performance of each sound, including the reference sound, is jointly 

contingent on the direction it comes from and the session it was presented in. The directional 

effect results primarily from the fact that the roof-top fans on buildings to the west were the 

predominant source of ambient noise, which more effectively masks vehicles approaching from the 

west (the “a-to-b” direction), compared with the reverse. However the directional effect is more 

pronounced where spectral peaks in the ambient noise (shown in Figure 21) happen to coincide 

with the peaks of a particular countermeasure sound. Furthermore, there were slight differences in 

the spectrum of the ambient sound caused by rustling in the leaves of the nearby trees in different 

wind conditions. Thus, each direction-by-session condition may be regarded as constituting an 

independent test of the performance of each countermeasure sound relative to the reference sound. 

The frequency of Type I errors should thus follow a binomial distribution. 

Based on this, the detectability of each countermeasure sound relative to the reference was 

evaluated by t-tests comparing each sound to the reference vehicle for the corresponding session 

and direction condition of each. With nine countermeasure sounds to compare to the reference, a 

Bonferroni post-hoc adjustment for family-wise error rate yielded a critical p-value of 0.0056 for 

deciding if a particular artificial sound is overall significantly different than the reference. Treating 

the four session-by-direction test conditions as independent samples of ambient sound, this critical 

p-value is achieved if at least two conditions show a significant difference at the 0.0311 level, 

according to binomial calculations. If this is observed then one can conclude that the sound has 

significantly different detectability than the reference sound. Table 22 shows the results of these ttests (*Significant sums at Bonferroni adjusted 0.0056 rate per sound; ns = not significant). 

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Session 2 

100 

a-b 

80

Detectability (ft) 

60 b-a 

40 2.5-sec 

20 stopping 

distance 

0 

Sound 

Session 3 

100 

a-b 

80 

60 b-a 

40 

2.5-sec 

20 stopping 

distance 

0 

Sound 

Detectability (ft) 

R A1 A2 A5 B C E1 E3 E4 D 
 

R A1 A2 A5 B C E1 E3 E4 D 
 

Chapter 7: Evaluation of Countermeasure Sounds 

Figure 50. Mean Detectability at low Amplitude for all Sounds by Session and Direction. 

The Horizontal Line Indicates the Stopping Distance (25.5 Ft) for a Vehicle Traveling at 6 

mph, Assuming a 2.5-sec Drivers Reaction Time 

As can be seen in Table 22, sounds A2, A5, E3, and E4 have significantly better detectability than 

the reference sound in at least two of the four conditions. These sounds never have significantly 

worse detectability in any condition. Thus, these sounds overall have better detectability than the 

reference sound. In contrast, sounds A1, B, C, D, and E1 all have significantly worse detectability 

than the reference sound in at least two of the four conditions. These sounds never have 

significantly better detectability in any condition. Thus, these sounds overall have worse 

detectability than the reference sound. 

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Chapter 7: Evaluation of Countermeasure Sounds 

Table 22. Comparison of Countermeasure Sounds to Reference Sound Across Conditions 

at p = 0.0311 Level. 

Session Direction A1 A2 A5 B C E1 E3 E4 D 

2 a-b Worse ns ns Worse Worse Worse Better Better Worse 

2 b-a ns Better Better ns ns Worse ns Better Worse 

3 a-b ns Better ns Worse Worse Better ns Better Worse 

3 b-a Worse Better Better Worse ns Worse Better Ns Worse 

Total Better 0 3* 2* 0 0 1 2* 3* 0 

Total Worse 2* 0 0 3* 2* 3* 0 0 4* 

(* indicates significant sums at Bonferroni adjusted 0.0056 rate per sound; ns indicates not 

significant). 

7.9.3 Ranking of Countermeasure Sounds 

To compare the detectability of the countermeasure sounds to each other, a mixed design ANOVA 

was performed on detectability with session and vision as between-subjects independent variables, 

and sound, direction, and amplitude as within-subject independent variables. The reference sound 

‘R’ and sound ‘D’ were excluded from this analysis since they did not differ in amplitude. In any 

case, sound ‘D’ was significantly worse than the reference sound, so it is of minor interest for 

subsequent analyses. 

The analysis indicated significant main effects of sound (F(3.6, 132.0) = 78.3, p < 0.001, 

Greenhouse-Geisser corrected for sphericity), but, more importantly, a significant four-way 

interaction of session, sound, direction, and amplitude (F(5.2, 192.8) = 4.5, p = 0.001, 

Greenhouse-Geisser corrected for sphericity). This interaction is depicted in Figure 51 and Figure 

52, where ‘Lo’ refers to the low amplitude tested (59.5 dB(A)) and ‘Hi’ refers to the high 

amplitude tested (63.5 dB(A). 

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100 

60 

80 Lo a-b 

Lo b-a 

Hi a-b 

20 

40 Hi b-a 

ICE a-b 

ICE b-a 

0 

A1 A2 A5 B C E1 E3 E4
 

Detectability (ft) 

Sound 

Figure 51. Mean Detectability of Artificial Sounds by Session, Direction, and Amplitude 

(Session 2) 

Session 3 

100 

Lo a-b 

Detectability (ft)80
 

A1 A2 A5 B C E1 E3 E4
 

Lo b-a 

60 Hi a-b 

Hi b-a 40 

ICE a-b 

20 ICE b-a 

0 

Sound 

Chapter 7: Evaluation of Countermeasure Sounds 

Session 2 

Figure 52. Mean Detectability of Artificial Sounds by Session, Direction, and Amplitude 

(Session 3) 

This implies that the relative performance of each artificial sound is jointly contingent on the 

direction it comes from, the session it was presented in, and the amplitude that was used. Similar to 

when comparing only the low amplitude sounds, this is likely due to differences in the spectrum of 

the ambient sound among the directions and sessions interacting with the different amplitudes. 

Likewise, each condition may be regarded as an independent test of the performance of each 

artificial sound. 

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Chapter 7: Evaluation of Countermeasure Sounds 

With this in mind, sounds are ranked by comparing each to the other with t-tests for each sessionby-direction-by-amplitude condition. To assist in the control for family-wise error rate, the 

analyses only included the four sounds shown in the previous section to be superior to the 

reference sound. With four sounds to compare to each other, a Bonferroni post-hoc adjustment for 

family-wise error rate yielded a critical p-value of 0.00833 for deciding if a particular sound is 

overall significantly different than another sound. Treating the eight session-by-direction-byamplitude test conditions as independent samples, this critical p-value is achieved if at least two 

conditions show a significant difference at the 0.0178 level, according to binomial calculations. If 

this is observed then one can conclude that the sound has significantly different detectability than 

the reference sound. Table 23 and Table 24 show the results of these t-tests; * significant sums at 

Bonferroni adjusted 0.0083 rate per comparison. 

Table 23. Comparison of Artificial Sounds to Each Other Across Conditions at p = 0.0178 

Level. 

Amplitude Session Direction A5 

A2 Versus 

E3 E4 A2 

A5 Versus 

E3 E4 

Lo 2 a-b ns ns Worse ns Worse Worse 

Lo 2 b-a ns ns ns ns Better ns 

Lo 3 a-b Better ns Worse Worse ns Worse 

Lo 3 b-a ns ns Better ns ns ns 

Hi 2 a-b Better ns Worse Worse Worse Worse 

Hi 2 b-a Better Better ns Worse Worse Worse 

Hi 3 a-b Better ns ns Worse Worse Worse 

Hi 3 b-a Better ns ns Worse Worse Worse 

Total Better 5* 1 1 0 1 0 

Total Worse 0 0 3* 5* 5* 6* 

Table 24. Comparison of Artificial Sounds to Each Other Across Conditions at p = 0.0178 

Level (Continuation). 

Amplitude Session Direction 

A2 

E3 Versus 

A5 E4 A2 

E4 Versus 

A5 E3 

Lo 2 a-b ns Better Worse Better Better Better 

Lo 2 b-a ns Worse Worse ns ns Better 

Lo 3 a-b ns ns Worse Better Better Better 

Lo 3 b-a ns ns ns Worse ns ns 

Hi 2 a-b ns Better Worse Better Better Better 

Hi 2 b-a Worse Better ns ns Better ns 

Hi 3 a-b ns Better Worse ns Better Better 

Hi 3 b-a ns Better ns ns Better ns 

Total Better 0 5* 0 3* 6* 5* 

Total Worse 1 1 5* 1 0 0 

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Chapter 7: Evaluation of Countermeasure Sounds 

As can be seen in Table 23, sounds E4 has overall significantly better detectability than the other 

sounds, and within each condition it is never worse than any other sound, except for one condition 

when compared to A2. Sounds A2 and E3 are overall not significantly different than each other, 

showing only a difference in a single condition. Sound A5 has overall significantly worse 

detectability than the other sounds, and within each condition is it never better, except for one 

condition when compared to E3. The overall ranking of the sounds from most to least detectable is 

therefore: 

 E4 

 A2 and E3 

 A5 

7.10 Vision Effects 

None of the analyses reported above found a significant effect of vision ability. Blind participants, 

on average, were no better or worse than sighted participants in detecting the approach sounds 

(smallest p = 0.636). 

7.11 Idle, Acceleration, and Start-up Sounds 

Discussions with blind individuals during Phase 1 of this research program led to the identification 

of three types of sounds emitted by motor vehicles that often prove useful for orientation and for 

warning of potentially threatening vehicle movements: 

 Start-up sounds; 

 Idling sounds; and 

 Acceleration-from-stationary-position sounds. 

Extensive, full-factorial, human-subject testing of such sounds from all of the vendors of 

countermeasure systems was contemplated for Phase 2 of this research program. However, none 

of the five vendors provided a system that generates a start-up sound. The two systems that were 

tested as pre-production prototypes fully integrated with a vehicle both mute the sound when a 

vehicle is idling in park, neutral, or drive, but one of them emits sound while idling in reverse. The 

demo hardware from the other vendors might or might not emit sound while the vehicle is 

stationary, depending on how it is programmed. All five systems emit sound during acceleration 

from 0 to 10 mph, with three of the five emitting at higher amplitude during the first few seconds 

of such acceleration. 

During the first experimental session, it became clear that it would not be possible to conduct 

enough trials for both pass-by and stationary sounds within the two-hour fatigue limit for such 

testing. Furthermore, the generally louder startup and acceleration noises seemed to bias subject 

expectations in a way that detracted from prompt detection of vehicles in the pass-by trials. Hence, 

only a dozen trials of stationary sounds were conducted – all in the first experimental session. 

In actual ICE vehicles, startup sounds and acceleration from a stationary position are relatively 

noisy, with mean LAeq .1/2 sec values of 70.7 and 66.7 dB (A) respectively (see Table 7 in Section 

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Chapter 7: Evaluation of Countermeasure Sounds 

5.1.3) There is no doubt that such sounds would be immediately detected; rather, the experiment 

was designed to explore how much below the normal level of startup and acceleration sounds 

would synthetic sounds provide effective alerts to pedestrians at a distance of 25 feet. On the other 

hand, idle sounds in current ICEs are often difficult to perceive in urban ambients, since their mean 

LAeq .1/2 sec values are only 55.7 dB(A). Thus the experiment was designed to provide an indication 

as to how much above average ICE values idle sounds need to be to be detected at 25 feet. Note: 

The six speakers used to generate the sounds for idle, start-up and initial acceleration were 

positioned along a line parallel with the row of subjects, but separated from it by 25 feet in order to 

provide a safe clearance for the vehicles to pass through. The 25-foot separation also helped to 

make the SPL values at the subjects’ ears relatively uniform, i.e., within 1 dB. The significant 

independent variable in these trials was the sound pressure level as measured at the subjects’ 

location, not the distance to the source. 

Although only a limited set of data from Session 1 (n=27) is available, the conclusion is that 

virtually everyone (more than 95%) can detect start-up sounds, even when they are presented at 

levels almost 10 dB lower than average actual ICEs, because even at those levels, they are 

approximately equal to or greater than the ambient noise. Conversely, broadband sounds that are 

presented about 3 dB below the ambient level may be missed by a large percentage of participants, 

and those 5 dB below ambient will be missed by almost everyone. 

Table 25. Missed Detections for Various Start-up, Idle, and Acceleration Sounds 

Maneuver Sound LAeq .5 

Value 

Missed Detections out of 

27 participants Block A 

Missed Detections out of 

27 participants Block B 

Start-up actual ICE 58 0 N/A 

Start-up actual ICE 62 2 1 

Idling A1 59 1 1 

Idling A1 63 1 1 

0 to 10 mph A5 54 25 N/A 

0 to 10 mph E3 57 N/A 12, 8 

0 to 10 mph A5 58 1 N/A 

0 to 10 mph E3 60 N/A 5 

7.12 Summary: Countermeasure Evaluation 

 The ranking of the countermeasure sounds by peak Specific Loudness is not well 

correlated with the average detection distances measured in the human-subject testing. 

 The relative performance of each sound, including the reference ICE sound, was jointly 

contingent on the direction it came from and the session it was presented in. 

 Synthetic sounds that resemble those of an ICE produce similar detection distances as 

actual ICE vehicles. 

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Chapter 7: Evaluation of Countermeasure Sounds 

 In some instances, synthetic sounds designed according to psychoacoustic principles can 

produce double the detection distances at slightly lower overall noise levels. 

 Synthetic sounds that contain only the fundamental combustion noise are relatively 

ineffective. 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

8. DISCUSSION: POTENTIAL SPECIFICATIONS FOR 

VEHICLE SOUNDS 

For the reasons given in the Phase 1 report on this research [DOT HS 811 304], the Phase 2 work 

focused initially on the following ideas: 1) the lack of detectability of quieter vehicles could be 

remediated if they are fitted with synthetic sound generators that emulate the sound of typical 

ICEs; and 2) the specifications for the vehicle sounds can be defined in terms of objective 

parameters – namely, overall sound output as measured by the SAE J2889-1 procedure and 

spectral distribution specifications for the minimum amount of sound level in one-third-octave 

bands. However, recognizability is more complex. Most sounds, and sounds as complex as those 

emitted by an ICE, have numerous properties in addition to loudness and spectral distribution that 

affect human perception. Among these properties are rise time, decay time, repetition rates, 

variations in pitch and loudness, and phase relations among various components of the sound. It 

can be demonstrated, for example, that changes in these properties can render a sound 

unrecognizable even though loudness and spectral distribution are unchanged by playing a 

recording of the sound backwards. There are no established quantitative metrics for many qualities 

of a sound that a person might use for recognition. 

Some of the prototype sounds that were tested in this research were synthesized to resemble ICE 

sounds, while others were not. Some sounds were comprised of entirely synthetic, non-ICE sounds 

generated by a digital-signal-processor (DSP), and demonstrated that, in some instances, it is 

possible to achieve significantly better detection distances, at the same sound level (as measured by 

the SAE 2889-1 draft procedure), compared with ICE-like sounds. Sample sounds constructed 

according to established principles for auditory warnings were recognized on first hearing in the 

human-subject tests when they were played at a level approximately equal to or higher than the 

ambient noise level. These sounds lack the low-frequency, fundamental combustion noise of an 

ICE, which allows them to be played through small, well-sealed speakers similar to those used in 

back-up warning devices. However, an objective specification for non-ICE-like sounds is more 

difficult to develop than one for synthetic sound generators that emulate the sound of typical ICEs. 

This former approach could result in a wider variety of sounds, some of which might be not 

recognized as a vehicle or which might be perceived as annoying. Most of the sounds offered for 

testing by manufacturers were chosen through a jurying process. 

In this section we discuss four potential options for defining specification for countermeasure 

sounds to alert pedestrians to the approach or proximity of vehicles operating in electric only mode 

while operating in the specified conditions. Note that these options have been derived with the 

intent of not increasing overall community noise by using the same or slightly lower level as that 

of current ICE vehicles. 

The four options to define specifications for vehicle sounds are: 

 Recording(s) of Actual ICE Sounds (Option A). 

1. Defined by overall A-weighted levels. 

2. Defined by one-third octave band average spectra. 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

3. Defined by one-third octave band minima spectra. 

4. Combinations of the above. 

 Synthesized ICE-Equivalent Sounds (Option B). 

1. Defined by overall A-weighted levels. 

2. Defined by one-third octave band average spectra. 

3. Defined by one-third octave band minima spectra. 

4. Combinations of the above. 

 Alternative, non ICE-like Sounds Designed for Detectability (Option C). 

 Hybrid of options listed above (Option D). 

8.1 Scope 

Previous research by NHTSA (Phase 1) shows that the overall sound levels for the HEVs tested are 

noticeably lower at low speeds than for the ICE vehicles tested. Considering the results of acoustic 

measurements and human subject data, countermeasures are only needed when vehicles are 

operated at speeds generally less than 10 to 20 mph where tire noise is not dominant. At the present 

time, synthetic vehicle sounds appear to be the only countermeasure that is useful in providing 

relevant sound cues needed by pedestrians. Relevant sound cues provide information about vehicle 

presence, speed, and rate of change in speed. Groups representing people who are blind have 

expressed a preference for sound(s) that will be recognized as that of an approaching vehicle so 

that it will be intuitive for all pedestrians. 

Therefore, the elements of a specification for vehicle sounds should address the following issues: 

 Sound output levels; 

 Pitch changes that convey changes in vehicle speed; and 

 Acoustic qualities that determine whether the sound is perceived as a vehicle. 

The options discussed in this chapter (options A-D) assume that the vehicle acoustic 

countermeasure should: 

 Alert pedestrians, including blind pedestrians, to vehicle presence and 

operation below 20 mph; 

 Provide information at least equivalent to that provided by ICE vehicles, 

including speed change; 

 Apply to vehicles operating in electric only mode while operating in the 

specified conditions; and 

 Provide for detection of a vehicle in residential, commercial and other 

suburban and urban environments. Note: Human-subject tests cited in this 

chapter were conducted in an ambient level of approximately 58-61 dB(A). 

Acoustic data acquired from a sample of ICE vehicles were used to determine the sound levels at 

which ICE-like synthetic vehicle sounds, developed as countermeasures, would be set. 

Measurements include overall sound levels and spectral content (i.e., one-third octave-band data). 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

Specifications discussed here also include input from psychoacoustic model(s) and human-subject 

testing. Psychoacoustic model(s) and human-subject testing were used to explore issues of 

detectability, masking, and recognition of alternative, non-ICE-like sound countermeasures. 

Acoustic measurements were collected for the following vehicle operating modes: 

 Low-speed pass-by forward at 6, 10, 15 and 20 mph; 

 Low-speed pass-by reverse at 6 mph; 

 Acceleration (from 0 to 10 mph); 

 Idle; and 

 Start-up. 

One of the operating modes, deceleration from 20 to 10 mph (as if to turn right), tested in Phase 1 

of the Quieter Cars and the Safety of Blind Pedestrian research program, could not be included in 

the Phase 2. Deceleration from 20 to 10 mph (as if to turn right) was not included in Phase 2 

because a location of sufficient length to test vehicle deceleration was not readily available. In 

addition, to conduct a valid test of the detectability of decelerations with real vehicles, it would be 

necessary to limit the number of subjects being tested to only one or two in a single experimental 

session, so that they would hear the same thing. Conducting numerous additional sessions, or 

extending session durations to approximately six hours so that deceleration could be included was 

inadvisable for reasons of cost, timeliness of project completion, and the health and comfort of the 

subjects. Deceleration is a safety scenario identified in Phase 1 of our research and the noiseemission characteristics of a vehicle decelerating from 20 to 10 mph can be approximated by using 

the measured values for 20- and 10- mph pass-bys and programming the DSP to interpolate 

between these values as the vehicle decelerates. 

Overall sound level by itself does not determine how well a sound is detected; spectral content has 

a significant impact on detectability. If sounds “X” and “Y” are similar, broad-band sounds (e.g., 

engine sounds) coming from the same direction, “X” may need to have a higher SPL than “Y” in 

order to be detected. If “X” and “Y” are different, comparisons are more complicated. For 

example, if “X” is a narrow-band sound, it may be detected even if its SPL is significantly lower 

than that of “Y,” provided that narrow-band sound is not in a frequency range where the ambient is 

especially strong. (“Narrow-band” sounds are those composed of a relatively small range of 

frequencies – a few octaves or less. “Wide-band” sounds are those containing frequencies from all 

or most of the ten-octave range of human hearing.) On the other hand, if “X” is a narrow-band 

sound that is in a frequency range where the ambient is especially strong, then “X” may not be 

detected even if its SPL is significantly higher than that of “Y”. In general, if sounds have similar 

spectral content and temporal variations, then their detectability will be similar. If the sounds vary 

in spectral content, then, detectability could be very different. 

Sound characteristics such as rise time, decay time, repetition rate, and variation in pitch and 

loudness can affect recognizability without necessarily changing spectral balance. Duplicating the 

spectral balance of an ICE does not guarantee that the sound will be recognized as an ICE. This is a 

caveat to any specification. 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

Pitch-shifting is perhaps the easiest to specify and test for compliance. The discussion in the 

following section on pitch shifting applies to all of the alternatives described in subsequent sections 

relating to options for specifications related to sound levels, spectral content and recognizability. 

8.2 Pitch-Shifting Specifications 

Prototypes supplied by OEMs for this study shift the pitch of the sounds (generated or processed 

by a DSP) in proportion to vehicle speed. Testing to verify that a sound has pitch shifting is 

relatively insensitive to test conditions. One possible method to test for pitch shifting is to record 

the countermeasure sound at various speeds of interest and play them back in a spectrum analyzer. 

If the frequencies of spectral peaks identified as contributing to pitch increase about 40 percent 

over the speed range from 0 to 20 mph, which is typical of an actual ICE (e.g., tachometer goes 

from about 800 to 1100 RPM), then there is adequate pitch shifting. (Note: Not all peaks will 

positively contribute to pitch strength. In general, peaks that are harmonically related increase 

pitch strength while those that are not harmonically related decrease pitch strength.) A sample of 

pitch shifting analysis is shown in the graphs of Figure 53, which show the spectrum of sound A5 

with engine RPM at idle and at about 20 mph. Figure 53 shows the frequency of the highest peak 

in the spectrum increases from 396 Hz at 0 mph to 653 Hz at 20 mph, an increase of 53 percent, 

which would satisfy the proposed criterion. The smaller peaks also shift in the same proportion. 

Figure 53. Spectra of Sound A5 at 0 and 20 mph 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

8.3 Minimum Sound Pressure Level and Spectral Content 

Specifications Based on ICE Vehicle Acoustic 

Characteristics 

8.3.1 Performance Metrics 

The following potential specifications for countermeasure sounds are based on the assumption that 

the ICE vehicles measured in this study are typical of the current fleet and emit an acceptable 

amount of noise during low-speed operations and that some (e.g., ICE-like) countermeasure sounds 

can be based on the statistical average of real-vehicle spectral characteristics. The actual ICE 

vehicle used in the human-subject experiments, which emitted the same noise level as the average 

of the 10 vehicles tested, was detected at a mean distance of 41 feet, or 4.66 seconds in a 6-mph 

pass-by. This amount of detection time is in close agreement with a 5-second value that has been 

suggested as safety guideline by some discussion groups. This means that the test vehicle traveling 

at 6mph was detected approximately 5 seconds before it arrived at the pedestrian’s location. 

 These specifications represent the minimum overall A-weighted sound pressure level (Table 26), 

and the minimum level in each one-third octave-band (Table 27) for ICE-vehicle countermeasure 

sound. The derivations of these data are given in Section 5. 

The A-weighted 1⁄2-second equivalent sound pressure level is defined as (IEC 61672-1, 2002): 

࡭࢖ ૚,ࢗࢋ࡭ࡸ

૛ሺࣀሻࣀࢊቃ

 ࢀି࢚ ૛ ࢉࢋ࢙ ׬ ሻࢀ/૚ቂሺ ૚૙ ൜ࢍ࢕࢒ ૙૛ ൌ ࢚ (A(dB૙ ࢖ ൗൠ૛ /૚

஺ሺߞሻ is the squared, A-weighted instantaneous sound pressure. ଶ where ݌

Table 26 gives the specification for the broadband A-weighted sound pressure level for a vehicle 

generating a countermeasure sound. 

Table 26. Minimum Overall A-weighted Level (LAeq, 1⁄2 sec) by Vehicle Operation 

Vehicle Operation LAeq, 1/2 sec, dB(A) 

6 mph 61.1 

10 mph 63.6 

15 mph 68.1 

20 mph 70.2 

Acceleration 66.7 

Start-up 70.7 

Idle 55.2 

8.3.1.1 Sound Pressure Levels for A-weighted One-Third Octave Band Spectra 

Table 27 shows the corresponding minimum A-weighted one-third-octave-band spectra for each 

operating mode that could apply to options A and B in the following sections. Each of these 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

minima must be met in order to assure that the countermeasure sound emits sound at a level at least 

as great as the estimated typical ICE vehicle. The values in this table were derived by the 

procedures described in Section 5. 

Table 27. A-weighted One-Third-Octave-Band Spectra at Microphone Line LAeq,1/2sec 

1/3 Octave Band Center 

Frequency, Hz 

6 mph 10 mph 15 mph 20 mph Acceleration Startup Idle 

100 to 20k 61.1 63.6 68.1 70.2 66.7 70.7 55.2 

315 43.9 46.9 50.2 52.5 49.8 44.2 37.3 

400 46.5 48.7 53.0 54.1 51.4 46.6 39.0 

500 47.9 51.2 55.6 57.1 53.4 51.8 42.1 

630 49.0 52.5 56.9 59.1 54.6 52.4 42.3 

800 51.1 54.6 59.5 62.3 55.1 55.2 43.2 

1k 51.4 55.2 60.2 63.2 55.6 57.8 44.9 

1.25k 52.2 54.6 59.6 62.2 57.2 60.5 46.3 

1.6k 52.0 54.3 58.8 61.3 57.0 61.1 45.4 

2k 50.3 52.0 56.1 57.9 55.7 60.5 44.6 

2.5k 49.1 50.3 53.9 54.9 55.1 61.1 43.8 

3.15k 48.6 49.2 52.4 52.1 54.9 61.6 44.1 

4k 46.9 47.5 50.5 49.5 53.2 60.9 42.4 

5k 44.1 45.0 47.8 46.4 50.8 59.2 40.3 

8.3.1.2 One-Third Octave Band Restrictions 

ICE vehicles have energy components in all frequencies (e.g., 100 to 20k Hz), however, the 

psychoacoustic models show that energy components in the one-third octave bands ranging from 

1600 Hz to 5000 Hz contributed the most to detection, and those ranging from 315 Hz to 1600 

Hz contributed additional detection and pitch information. These spectral distribution limits are 

derived from the procedures described in Section 6, and depicted in Figure 38, in which all of the 

ICE sounds are masked by the ambient noise below 8 ERBs (which corresponds to 315 Hz). The 

implication of Figure 38 with the fact that many people have high frequency hearing loss is that 

frequencies from 300-5000 Hz are the ones most useful for detection. Very high frequency 

sounds – above 5 kHz – are not masked, but many older individuals cannot hear these 

frequencies well, and loudspeakers recessed behind the grill of an automobile may not radiate 

such sounds well. Therefore, a possible refinement to a countermeasure sound specification 

would be to limit the one-third octave band specification to cover the range of 315 Hz to 5000 

Hz with the levels specified in other one-third octave bands (see Table 44) which could be 

limited to usage for guidance purposes only (based on psychoacoustic analyses/models). 

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8.4 Option A: Recordings of Actual ICE Sounds 

The purpose of this option is to provide a method to generate countermeasure sounds that closely 

approximate the sound of typical motor vehicles. 

8.4.1 Sound 

In this approach, recordings of actual ICE sounds are used. Recordings are made when the 

vehicle is operating at constant speeds, forward from 0 to 20 mph and in reverse at 6 mph. Other 

components of the vehicles noise output (tire noise, aerodynamic noise, AC fan noise, etc.). will 

be emitted regardless of whether an ICE is in use and would not be included in these recordings. 

8.4.2 System 

Sound generation systems with signal processing capabilities would be used to continuously and 

monotonically vary the sounds from one operating condition to the next according to vehicle input 

(e.g. vehicle speed sensors, throttle sensors, etc.). 

8.4.3 Test Methods 

In this option, emitted sounds would be based on approved, standardized recordings (approval and 

standardization process to be determined) with processing limited to pitch shifting in proportion to 

vehicle speed and interpolation between sounds. 

Option A1: Minimum overall A-weighted levels, similar to those listed in Table 26, would be met 

at the measurement location specified by SAE J2889-1 (or similar test method). 

Option A2: Average one-third octave band levels, similar to those listed in Table 27, would be met 

at the measurement location specified by SAE J2889-1 (or similar test method) for one-third 

octave bands from 315 Hz to 5000 Hz. 

Option A3: Minimum one-third octave band levels, similar to those listed in Table 27, but lower by 

5 dB to allow for variation of individual vehicles, would be met at the measurement location 

specified by SAE J2889-1 (or similar test method) for one-third octave bands from 315 Hz to 5000 

Hz. 

Option A4: Combination of options A1 and A2 or options A1 and A3. Both of these level 

specifications (overall and one-third octave bands) are based on the total noise output of the test 

vehicle, i.e. sound generator plus tire and aerodynamic noises. In the higher portions of the speed 

range (10 to 20 mph), most of the noise emission is from tires, which would not be reproduced by 

the sound generator, because so doing would effectively double the noise output of the vehicle. 

8.4.4 Assumptions 

Option A assumes that vehicles used to create the standardized recordings have sound-emission 

characteristics that provide adequate recognizability for pedestrians. 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

Option A assumes that band-limited (315 Hz to 5000 Hz) ICE-like sounds will be recognizable 

as motor vehicles. 

8.4.5 Observations 

Observations for this approach include: 

 It is driven by existing ICE-vehicle sound characteristics and levels, this approach 

helps with recognizability of the sound source (e.g., recognized as a vehicle). 

 It reduces the possibility of having a wider variety of sounds, some of which 

might be not recognized as a vehicle or which might be perceived as annoying. 

 More than one set of standardized recordings could be used. 

 New recordings could be developed for future ICE vehicles. 

 The countermeasure specification could be updated periodically, as needed, by 

providing a new set of standardized ICE recordings over time. 

 This option precludes the use of sounds that may be more unique and effective 

than the basic ICE sound. 

 It may provide fewer cues for blind pedestrians than other options. More cues 

may be advantageous since not all people respond to the same cues in the same
 

way or use the same cues to navigate. 

 It is difficult to make recordings of actual vehicles operating under actual road 

load without also introducing their tire noise, which should not be emitted by the 

sound generator, because the tires of the electric or hybrid vehicle will provide it. 

8.4.6 Suggestions for Further Research 

 Develop directivity specifications, and refine methods to test pitch-shifting, 

overall A-weighted levels, and one-third octave band levels of the engine sound 

alone. 

 Produce and evaluate standardized recordings of actual ICE sounds operating at 

constant speeds, from 0 to 20 mph to validate the practicality of implementing 

countermeasure sounds in vehicles. Investigate the frequency response 

characteristics of the actual speakers that could be used for countermeasure 

sounds and the effect of their placement under the hood of vehicles. 

8.5 Option B: Synthesized ICE-Equivalent Sounds 

The purpose of this option is to provide a method to generate countermeasure sounds that closely 

approximate the sound of typical motor vehicles. Additional consideration has been given to 

make this option more flexible than Option A. 

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8.5.1 Sound 

In this option, simulated ICE sounds would be synthesized directly by a digital-signal processor 

(DSP) programmed to create ICE-like sounds (based on actual target sounds) that would vary pitch 

and loudness depending on vehicle inputs. Target sounds are recordings of actual ICEs. This is in 

contrast to Option A in which the sounds come directly from recordings of actual vehicles, and the 

processor must store and interpolate among files representing every mode of operation and for 

every speed within the 0 to 20-mph range. Here, the resulting synthesized sounds would resemble 

those of Option A, but have fewer spectral components. A synthesizer could be simpler and 

cheaper than a sound generator based on real ICE sounds. 

For this option, target sounds, recorded from actual vehicles for the operations specified in 

Section 8.1 would be used. The synthesized sounds would then be developed to match the spectral 

shape of these target sounds within a tolerance of 3 dB per spectral line when comparing power 

spectra. (Note: by definition, power-spectra spectral lines have a resolution of 1 Hz). The tolerance 

here is for comparison between the synthesized sound and target sound. In Option A, no such 

additional tolerance is required since the sound file being played comes directly from a recorded 

ICE sound. 

8.5.2 System 

Sound generation systems with signal processing capabilities would be used to continuously and 

monotonically vary the sounds from one operating condition to the next according to vehicle input 

(e.g. vehicle speed sensors, throttle sensors, etc.) and the synthesis algorithms developed for their 

sounds. 

8.5.3 Test Methods 

Emitted synthesized sounds would be based entirely on ICE recordings with processing limited to 

pitch shifting in proportion to vehicle speed and interpolation between sounds. 

Option B1: Minimum overall A-weighted levels, similar to those listed in Table 26, would be met 

at the measurement location specified by SAE J2889-1 (or similar test method). 

Option B2: Average one-third octave band levels, similar to those listed in Table 27, would be met 

at the measurement location specified by SAE J2889-1 (or similar test method) for one-third 

octave bands from 315 Hz to 5000 Hz. 

Option B3: Minimum one-third octave band levels, similar to those listed in Table 27, but lower by 

5 dB to allow for variation of individual vehicles, would be met at the measurement location 

specified by SAE J2889-1 (or similar test method) for one-third octave bands from 315 Hz to 

5000 Hz. 

Option B4: Combination of options B1 and B2 or options B1 and B3. Both of these level 

specifications (overall and one-third octave bands) are based on the total noise output of the test 

vehicle, i.e. sound generator plus tire and aerodynamic noises. In the higher portions of the speed 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

range (10 to 20 mph), most of the noise emission is from tires, which would not be reproduced by 

the sound generator, because so doing would effectively double the noise output of the vehicle. 

8.5.4 Assumptions 

Option B assumes that vehicles used to provide the templates for the synthesized sounds have 

sound-emission characteristics that provide adequate recognizability for pedestrians. 

In addition, this option assumes that band-limited (315 Hz to 5000 Hz) ICE-like sounds will be 

recognizable as motor vehicles. 

8.5.5 Observations 

Observations for this approach include: 

 It provides ICE-equivalent sounds which help with recognizability. 

 Sounds could be developed to match a particular ICE vehicle make/model (e.g., 

sound signature) which would help with recognizability of the sound. 

 It may be less expensive to build the sound emission systems electronics than 

Option A. 

 Recordings are not needed a priori because the ICE recordings are not 

standardized in this option. 

 If a poor recording was chosen, for example, one with too much engine exhaust, it 

might not have sufficient high-frequency components to ensure adequate 

detection. 

 This option precludes the use of sounds that may be more unique and effective 

than the basic ICE sound. 

 It may provide fewer cues for blind pedestrians than other options. The more cues 

the better since not all people respond to the same cues in the same way or uses 

the same cues to navigate. 

8.5.6 Suggestions for Further Research 

 Develop directivity specifications. 

 Develop methods to test pitch-shifting, and refine test methods to record overall Aweighted sound levels and one-third octave band levels. 

8.6 Option C: Alternative, non-ICE-like Sounds Designed for 

Detectability 

Alternative countermeasure sounds with acoustic characteristics different from ICE vehicles were 

also examined. These alternative countermeasure sounds were shown to be effective for vehicle 

detection during human-subject experiments. At the same overall SPL values as ICE-like sounds, 

some non-ICE sounds were detected at almost double the distance of the ICE reference vehicle. 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

Alternatively, these sounds can be played at levels as much as several decibels below the noise 

output of typical ICEs while providing the same detectability, thus reducing community noise 

impact. Furthermore, these sounds may be cheaper to reproduce than ICE-like sounds, which have 

greater bandwidth specifications, necessitating more expensive loudspeakers. The design of a nonICE sound involves a complex tradeoff among: 

 Detectability (detection distance or detection time); 

 Overall SPL value; 

 Annoyance; and 

 Cost. 

While the required SPL values for non-ICE-like sounds will generally be lower than for ICE-like 

sounds for the same detection distance, there is no objective basis upon which to calculate the 

difference in SPL values for the class of non-ICE sounds as a whole. Rather, the equivalentdetectability-SPL value for a particular non-ICE sound must initially be determined experimentally 

by a jurying process that rates detectability. As psychoacoustic models improve, it may be 

possible to use them in place of jury testing to determine minimum SPL specifications for these 

sounds, but that approach is not yet sufficiently accurate. 

8.6.1 Sound 

The following characteristics can improve detectability of an alerting sound and are likely to be 

interpreted as hazard warnings on first hearing: 

 Pitch shifting denoting vehicle speed change (approximately 40% pitch shift for 0 

to 20 mph speed change and linear within that speed range). 

 Pulsating quality with pulse widths of 100 to 200 msec; about 3 to 10 per second. 

 Inter-pulse intervals of no more than 150 msec. 

 Fundamental tonal component in the 150 to 1000 Hz range. 

 At least three prominent harmonics in the 1 to 4 kHz range. 

 4 or more components (peaks in spectral distribution) exceeding 50 dB(A). 

Sounds that conform to the guidelines above may not resemble the sound of an ICE, although 

recordings of ICE noise can be processed through a DSP to conform to the guidelines above while 

retaining a quality that immediately suggests an ICE. (Other kinds of sounds, such as, music or 

movie sound effects, could also be processed to give them the properties of effective auditory 

warnings.) Whether the original character of such sounds could still be recognized as a vehicle 

after processing could only be determined on a case-by-case basis through jury testing (see test 

methods below). In this option, any sounds constructed according to psychoacoustic principles for 

effective alerts, as listed above, would be permissible. Sounds could also include ICE-like sounds 

at reduced levels to enhance recognizability. 

Additionally, sounds could be designed to include additional features to enhance detectability in 

certain situations, such as: 

 Increased amplitude when driving in reverse. 

 Briefly increased amplitude when accelerating from a stationary position. 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

8.6.2 System 

Sound generation systems with signal processing capabilities would be used to continuously and 

monotonically vary the pitch and amplitude of sounds as appropriate to operating conditions 

according to vehicle inputs (e.g. vehicle speed sensors, throttle sensors, etc.). The appropriate 

relationship between sound amplitude and throttle position would need to be determined. 

8.6.3 Test Methods 

The detectability of a specific non-ICE sound can be best determined only through human-subjects 

testing, at the present state of the art. Because such testing is expensive, two alternative versions of 

this option are proposed. The first minimizes testing costs by constraining the number of 

permissible sounds. The second allows any sound that passes the jurying test, so long as the 

developers bear the cost of such testing. 

Option C1: A single non-ICE sound, or a limited set of such sounds, might be tested by a jurying 

process to determine the SPL values at which they produce equivalent detection distances to ICE 

sounds. 

Option C2: Applications of non-ICE sounds other than those developed in Option 1 would be 

required to conduct jury testing to establish the required playback levels for those sounds that 

would ensure equivalence to ICE-like sounds in terms of detection distance or time. 

Jury testing is widely used in the automotive industry to make decisions about how vehicles should 

sound (Otto, Amman, Eaton, and Lake, 2001). For countermeasure sounds, juries could rate 

candidate sounds in terms of detection distance with speed (or detection time), recognizability, and 

annoyance to discover which sounds are optimal. An SAE-type test method would not be 

sufficient because SPL values for non-ICE sounds are only loosely correlated with detection 

distance/time and the SAE test is totally insensitive to annoyance. Although there are multiple 

experimental paradigms for the jury approach, the least problematic would be one in which the 

jury members register their ability to detect a test sound by pressing a button linked to an automatic 

data-collection system that records detection time for each member in each trial. By this means, it 

can be determined whether a given test sound has the same or greater detectability and 

recognizability as a reference ICE sound, when both sounds are played in identical and 

representative background noise conditions. 

Further experimental work is required to determine: 

 Ambient-noise specifications for the test. 

 Repeatability of the jury. 

In order to make jury-testing results more repeatable, synthetic ambient noise with a spectral 

characteristic that resembles typical urban conditions is likely to be used for jury testing. 

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8.6.4 Assumptions 

Option C assumes that the jury process will screen out sounds that could be confused with nonvehicular threats to pedestrians and sounds that are annoying or offensive. 

8.6.5 Observations 

Some observations for this approach include: 

 It could provide improved detectability over ICE-equivalent sounds. 

 The limited frequency range works well for small speakers similar in size and 

construction to those currently used for backup warnings on commercial vehicles 

and some vehicle alerting systems. 

 To survive the jury-screening process, sounds must be detectable while 

minimizing confusion and annoyance. 

 This approach is very open-ended (e.g., can produce a wide variety of sounds, 

which could be confusing to pedestrians). 

 In this approach, blind pedestrians must recognize a sound that has the properties 

of a warning sound (as described in 8.6.1) which are different from the properties 

of ICE-equivalent sounds. Nearly all subjects seemed to recognize a warning 

sound as such on first hearing, but during debriefing, some subjects expressed 

concern that there could be uncertainty if there are too many different warning 

sounds. Further research is needed on this issue. 

8.6.6 Suggestions for Further Research 

 Develop directivity specifications. 

 Determine acceptability of jury testing. 

 Develop methods to conduct jury tests including specifications for ambient noise 

conditions. 

 Develop one or more standardized non-ICE sounds. 

 Since these are novel sounds, further investigation into the negative impacts of these 

sounds should be conducted. These impacts include but are not limited to: annoyance to 

driver, pedestrians, and communities; confusion; and reduced recognizability. 

8.7 Option D: Hybrid of Options Listed Above 

8.7.1 Sound 

This option is comprised of a combination of Options B and C in any of various proportions. For 

the portion of the sound derived by Option B, simulated ICE sounds would be generated directly 

by a digital-signal processor (DSP) programmed to create ICE-like sounds, which would vary pitch 

and loudness depending on vehicle inputs. The resulting sounds would resemble those of Option 

A, however the level of the ICE-engine sounds might be considerably lower than in Options A or 

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Chapter 8: Discussion: Potential Specifications for Vehicle Sounds 

B. The portion derived from Option C would conform to the characteristics specified in Section 

8.6.1. 

8.7.2 System 

Sound generation systems would be used with signal processing capabilities to continuously and 

monotonically vary the pitch and amplitude of sounds as appropriate to operating conditions 

according to vehicle inputs. This system would simultaneously generate both ICE-like sounds at a 

lower level than in Options A and B (levels to be determined by jury testing) and synthetic sounds 

designed for optimal combinations of alerting potential and minimal annoyance. The ICE-like 

sound components may not be heard in higher urban ambient-noise conditions, but their 

association with the alerting sound is learned during exposure in quieter environments. 

8.7.3 Test Methods 

The detectability of a specific non-ICE sound can be best determined only through human-subjects 

testing, at the present state of the art. Because such testing is expensive, two alternative tests 

methods are proposed. 

Option D1: A single non-ICE sound, or a limited set of such sounds, might be tested by a jurying 

process to determine the SPL values at which they produce equivalent detection distances to ICE 

sounds. 

Option D2: Applications of non-ICE sounds other than those developed in Option D1 would be 

based on jury testing to establish the required playback levels for those sounds that would ensure 

equivalence to ICE-like sounds in terms of detection distance or time. 

Jury testing is widely used in the automotive industry to make decisions about how vehicles should 

sound (Otto, Amman, Eaton, and Lake, 2001). For countermeasure sounds, juries could rate 

candidate sounds in terms of detection distance with speed (or detection time), recognizability, and 

annoyance to discover which sounds are optimal. An SAE-type test method would not be 

sufficient because SPL values for non-ICE sounds are only loosely correlated with detection 

distance/time and the SAE test is totally insensitive to annoyance. Although there are multiple 

experimental paradigms for the jury approach, the least problematic would be one in which the 

jury members register their ability to detect a test sound by pressing a button linked to an automatic 

data-collection system that records detection time for each member in each trial. By this means, it 

can be determined whether a given test sound has the same or greater detectability and 

recognizability as a reference ICE sound, when both sounds are played in identical and 

representative background noise conditions. 

Further experimental work is required to determine: 

 Ambient-noise specifications for the test. 

 Repeatability of the jury results. 

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In order to make jury-testing results more repeatable, synthetic ambient noise with a spectral 

characteristic that resembles typical urban conditions is likely to be used for jury testing. 

8.7.4 Assumptions 

Option D assumes that the jury process will also screen out sounds that could be confused with 

non-vehicular threats to pedestrians and sounds that are annoying or offensive. 

8.7.5 Observations 

Similar to Option C described in Section 8.6.5. However, the addition of ICE-like spectral content 

may facilitate recognition, thereby enhancing the appeal of these sounds to juries. This approach 

was used in some of the most effective sounds tested in this research (in terms of detection 

distance), for example, sounds A5 and E3. Since these sounds combine characteristics from both 

options B and C, the potential negative impacts (in terms of recognition) may be less than for 

Option C. 

8.7.6 Suggestions for Further Research 

 Develop directivity specifications. 

 Develop methods to conduct jury tests, including specifications for ambient noise 

conditions. 

 Develop one or more standardized non-ICE sounds. 

 Since these are novel sounds, conduct further investigation into the negative impacts of 

these sounds. These impacts include but are not limited to: annoyance to driver, 

pedestrians, and communities; confusion; and reduced recognizability. 

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Chapter 9: Summary of Findings and Considerations for Further Research 

9. SUMMARY OF FINDINGS AND CONSIDERATIONS 

FOR FURTHER RESEARCH 

9.1 Summary of Findings 

Acoustic countermeasures should alert pedestrians, including blind pedestrians, of vehicle presence 

and operation. Information at least equivalent to the cues provided by ICE vehicles, including 

ideally, speed change, should be provided. In this research, acoustic data acquired from a sample of 

ICE vehicles was used to determine the sound levels at which synthetic vehicle sounds, developed 

as countermeasures, should be set. ICE-equivalent sounds were specified as overall A-weighted 

sound levels and spectral content, one-third octave band. Psychoacoustics models showed that 

frequency components between 1600 and 5000 Hz were more detectable due to strong signal 

strength and relatively low ambient levels in this range and frequency components below 315 Hz 

were often masked by urban ambient noise. Therefore, it is reasonable to specify minimum onethird octave band levels from 315 Hz to 5000 Hz. Minimum overall A-weighted levels and 

minimum one-third octave band levels (refer to Table 27 and Table 28) were developed and could 

form the basis for specifications to be met at the measurement location specified by SAE J2889-1. 

For recognizability as a vehicle under Options A and B, sounds should be either an ICE recording 

or modeled on a recording and include pitch shifting to denote vehicle speed. ICEs normally emit 

louder sounds during acceleration. This additional-loudness cue is most useful to blind pedestrians 

when a stationary vehicle first starts moving, but could be included over the whole 0 to 20 mph 

range by providing an input to the sound generator from the throttle-position sensor. 

The disparity in noise emission levels between moving ICEs and vehicles in pure electric mode is 

greatest at about 6 mph (Garay-Vega, et. al, 2010), hence 6-mph pass-by scenarios were chosen for 

human-subject testing of the detectability of vehicles operating in EV mode with and without 

synthetic sound generators. At 6 mph, a typical ICE vehicle generated sufficient noise (60-61 dB 

(A)) to be detectable at an average distance of about 41feet in a typical urban noise environment 

with average noise level of 58 – 61 dB (A). This provides a detection time of slightly less than 5 

seconds before the vehicle might collide with a pedestrian who ventured into its path. 

Various synthetic sounds, along with a reference ICE vehicle and a no-vehicle condition were 

evaluated through the use of panels of human subjects to determine the distances at which the 

participants could detect and recognize an approaching vehicle. The synthetic sounds were played 

at two different levels with values for LAeq ,1/2 sec of 59.5 (slightly below the typical ICE vehicle) 

and 63.5 (about 3 dB louder than a typical ICE. Each of the nine different synthetic sounds could 

be described as belonging to one of five categories: 

 Sounds that emulate the sound of an ICE, including combustion noise, valve noise 

and fan noise. 

 Sounds that emulate only the fundamental combustion noise (generally below 250 

Hz for the low speeds of interest).
 

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Chapter 9: Summary of Findings and Considerations for Further Research 

 Sounds based on ICE sounds but with exaggerated high-frequencies. Entirely 

artificial sounds designed to maximize detection range while minimizing overall 

loudness and annoyance. 

 Hybrid sounds combining categories listed above to enhance both detection 

distance and recognizability.
 

The relative effectiveness (detection distance) of the tested sounds in these different categories of 

sounds may be summarized as follows (see also Table E2 and Figure 54): 

72 

57 

52 

50 

41 

35 

32 

30 

20 

19 

0 20 40 60 80 

Detection Distance (feet) 

D 

B 

E1 

C 

A1 

ICE Vehicle 

A5 

E3 

A2 

E4 

Figure 54. Average Detection Distance (for 6 mph Forward Pass-By & Sound Level = 59.5 

dB(A) 

 Figure 54 shows the mean detection distances for the sounds evaluated in the 

human-subject studies. Sounds at the top of the list can be described as ICE-like 

sounds with only the fundamental combustion noise or otherwise lacking in the 

qualities that support detectability. Sounds at the end of the list can be described 

as alternative sounds designed according to psychoacoustic principles.
 

 Sounds designed according to principles for auditory warnings (e.g. sound ‘E4’) 

with a substantial impulsive component worked best, sometimes providing 

detection at distances about double those of the ICE reference vehicle when 

played at the slightly lower (59.5 dB(A)) level. 

 Next most effective were ICE-like sounds with exaggerated high-frequency 

components (e.g., sounds ‘A2’ and ‘E3’). 

 Sounds that resembled normal ICEs in both timbre and loudness (e.g. ‘A1’ and 

‘E1’) were detected at distances roughly similar to those of the ICE reference 

vehicle. 

 Sounds bearing no resemblance to a piston engine or an auditory warning were 

detected at distances slightly, but not significantly, greater than those of the ICE 

reference vehicle, if played at the 63.5 dB(A) level (e.g., sound ‘C’). At the lower 

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Chapter 9: Summary of Findings and Considerations for Further Research 

playback level, 59.5 dB(A), sound ‘C’ was slightly inferior to the reference 

vehicle, while sound ‘D’ was detected at about half the distance of the reference 

vehicle. 

 Sounds containing only combustion noise frequencies (e.g., sound ‘B’) were 

detected at about half the distance of the ICE reference vehicle. 

Among the factors contributing to the superior detectability of sound ‘E4’ are the following: 

 Most of the energy in the sound lies in the 500 Hz to 3 kHz region where human 

hearing is most sensitive. 

 The impulsive quality of this sound means that the peak levels of the sound can be 

several dB louder than a continuous sound with the same LAeq, 1⁄2 sec. 

 Even if the peak level of a pulsating sound is equal to that of a continuous sound 

of similar spectral content, the pulsating sound will have a higher alerting 

potential.
 

The rapid bursts of high-frequency sound in ‘E4’ bear no resemblance to any ICE-like sound, yet 

many study participants indicated that they recognized it as an alert to an approaching vehicle 

immediately. Furthermore, in a practice trial, with all sounds played at the 63.5 dB (A) level, all 

but one of 24 participants detected the ‘E4’ sound on first hearing, and every subject detected it on 

second hearing. In contrast, 22 of the 24 subjects in the practice trial correctly made no response to 

a no-vehicle condition on the first exposure. 

The ICE-like components of sound ‘E4’ are emitted at a level that is about 10 dB below the peak 

of the other component (i.e., the rapid bursts of high-frequency)and are not audible at the distances 

that participants detected this sound in the presence of the Volpe Center ambient noise. The ICElike sounds became audible as the vehicle passed the participants, and may thus reinforce the 

association of the rapid bursts of high-frequency sound in ‘E4’with an approaching vehicle. 

However, there is insufficient evidence to conclude that they are necessary, since all but one 

subject recognized the ‘E4’ sound as an approaching threat on first hearing. 

The implication of these experiments and the acoustic analysis of the sounds used is that 

countermeasures which emulate ICE sounds over the 300 to 5k Hz range can provide detection 

distances equivalent to typical ICE vehicles. However, synthetic sounds that are specifically 

designed for detectability have the potential to provide double the detection distance at the same 

average noise level. 

The criteria for detectability can be met through various options to specifications, such as: 

 Recording(s) of Actual ICE Sounds (Option A). 

1. Defined by overall A-weighted levels. 

2. Defined by one-third octave band average spectra. 

3. Defined by one-third octave band minima spectra. 

4. Combinations of the above. 

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Chapter 9: Summary of Findings and Considerations for Further Research 

 Synthesized ICE-Equivalent Sounds (Option B). 

1. Defined by overall A-weighted levels. 

2. Defined by one-third octave band average spectra. 

3. Defined by one-third octave band minima spectra. 

4. Combinations of the above. 

 Alternative, non ICE-like Sounds Designed for Detectability (Option C). 

 Hybrid of options listed above (Option D). 

Other factors that must be considered, but that were not explored in this research include: 

effectiveness of the alerting sound when multiple vehicles are present; acceptance and annoyance 

of the countermeasure sounds; variability in the directivity of speakers systems; and alternative 

(lower) sound levels for hybrid sounds and for sounds that do not resemble ICE sounds. 

9.2 Considerations for Further Research 

Countermeasure sounds that emulate ICEs are expected to provide the same auditory cues to 

pedestrians and the same or lower community-noise impacts compared to current vehicles. In 

order to ensure that these sounds are emitted in a manner consistent with ICE vehicle emissions, it 

is necessary that the directivity of the ICE and countermeasure sources are similar. To this end, it is 

recommended that the directivity of ICE and vehicle sound countermeasure systems be 

determined. If these vary significantly, then steps will need to be taken to ensure that the 

countermeasure sounds are at least as loud as the ICE sounds at the location that the ICE sound 

was detected. Alerting sounds that work better to signal the approach of a single vehicle may, or 

may not, be more difficult for blind pedestrians to interpret in a multi-vehicle situation. 

Furthermore, even if their contribution to overall average noise levels is less than ICEs, their 

alerting characteristics may be more annoying than those of ICEs. 

The issue of whether sounds designed for maximum detectability degrade pedestrian safety in 

multi-vehicle situations could possibly be addressed experimentally. The experiment could recreate the sounds heard by a pedestrian at a complex intersection with numerous vehicles moving 

at the same time. Conventional ICE sounds, synthetic sounds designed for maximum detectability, 

and various combinations could be presented to participants to determine if there is any difference 

in their ability to determine when it is safe to cross. 

Annoyance is potentially a significant issue. The developers of countermeasure sounds use a jury 

process to determine which particular sounds combine good detectability with minimal annoyance. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

101 
 

 

Bibliography 

10. BIBLIOGRAPHY 

AASHTO. (2004). A Policy on Geometric Design of Highways and Streets. Washington, DC: 

American Association of State Highway and Transportation Officials, Retrieved from: 

https://bookstore.transportation.org/category_item.aspx?id=DS. 

ANSI S3.4-2007. American National Standard Procedure for the Computation of Loudness of 

Steady Sound. 

DIN 45 631, Procedure for Calculating Loudness Level and Loudness, Deutsches Institut für 

Normung. 

Fletcher, H. and Munson, W. A. (1933). Loudness, Its Definition, Measurement, and Calculation. 

Journal of the Acoustic Society of America. Volume 5, Issue 1. pp. 82-108. 

Fleming, Plotkin, Roof, Ikelheimer, and Senzig. (2005). Assessment of Tools for Modeling 

Aircraft Noise in the National Parks. Federal Interagency Committee on Aviation Noise 

(FICAN) Federal Aviation Administration.. Retrieved from: 

http://www.fican.org/pages/findings.html 

Garay-Vega, L; Hastings, A.; Pollard, J.K.; Zuschlag, M. & Stearns, M. (2010, April). Quieter 

Cars and the Safety of Blind. Pedestrians: Phase 1. DOT HS 811 304 Washington, DC: 

National Highway Traffic Safety Administration. 

Giguère, C.; Laroche, C.; Al Osman, R. and Zheng, Y. (2008). Optimal Installation of Audible 

Warning Systems in the Noisy Workplace, 9th International Congress on Noise as a Public 

Health Problem (ICBEN) 2008, Foxwoods, CT. Retrieved from: 

http://www.icben.org/proceedings2008/PDFs/Giguere_et_al.pdf. 

Goodes, P.; Bai, Y.B. and Meyer, E. (2009). Investigation into the Detection of a Quiet Vehicle 

by the Blind Community and the Application of an External Noise Emitting System. SAE 

2009-01-2189. 

Hanna, R. (2009, September). Incidence of Pedestrians and Bicyclists Crashes by Hybrid 

Passenger Vehicles: National Center for Statistical Analysis Technical Report. DOT HS 811 

204. Washington, DC: National Highway Traffic Safety Administration. 

H.R. 5381--111th Congress: Motor Vehicle Safety Act of 2010. (2010). In GovTrack.us (database 

of federal legislation). Retrieved December 8, 2010, from 

http://www.govtrack.us/congress/bill.xpd?bill=h111-5381 

IEC 61672-1 Electroacoustics - Sound Level Meters - Part 1: Specifications, International 

Electrotechnical Commission, 2002. 

ISO 532-B. Acoustics - Method for Calculating Loudness Level, International Organization for 

Standardization. 

Japanese Automobile Standards Internationalization Centre (JASIC). (2009). A Study on 

Approach Warning Systems for Hybrid Vehicle in Motor Mode. Presented at the 49th World 

Forum for Harmonization of Vehicle Regulation (WP.29) Working Group on Noise (GRB)­

49. February 16-18, 2009. Document Number: GRB-49-10. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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Bibliography 

Kerber, S. & Fastl, H. (2008). Prediction of Perceptibility of Vehicle Exterior Noise in Ambient 

Noise. Proceedings DAGA, Dresden. 

Maurer, M. (2008). The Danger Posed by Silent Vehicles. National Federation of the Blind. 

Remarks made for the United Nations Economic Commission for Europe, Working Party on 

Noise. 47th GRB session February 19, 2008 Geneva. Informal Document No. GRB-47-10. 

http://www.unece.org/trans/doc/2008/wp29grb/ECE-TRANS-WP29-GRB-47-inf10e.pdf 

Moore B.C.J. (2003). An Introduction to the Psychology of Hearing, Fifth Edition, Academic 

Press”. 

Moore & Glasberg, (1997). A Model of Loudness Perception Applied to Cochlear Hearing Loss, 

Auditory Neuroscience Vol. 3 pp.289-311. 

Moore, B. C. J., Glasberg, B. R., and Baer, T. (1997). A Model for the Prediction of Thresholds, 

Loudness and Partial Loudness, J. Audio Eng. Soc. 45, 224-240. 

MLIT and JASIC (2010). Guidelines for Measure Against Quietness Problem of HV. GRB 

Informal group on Quiet Road Transport Vehicles (QRTV) Working papers of the 3rd 

informal meeting. Tokyo, 13-15 July 2010. Retrieved from: 

http://www.unece.org/trans/main/wp29/wp29wgs/wp29grb/QRTV_3.html. 

Otto, N., Amman, S., Eaton, C. and Lake, S. Guidelines for Jury Evaluations of Automotive 

Sounds. Sound and Vibration. Volume 35(4). 

Public Law 111-373: Pedestrian Safety Enhancement Act of 2010. (124 stat.4086; Date: 1/4/11; 

enacted S.841). 

Rosenblum, L. (2008), Hybrid Cars are Harder to Hear. University of California Riverside 

Newsroom April 28, 2008. 

Rosenblum, L. (2008). Sound Measurement and Mobility. Transcript of the Quiet Cars Public 

Meeting on June 23, 2008.Docket ID NHTSA-2008-0108-0023. pp. 53 -65. 

http://www.nhtsa.gov/DOT/NHTSA/NVS/Crash%20Avoidance/Technical%20Publications/2 

010/811304rev.pdf. 

SAE. Measurement of Minimum Noise Emitted by Road Vehicles. SAE J2889-1 Draft 2009. 

Society of Automotive Engineers 

Stanton, N. and Edworthy J. (Eds.). (1999). Human Factors in Auditory Warnings. 

QRTV (2010). Terms of Reference and Rules of Procedure for the GRB Informal Group on 

Quiet Road Transport Vehicles (QRTV). Retrieved from: 

http://www.unece.org/trans/main/wp29/wp29wgs/wp29grb/QRTV_1.html. 

Wall Emerson, Naghshineh, Happen and Wiener (unpublished).The Effect of Hybrid on Blind 

Pedestrian’s Crossing. Western Michigan University and Marquette University. Presented at 

SAE Vehicle Sound for Pedestrian Committee Meeting on April 23rd, 2009. Detroit, MI. 

Zheng Y, Giguère C, Laroche C, Sabourin C, Gagné A, Elyea M., A Psychoacoustical Model for 

Specifying the Level and Spectrum of Acoustic Warning Signals in the Workplace, J Occup 

Environ Hyg. 2007 Feb;4(2):87-98. 

Zwicker, E. and H. Fastl (1999). Psychoacoustics: Facts and Models. Berlin, Springer-Verlag. 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

104 
 

Appendix A: Supplemental Data and Information 

APPENDIX A. SUPPLEMENTAL DATA AND 

INFORMATION 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

105 
 

 

 

 

 

 

Appendix A.1: Intersections Used for Ambient Measurements for Countermeasure Testing 

A.1 Intersections Used for Ambient 

Measurements for Countermeasure Testing 

Table 28. Intersections Used for Ambient Measurements 

ID Location Geometry and Traffic Control 

1 Sergeant Street, Cabot Street, and Centre Street “+” shaped, signalized 

2 Richmond Road & Centre Street T-shaped, stop sign 

3 Colby Road & Centre Street T-Shaped, stop sign 

4 Boston College driveway & Centre Street T-shaped, sidewalk set back 25ft from Centre Street 

5 Mill Street & Centre Street T-Shaped, stop sign 

6 Ward Street & Centre Street T-Shaped, stop sign 

7 Nathan Road. & Centre Street T-shaped, stop sign 

8 Grafton Street & Centre Street 1-way street, enter from Centre Street 

9 Commonwealth Avenue & Center Street “+” shaped, signalized 

10 Elmore Street & Ward Street T-shaped 

11 Langdon Street & Winchester Road T-shaped 

12 Cabot Street & Harvard Street 4-way stop 

13 Atwood Avenue & Harvard Circle Y-shaped 

14 Gay Street & Frederick Street T-shaped 

15 Bonwood Street & Frederick Street T-shaped 

16 Harvard Street & Norwood Avenue T-shaped 

17 Walnut Street & Centre Street T-shaped, signalized 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

A.2 Acoustic Measurement ResultsTabulated Data 

Table 29. Overall A-weighted Uncorrected Sound Pressure Levels: 6 mph, Reverse 

Vehicle LAeq LAF, max LAF, min 

1 56.3 56.7 55.3 

1 58.4 58.9 57.6 

1 60.4 60.8 60.2 

1 61.2 61.5 60.9 

2 56.6 57.0 55.7 

2 57.3 58.4 57.0 

2 57.5 57.9 57.1 

2 57.9 58.3 57.3 

2 58.3 58.5 58.2 

2 59.0 59.4 58.7 

3 59.3 59.4 59.0 

3 59.5 60.0 58.8 

3 59.9 60.1 59.4 

3 60.0 60.5 59.5 

3 60.4 60.9 59.8 

3 61.4 61.7 60.5 

4 54.9 55.2 54.6 

4 55.3 55.6 55.1 

4 55.9 56.4 55.5 

4 56.3 56.5 56.1 

4 56.8 57.1 56.3 

4 57.2 57.4 56.7 

5 56.0 56.2 55.5 

5 57.0 57.4 56.1 

5 57.3 57.5 57.2 

5 57.8 58.1 57.6 

5 59.0 59.4 58.9 

5 59.4 59.5 59.2 

6 61.0 61.4 60.4 

6 61.9 62.3 61.3 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Vehicle LAeq LAF, max LAF, min 

6 62.4 63.0 61.2 

6 64.8 65.5 63.6 

8 58.0 58.2 57.7 

8 58.0 58.4 57.6 

8 58.1 58.5 57.3 

8 58.6 59.0 58.4 

8 59.7 60.1 59.3 

8 59.8 60.2 59.5 

8 60.6 61.1 60.1 

8 60.7 61.0 60.3 

9 57.8 58.0 57.6 

9 58.5 58.8 57.9 

9 58.8 59.1 58.6 

9 60.0 60.3 59.3 

10 57.8 58.1 57.5 

10 58.1 58.2 57.6 

10 58.1 58.3 57.9 

10 58.3 58.4 58.1 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 30. Overall A-weighted Uncorrected Sound Pressure Levels: 6 mph, Forward 

Vehicle LAeq LAF, max LAF, min 

1 62.0 62.5 61.2 

1 62.3 62.7 61.7 

1 62.9 63.2 62.3 

1 63.1 63.6 62.9 

2 59.8 60.2 59.0 

2 60.1 60.5 59.5 

2 60.6 60.7 60.4 

2 60.8 61.3 60.5 

3 58.3 59.1 58.0 

3 58.6 59.1 57.9 

3 60.3 60.6 59.7 

3 61.1 61.2 60.5 

3 62.2 63.0 61.9 

3 62.7 62.9 62.4 

4 59.0 59.9 58.6 

4 59.5 59.7 59.3 

4 60.3 60.7 59.4 

4 61.2 61.6 61.2 

4 61.7 62.6 61.3 

4 63.2 63.7 62.8 

5 57.3 58.2 57.1 

5 58.7 59.1 58.3 

5 60.4 61.5 60.3 

5 61.4 61.9 60.1 

6 59.7 60.2 58.4 

6 60.5 60.8 60.3 

6 61.3 61.5 61.1 

6 63.9 63.9 63.8 

8 62.6 63.3 61.1 

8 62.8 63.2 61.7 

8 62.8 63.2 62.7 

8 63.3 64.1 62.8 

9 60.0 60.1 59.9 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Vehicle LAeq LAF, max LAF, min 

9 60.5 60.8 60.4 

9 62.2 62.4 61.9 

9 62.3 62.8 62.1 

10 61.6 61.8 61.5 

10 61.7 62.0 61.0 

10 62.5 63.1 60.9 

10 63.2 63.5 62.0 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 31. Overall A-weighted Uncorrected Sound Pressure Levels: 10 mph, Forward 

Vehicle LAeq LAF, max LAF, min 

1 62.9 63.4 62.2 

1 63.1 63.4 62.6 

1 63.6 64.2 62.4 

1 63.6 64.3 62.5 

2 60.0 60.9 59.7 

2 61.6 61.8 61.5 

2 61.8 62.9 61.3 

2 62.7 63.2 61.3 

3 63.1 63.7 62.8 

3 63.8 64.2 63.5 

3 64.3 64.7 64.1 

3 64.5 64.7 64.3 

3 64.9 65.2 64.6 

3 65.5 65.8 65.2 

4 62.4 62.8 62.3 

4 62.7 63.1 62.3 

4 63.5 64.0 63.2 

4 63.7 64.0 62.9 

5 63.3 63.7 62.7 

5 64.0 64.6 63.6 

5 64.1 64.5 63.8 

5 64.8 65.1 64.6 

6 62.9 64.2 62.4 

6 64.4 64.6 64.0 

6 65.5 66.2 63.9 

6 66.4 66.7 66.1 

8 63.0 63.7 61.5 

8 63.7 64.0 63.6 

8 64.2 64.9 63.9 

8 64.4 64.8 63.9 

9 62.2 62.4 61.9 

9 62.3 62.6 61.9 

9 62.8 63.2 61.9 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Vehicle LAeq LAF, max LAF, min 

9 62.9 63.3 61.6 

10 64.4 65.0 63.1 

10 64.7 65.0 64.0 

10 65.4 65.7 64.2 

10 65.9 66.4 65.4 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 32. Overall A-weighted Uncorrected Sound Pressure Levels: 15 mph, Forward 

Vehicle LAeq LAF, max LAF, min 

1 68.6 69.0 67.6 

1 68.6 70.1 67.7 

1 69.1 69.5 67.9 

1 70.3 70.6 69.4 

2 65.9 66.2 65.1 

2 66.5 67.2 65.1 

3 65.3 66.7 64.6 

3 67.1 67.8 65.6 

3 68.4 69.5 67.8 

3 69.3 69.8 67.9 

3 69.5 70.1 67.9 

3 69.9 70.6 69.5 

4 69.3 69.6 68.5 

4 69.9 70.1 68.6 

5 65.4 66.3 63.4 

5 66.7 67.9 64.3 

5 67.1 67.6 66.8 

5 67.4 68.1 67.0 

6 66.0 67.1 64.2 

6 68.8 69.0 67.8 

6 69.3 70.3 67.4 

6 70.2 70.4 69.0 

8 68.7 69.1 67.3 

8 70.2 70.4 69.9 

9 65.8 67.0 63.1 

9 67.7 68.1 66.7 

9 68.0 68.4 66.1 

9 68.5 68.8 68.1 

10 62.7 65.2 62.2 

10 67.9 68.2 67.6 

10 68.0 68.5 65.9 

10 69.4 69.5 69.2 

10 69.8 70.7 69.1 

10 71.2 71.6 69.8 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 33. Overall A-weighted Uncorrected Sound Pressure Levels: 20 mph, Forward 

Vehicle LAeq LAF, max LAF, min 

1 68.6 69.8 65.9 

1 69.2 70.1 67.1 

1 70.1 70.8 69.8 

1 70.3 70.5 69.5 

2 63.6 64.8 62.3 

2 67.1 68.1 64.8 

2 67.7 68.6 67.4 

2 69.4 69.9 68.4 

2 70.6 71.1 69.9 

2 71.0 71.4 70.6 

3 69.5 70.8 66.7 

3 69.8 70.6 67.8 

3 70.4 71.0 69.6 

3 70.6 71.0 70.2 

4 69.6 70.4 69.1 

4 70.0 70.5 68.7 

4 70.5 70.6 70.3 

4 70.8 71.0 70.3 

5 69.6 70.8 66.8 

5 69.7 71.1 66.4 

5 70.6 71.0 70.2 

5 71.2 72.2 70.4 

6 66.1 67.2 64.1 

6 71.0 71.9 69.1 

6 71.4 72.1 70.8 

6 72.9 73.7 70.5 

8 71.6 72.0 71.3 

8 71.8 72.9 70.7 

8 72.6 72.6 72.2 

8 73.3 73.8 72.1 

9 67.6 70.6 66.5 

9 70.0 70.4 68.8 

9 70.0 70.8 67.5 

9 71.3 71.9 70.7 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Vehicle LAeq LAF, max LAF, min 

10 69.8 71.1 67.0 

10 72.3 73.4 71.5 

10 73.6 74.1 73.2 

10 73.9 74.3 72.7 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 34. Overall A-weighted Uncorrected Sound Pressure Levels: Acceleration 

Vehicle LAeq LAF, max LAF, min 

1 68.8 69.4 68.2 

1 69.6 70.5 68.2 

1 70.1 70.5 69.5 

1 72.3 73.2 70.1 

2 64.7 65.7 64.3 

2 65.5 66.0 64.8 

2 65.8 66.4 64.7 

2 66.0 66.4 65.3 

3 63.6 63.8 63.5 

3 63.6 64.0 63.5 

3 64.5 65.1 64.2 

3 65.7 65.9 65.4 

4 60.8 61.2 60.6 

4 61.2 61.5 60.5 

4 63.6 63.9 63.1 

4 63.7 64.0 63.4 

4 64.8 65.3 64.6 

4 65.2 65.5 65.0 

5 62.3 62.7 62.2 

5 62.6 62.8 62.1 

5 63.2 63.5 62.4 

6 68.9 69.1 68.8 

6 69.4 69.7 69.0 

6 71.2 71.9 69.9 

6 72.9 73.6 71.3 

8 66.8 67.1 66.2 

8 66.9 67.4 65.2 

8 68.4 69.1 67.2 

8 68.4 69.2 67.4 

9 65.5 66.2 65.2 

9 66.8 67.1 66.7 

9 67.3 67.7 66.8 

9 68.2 68.6 67.4 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Vehicle LAeq LAF, max LAF, min 

10 67.0 67.2 66.9 

10 68.6 69.1 67.7 

10 68.7 69.1 68.3 

10 69.1 70.0 67.3 

10 69.4 69.7 68.5 

10 71.1 71.5 70.2 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 35. Overall A-weighted Uncorrected Sound Pressure Levels: Startup 

Vehicle LAeq LAF, max LAF, min 

1 74.6 77.2 49.3 

1 75.4 78.4 74.7 

1 76.4 77.3 74.8 

1 77.0 77.7 75.6 

2 66.6 67.4 65.4 

2 67.4 68.8 65.8 

2 67.8 69.8 65.3 

2 68.8 70.2 65.5 

3 65.9 71.3 65.6 

3 68.9 72.3 48.7 

3 74.2 77.5 48.9 

3 74.3 77.1 70.1 

4 65.5 67.8 64.8 

4 66.1 68.9 65.7 

4 66.2 68.2 65.8 

4 66.6 70.2 50.3 

5 68.1 70.8 46.1 

5 70.4 71.4 70.0 

5 70.5 71.2 70.0 

5 70.7 71.6 69.8 

6 69.8 71.6 68.5 

6 70.8 71.6 68.6 

6 72.0 73.4 69.5 

6 72.0 75.8 70.2 

8 74.2 75.3 73.6 

8 74.2 75.6 73.1 

8 74.5 75.4 73.4 

8 74.8 76.5 74.0 

9 72.8 75.8 56.3 

9 75.8 78.0 72.9 

10 66.0 70.9 52.0 

10 66.4 70.6 65.5 

10 68.4 70.8 53.2 

10 70.8 72.9 67.2 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 36. Overall A-weighted Uncorrected Sound Pressure Levels: Idle 

Vehicle LAeq LAF, max LAF, min 

1 59.2 59.6 59.0 

1 59.4 59.9 59.2 

2 53.1 53.5 52.9 

2 53.9 54.4 53.7 

3 54.8 55.2 54.5 

3 54.8 55.2 54.6 

4 52.5 53.2 52.2 

4 53.7 54.4 53.4 

5 52.9 53.4 52.7 

5 53.1 53.5 52.9 

6 57.4 57.7 57.2 

6 60.1 60.4 59.9 

8 55.9 56.3 55.8 

8 56.4 56.8 56.3 

9 55.2 55.5 55.0 

9 55.5 55.8 55.3 

10 56.7 57.1 56.5 

10 57.4 57.8 57.3 

Table 37. Overall A-weighted Uncorrected Sound Pressure Levels: Ambient 

LAeq LAF, max LAF, min 

49.1 50.2 48.2 

47.8 48.9 47.0 

48.6 49.9 47.7 

47.5 48.5 46.8 

47.2 47.7 46.9 

46.8 47.4 46.5 

46.9 47.5 46.6 

46.5 47.1 46.1 

50.1 51.0 49.4 

50.1 51.2 49.3 

46.2 47.5 45.4 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

LAeq LAF, max LAF, min 

46.5 48.0 45.5 

48.4 48.8 48.0 

48.1 49.2 47.5 

53.4 54.0 53.0 

52.7 53.5 52.3 

53.2 53.9 52.9 

52.6 53.5 52.1 

53.0 53.4 52.8 

52.1 52.8 51.7 

50.7 51.2 50.5 

52.9 53.3 52.7 

52.0 52.6 51.5 

50.6 51.1 50.4 

51.7 52.2 51.5 

51.6 52.0 51.3 

51.5 51.9 51.2 

51.3 51.8 51.1 

50.9 51.3 50.6 

50.8 51.3 50.5 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 38. Ambient Levels at Cottage Farm Measurement Site during ICE Measurements, 

A-Weighted Levels, dB(A)
 

1/3 Octave Band 

Center Frequency, 

Hz 

Linear Average (1/3 

Octave Band) 

Minimum (Overall 

A-weighted) * 

Maximum (Overall 

A-weighted) 

Minimum (1/3 

Octave Band) ** 

Maximum 

(1/3 

Octave 

Band) 

Broadband 49.8 46.3 53.5 45.4 54.8 

Sum (100 to 20k) 49.6 46.1 53.4 45.3 54.7 

12.5 -8.3 -8.4 -9.6 -10.9 -3.6 

16 -2.6 -4.4 -2.8 -7.5 1.3 

20 2.0 0.3 2.4 -4.7 4.3 

25 8.4 5.8 9.1 2.0 10.2 

31.5 14.7 12.6 14.4 11.5 17.2 

40 19.0 17.5 19.1 16.1 21.4 

50 24.0 21.2 23.8 19.6 30.0 

63 29.2 25.5 28.7 25.5 34.4 

80 32.2 27.7 32.3 27.7 36.3 

100 34.6 30.7 34.1 30.7 38.4 

125 35.5 32.4 36.8 32.4 42.1 

160 36.1 32.1 37.9 32.0 41.5 

200 36.9 32.7 37.9 32.7 41.2 

250 36.5 33.9 38.1 33.1 40.7 

315 36.5 32.5 37.6 32.1 41.5 

400 36.0 31.9 38.1 31.8 39.7 

500 36.7 33.6 39.8 33.1 41.1 

630 38.2 34.4 41.7 34.0 42.2 

800 40.2 36.0 46.1 35.8 46.1 

1k 41.1 36.4 46.4 36.4 46.4 

1.25k 40.0 35.3 45.1 35.3 45.1 

1.6k 37.6 32.9 43.1 32.9 43.1 

2k 34.7 30.3 37.8 30.3 37.8 

2.5k 34.5 32.8 35.4 30.8 42.1 

3.15k 35.5 36.9 37.1 30.0 39.6 

4k 34.0 33.0 34.3 28.3 40.2 

5k 29.0 25.0 29.8 24.3 32.8 

6.3k 25.7 22.3 26.9 19.7 31.7 

8k 20.2 16.6 22.4 14.1 24.2 

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121 
 

 

Appendix A.2: Acoustic Measurements Results-Tabulated Data 

1/3 Octave Band 

Center Frequency, 

Hz 

Linear Average (1/3 

Octave Band) 

Minimum (Overall 

A-weighted) * 

Maximum (Overall 

A-weighted) 

Minimum (1/3 

Octave Band) ** 

Maximum 

(1/3 

Octave 

Band) 

10k 14.4 10.3 17.3 7.6 18.3 

12.5k 8.9 5.0 11.7 3.2 13.0 

16k 3.1 0.7 5.6 -0.8 8.7 

20k -1.9 -3.1 -0.4 -3.5 2.0 

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122 
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Table 39. Idle A-Weighted Levels, dB(A) for ICE Vehicles Measured (Uncorrected) at Cottag

A‐weighted Level, dB(A) for Vehicle Sample

Frequency, Hz 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1Broadband 53.1 53.9 53.1 52.9 54.8 54.8 53.7 52.5 59.2 59.4 56.7 57.4 57.4 60.1 55.2 55.5 55.9 56.4 55.

Sum (100 to

20k) 53.1 53.9 53.1 52.9 54.8 54.8 53.7 52.5 59.2 59.5 56.8 57.5 57.4 60.1 55.2 55.5 55.9 56.4 55.

12.5 ‐5.3

 ‐

5.7

 ‐

8.0

 ‐

8.6

 ‐

6.2

 ‐

6.6

 ‐

7.6 ‐7.6 ‐6.2 ‐6.6 ‐6.9 ‐6.8 ‐9.3 ‐8.9 ‐5.9 ‐5.5

 ‐

9.3

 ‐

9.3

 ‐

7

16 ‐1.4

 ‐

0.7

 ‐

3.7

 ‐

3.0

 ‐

0.6 0.0

 ‐

3.2 ‐2.8 ‐0.8 ‐0.8 ‐0.5 ‐0.7 ‐2.2 ‐2.1 0.4 0.1

 ‐

2.8

 ‐

3.8

 ‐

1

20 16.7 14.9 1.7 1.3 2.6 1.9 15.7 14.2 2.8 2.5 4.3 4.7 4.2 3.7 2.4 3.5 3.1 3.0 5.

25 23.5 21.7 7.6 7.9 7.5 8.0 17.8 16.4 8.7 8.2 11.0 10.0 25.9 23.6 23.5 25.1 9.7 9.3 14.

31.5 15.4 15.0 22.2 24.0 16.0 14.1 13.2 12.6 14.8 15.4 15.8 16.8 29.1 26.9 16.2 16.2 15.8 15.8 17.

40 18.6 18.6 31.1 33.0 22.7 19.8 19.3 18.6 18.9 20.1 21.8 22.2 19.7 19.8 20.2 20.6 20.1 19.6 21.50 25.1 25.0 22.5 21.7 22.8 22.4 22.9 22.6 24.0 23.8 25.6 25.2 28.4 27.0 31.1 28.9 27.7 27.5 25.63 27.8 29.5 28.2 28.0 28.5 27.9 26.2 26.0 26.8 27.3 29.5 29.3 30.1 29.9 29.0 28.9 29.0 28.5 28.80 31.9 31.1 31.5 31.9 31.9 30.6 30.7 30.8 36.4 35.1 34.4 34.0 33.8 33.1 34.8 34.2 35.4 35.5 33.

100 35.7 34.3 32.1 32.1 32.4 31.4 32.2 32.0 42.5 42.4 38.9 37.7 38.6 39.0 36.2 35.7 34.4 33.9 35

125 33.3 33.3 33.3 33.2 33.4 33.4 33.6 33.9 42.2 42.1 35.8 35.4 35.4 35.2 35.0 34.7 34.8 34.8 35.

160 33.9 33.9 35.0 34.0 34.8 35.1 34.8 34.9 40.8 41.5 39.5 38.1 38.5 37.9 38.3 37.0 38.2 36.6 36.

200 35.2 34.9 35.6 35.1 36.4 36.9 35.5 37.2 38.6 38.4 38.9 38.0 39.9 38.8 38.2 36.7 39.8 38.3 37

250 36.1 35.6 36.2 36.9 40.5 39.4 38.2 37.2 38.6 38.7 38.7 38.3 38.9 39.7 37.1 36.7 38.8 39.7 38

315 35.8 35.5 36.6 38.9 36.9 37.2 37.8 36.2 42.3 43.1 38.9 39.0 40.5 40.7 38.2 37.9 39.5 38.6 38

400 36.5 35.7 37.3 38.0 37.4 37.8 42.2 38.5 44.2 45.5 40.4 41.4 41.8 41.0 39.5 39.4 40.9 40.9 39

500 41.0 48.3 41.9 41.7 39.6 40.5 39.0 38.8 44.8 45.0 43.2 44.0 43.7 44.0 42.5 42.6 43.2 44.7 42.

630 40.1 39.5 44.1 44.2 41.8 38.5 40.7 40.5 46.4 47.0 43.8 44.3 44.5 44.5 42.7 43.5 44.3 44.0 43.

800 42.7 42.0 40.1 40.5 41.8 41.4 42.0 41.4 45.9 46.4 45.3 46.2 46.2 48.0 45.1 45.2 46.8 46.9 44.

1k 43.0 42.6 41.6 43.2 47.7 46.6 43.8 42.6 47.8 47.9 46.0 45.8 47.9 48.4 45.7 45.4 46.7 46.7 45

1.25k 43.2 43.8 42.1 42.5 47.9 49.3 46.5 44.0 50.6 50.0 48.1 48.1 48.3 50.4 44.5 45.2 48.7 48.2 46

1.6k 42.1 42.0 42.0 42.5 45.3 46.2 43.6 43.0 49.2 49.9 48.4 48.8 47.7 50.6 44.1 44.6 45.7 47.0 45

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123 
Appendix A.2: Acoustic Measurements Results-Tabulated Data 

A‐weighted Level, dB(A) for Vehicle Sample

Frequency, Hz 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1

2k 43.3 42.9 41.8 41.8 41.8 41.5 41.6 41.4 51.0 52.2 46.2 46.6 48.2 50.7 43.9 44.7 42.8 43.6 44.

2.5k 42.1 41.7 42.6 40.4 42.1 42.0 41.8 39.9 47.8 49.2 45.7 47.6 45.1 47.7 46.1 45.2 42.4 43.8 44.

3.15k 43.1 42.9 42.8 41.6 43.1 41.9 43.1 42.2 46.0 44.9 45.1 46.8 46.1 51.1 43.9 44.1 43.2 44.7 44.

4k 40.5 40.3 40.4 39.2 39.3 38.4 39.5 38.7 45.8 45.0 45.2 47.0 45.1 50.6 43.3 45.1 40.8 42.9 42.

5k 37.7 36.9 38.7 36.4 38.2 37.6 37.0 36.2 48.6 47.1 40.5 41.7 41.9 48.0 38.9 41.0 39.8 42.7 40

6.3k 35.5 35.0 36.5 34.4 37.6 36.1 33.4 31.8 41.5 41.6 38.9 39.9 39.1 45.7 37.8 39.5 36.6 37.9 37.

8k 32.0 31.2 33.9 31.0 32.8 31.8 32.1 30.0 36.5 36.6 35.4 36.3 37.7 44.6 33.8 36.2 34.8 36.1 34.

10k 28.0 26.9 30.2 27.9 30.0 29.0 29.5 27.9 33.6 33.7 33.4 32.2 31.4 38.8 29.5 31.4 32.0 32.6 31.

12.5k 25.2 25.5 27.2 24.1 23.9 22.5 24.1 22.3 28.7 29.5 28.8 27.8 26.7 33.6 25.9 27.3 27.2 28.4 26

16k 21.8 21.9 22.1 19.4 18.8 18.0 21.1 19.6 25.1 26.2 23.8 23.5 20.8 28.1 19.7 21.1 19.3 20.8 21

20k 13.6 15.9 12.9 12.4 10.4 10.4 10.5 10.7 17.8 19.9 16.2 15.6 13.6 19.8 12.9 12.8 12.9 14.1 14

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124 
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Table 40. 20 mph Pass-by Levels of ICE Vehicles Measured (Uncorrected) at Cottage Fa

A‐weighted Level, dB(A) for Vehicle Sample

Frequency, Hz 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1

Broadband 70.6 67.7 69.4 71.0 63.6 67.1 70.6 71.2 69.6 69.7 72.3 69.8 73.6 73.9 70.4 70.6 69.5 69.8 69

Sum (100 to

20k) 70.7 67.9 69.5 71.1 63.4 67.0 70.6 71.4 69.4 69.4 72.5 69.6 73.8 73.8 70.5 70.5 69.3 69.7 69

12.5 ‐13.0

 ‐

12.0

 ‐

3.4

 ‐

11.6

 ‐

7.6

 ‐

9.2

 ‐

4.8

 ‐

8.3

 ‐

10.6

 ‐

9.2

 ‐

11.6

 ‐

6.4

 ‐

12.3

 ‐

6.7

 ‐

12.3

 ‐

6.4

 ‐

10.6

 ‐

6.9

 ‐

17

16 ‐9.9

 ‐

1.5

 ‐

4.9

 ‐

7.3

 ‐

5.0 3.2

 ‐

6.6

 ‐

4.0

 ‐

1.9

 ‐

2.4

 ‐

4.1

 ‐

1.8

 ‐

1.1

 ‐

5.8

 ‐

3.5

 ‐

0.2

 ‐

1.4

 ‐

0.3

 ‐

5

20 0.1 10.1 ‐4.0 1.0 5.7 0.6 0.5 3.1 3.5 1.8 2.9 3.0 3.8 2.4 3.3 9.1 5.3 4.5 0

25 11.7 7.8 7.9 8.2 5.7 7.4 5.9 11.4 9.1 11.9 4.4 11.2 6.8 6.5 11.2 9.8 10.6 3.1 11

31.5 18.0 14.2 14.7 16.6 12.3 12.3 14.8 17.8 13.6 16.8 19.6 16.6 14.1 19.6 10.7 13.7 15.1 11.2 23

40 27.4 32.8 27.8 31.6 27.2 24.3 17.5 24.9 18.0 19.0 23.8 17.6 21.0 23.2 19.6 23.2 17.6 20.3 21

50 25.7 34.5 30.6 30.0 21.3 25.7 31.4 26.8 26.2 27.1 24.0 22.3 25.9 25.2 21.7 24.6 23.2 24.8 21

63 34.5 31.6 33.0 33.9 34.1 35.7 28.2 33.2 31.6 33.7 35.2 30.3 35.5 31.2 34.1 29.7 32.5 28.5 27

80 36.8 39.2 37.1 37.9 36.7 33.5 34.9 39.3 34.9 36.4 37.5 37.1 34.7 38.3 36.5 35.4 34.3 36.2 31

100 38.8 39.8 38.1 41.0 34.6 35.4 36.8 41.6 35.3 39.7 40.7 37.7 41.1 43.0 39.9 39.8 38.1 37.9 40

125 44.4 38.8 41.3 44.3 41.6 38.5 39.5 39.0 40.2 37.8 40.8 40.1 43.2 44.8 44.0 45.8 42.0 46.3 44

160 45.1 44.6 44.5 47.0 45.8 42.0 44.9 45.6 39.5 40.4 48.0 43.8 47.3 47.0 46.8 48.1 50.9 49.1 45

200 49.1 47.6 47.8 49.6 43.2 44.9 49.2 48.5 44.9 43.6 50.3 45.5 51.6 50.3 48.9 48.4 51.4 48.8 51

250 51.7 49.6 51.1 51.6 44.7 47.7 53.7 55.2 51.7 48.8 54.1 50.8 56.1 55.5 51.1 51.1 49.9 49.1 54

315 53.2 51.4 52.1 54.3 45.6 46.5 52.1 54.1 51.6 51.4 55.7 53.0 57.7 54.7 52.4 50.0 50.6 51.7 52

400 54.3 52.6 53.9 55.5 46.6 50.6 54.3 54.0 52.4 50.5 56.5 51.7 57.4 58.1 56.5 53.9 54.9 54.5 56

500 57.0 56.1 56.6 57.9 49.4 53.1 59.6 60.3 56.8 55.7 60.8 55.6 59.6 60.7 58.9 58.7 55.3 56.8 57

630 59.5 55.6 57.9 60.6 53.1 56.7 58.9 59.8 56.8 56.8 61.8 58.6 63.3 63.1 60.7 61.2 59.1 59.4 58

800 64.5 60.5 63.6 64.5 56.1 59.6 62.1 61.4 60.0 59.7 64.5 63.0 66.8 67.1 64.9 64.8 62.0 63.5 60

1k 63.5 60.4 62.1 64.3 55.7 59.8 63.8 64.0 62.3 62.6 66.4 62.9 67.4 67.5 61.9 62.6 61.9 62.2 63

1.25k 62.2 59.4 60.7 62.0 55.7 58.9 62.4 64.4 62.0 61.9 64.6 60.7 66.0 65.0 62.0 62.0 61.3 61.7 61

1.6k 61.3 58.2 59.5 61.6 54.3 58.2 62.4 63.2 62.2 62.6 63.2 61.0 64.2 64.5 61.0 60.7 60.3 60.1 60

2k 59.0 56.4 57.1 58.5 51.6 55.3 59.3 60.8 58.8 58.8 59.0 57.3 59.7 60.2 56.5 56.8 56.2 56.2 57

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125 
Appendix A.2: Acoustic Measurements Results-Tabulated Data 

A‐weighted Level, dB(A) for Vehicle Sample

Frequency, Hz 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1

2.5k 56.2 54.8 54.8 55.9 48.8 52.1 55.2 56.9 55.8 55.4 54.8 52.7 57.0 58.4 54.0 54.3 54.3 53.6 52

3.15k 53.2 51.9 52.4 53.5 46.0 49.8 51.9 53.1 51.4 52.7 53.8 52.8 55.4 57.2 50.9 50.2 50.8 50.5 49

4k 50.2 49.6 49.1 50.6 43.6 47.9 48.8 49.7 47.6 48.6 50.5 49.8 52.5 54.6 47.7 47.2 48.4 47.9 46

5k 46.6 46.0 45.6 47.2 40.5 44.3 45.3 46.2 44.2 46.2 47.8 45.9 48.8 50.6 45.4 44.8 45.7 44.9 43

6.3k 43.9 43.5 42.6 45.3 37.4 40.9 42.7 44.0 41.5 44.8 47.3 44.6 46.7 49.1 42.8 41.8 47.1 41.5 39

8k 40.9 40.7 40.8 41.9 34.3 38.3 40.2 41.0 38.0 40.9 43.6 41.6 43.1 45.2 39.7 38.4 39.8 38.2 37

10k 37.4 37.1 36.2 38.7 30.5 34.8 38.5 37.8 34.2 38.0 40.7 36.9 38.3 40.1 35.7 35.0 34.9 35.3 34

12.5k 33.0 32.6 31.9 34.4 25.9 31.1 34.5 32.7 29.4 33.2 37.0 33.2 34.3 36.7 30.4 29.8 29.0 29.6 31

16k 28.3 26.7 27.0 29.5 18.7 24.4 30.5 27.9 23.8 27.5 32.1 27.7 30.1 32.5 25.6 25.0 23.0 24.4 27

20k 21.5 19.7 20.1 21.5 9.2 16.4 24.6 21.4 15.5 19.7 24.4 19.8 22.3 24.5 18.7 17.8 14.5 16.8 20

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126 
Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 41. 20 mph Pass-by Levels of ICE Vehicles Measured (Uncorrected) at Cottage Fa

A‐weighted Level, dB(A) for Vehicle Sample

Frequency, Hz 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 3Broadband 70.0 70.5 70.8 71.4 71.0 72.9 66.1 67.6 70.0 71.3 70.0 71.6 71.8 72.6 73.3 70.1 68.6 70.3 69.

Sum (100 to

20k) 69.9 70.4 70.8 71.5 71.0 72.8 66.0 67.9 69.9 71.4 70.0 71.7 71.9 72.6 73.3 70.2 68.4 70.1 69.

12.5 ‐14.0

 ‐

13.3

 ‐

12.6

 ‐

6.6

 ‐

11.3

 ‐

4.1

 ‐

7.6

 ‐

8.0

 ‐

7.3

 ‐

5.1

 ‐

7.0

 ‐

8.3

 ‐

16.5

 ‐

9.2

 ‐

14.5

 ‐

11.1

 ‐

10.5

 ‐

15.2

 ‐

11.

16 ‐2.5

 ‐

6.8

 ‐

6.1

 ‐

1.6

 ‐

1.5 0.7 2.1

 ‐

2.8 1.6

 ‐

8.0 0.9 0.7

 ‐

2.1

 ‐

0.1

 ‐

5.2

 ‐

2.2

 ‐

1.0

 ‐

9.2

 ‐

9.

20 ‐1.0 ‐2.4 ‐0.1 5.3 2.8 4.0 6.0 3.1 4.0 2.3 ‐1.8 2.4 7.9 3.1 4.9 2.0 ‐1.0 ‐3.6 0.

25 8.5 17.5 8.9 11.7 7.8 10.3 11.8 7.8 7.1 5.5 9.0 16.2 7.2 12.7 7.6 7.5 5.4 10.5 7.

31.5 21.3 26.1 26.8 17.0 16.2 15.3 15.9 18.0 14.6 11.8 13.7 16.5 17.8 14.6 16.6 18.3 16.4 15.1 18.

40 18.2 23.3 19.4 21.4 20.2 22.3 19.1 26.2 24.5 21.0 23.2 18.1 23.2 23.9 21.6 19.5 23.3 24.4 23.

50 23.7 19.9 21.8 41.0 38.4 36.4 37.4 46.5 35.3 35.1 36.9 31.1 26.8 29.3 32.1 28.2 25.7 24.6 26.

63 29.5 31.6 33.3 29.4 31.6 26.9 32.9 31.3 29.1 28.3 29.2 29.8 37.6 30.7 34.9 31.2 29.8 28.0 28.

80 32.7 34.9 33.1 37.2 36.3 34.6 42.2 38.4 34.2 38.3 34.9 36.3 42.3 37.9 35.0 37.5 35.3 36.8 35.

100 38.1 39.2 37.0 47.0 43.7 40.5 41.7 47.4 39.2 44.4 39.3 39.6 39.8 37.0 36.4 46.1 38.2 41.9 39.

125 41.1 44.0 43.7 46.1 42.5 41.4 39.3 44.3 39.8 44.9 41.7 40.1 39.8 40.1 38.8 54.7 38.7 44.1 39.

160 42.0 44.3 43.4 48.5 45.5 44.9 43.1 49.6 46.8 47.4 46.1 43.9 44.4 45.4 45.2 56.1 49.0 51.9 51.

200 50.3 52.1 52.7 51.1 49.0 48.5 44.8 50.0 50.4 49.5 50.3 46.7 47.8 47.5 47.4 51.8 48.3 50.3 49.

250 55.0 51.8 54.0 59.3 55.4 55.0 49.7 53.6 54.0 56.3 55.2 55.1 55.5 54.1 53.2 48.3 46.4 50.4 45.

315 50.5 53.5 53.1 53.8 51.0 53.2 48.6 54.5 53.8 54.8 52.7 54.5 55.9 53.3 55.0 53.0 50.3 55.6 50.

400 54.0 55.9 55.4 53.8 53.4 53.7 49.3 53.0 54.1 56.6 53.3 55.2 54.6 56.0 56.9 56.0 51.0 56.6 52.

500 57.2 58.1 56.8 57.8 56.6 57.1 50.7 55.8 57.0 59.6 55.9 58.6 58.2 58.8 61.4 58.0 52.9 57.8 54.

630 58.0 59.0 59.2 58.7 57.4 59.6 51.9 55.7 58.2 61.0 57.7 62.3 60.7 62.5 61.9 61.3 58.4 61.3 60.

800 61.6 62.4 62.7 63.7 62.9 63.5 56.1 58.2 60.7 62.7 60.5 64.0 64.3 65.2 65.6 63.1 57.4 62.5 60.

1k 63.5 64.4 65.0 65.0 63.1 65.5 59.2 60.6 63.8 64.5 63.7 64.8 65.2 66.4 66.2 61.0 61.2 61.0 61.

1.25k 61.3 62.3 63.4 63.1 62.7 65.2 57.8 59.4 61.5 63.4 62.8 62.7 63.5 64.8 64.8 62.3 63.3 63.1 62.

1.6k 62.6 61.6 61.9 61.7 61.7 63.2 57.0 59.5 61.3 62.3 60.8 63.4 63.4 63.9 65.5 59.9 59.7 60.3 59.

2k 57.9 57.6 58.1 59.0 60.4 60.9 55.1 56.8 58.3 58.8 57.8 61.1 60.8 59.6 61.1 56.4 55.8 56.5 55.

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127 
Appendix A.2: Acoustic Measurements Results-Tabulated Data 

A‐weighted Level, dB(A) for Vehicle Sample

Frequency, Hz 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 3

2.5k 53.0 53.6 52.9 57.2 57.8 60.1 53.7 53.3 55.2 56.2 55.1 55.6 56.4 56.0 58.0 54.2 52.2 54.5 52.

3.15k 48.4 50.2 49.3 56.0 58.2 60.8 52.8 49.6 51.9 53.8 52.7 51.7 53.1 53.1 55.8 51.6 47.3 50.8 48.

4k 45.6 48.4 47.0 53.5 56.2 59.4 51.2 46.3 50.9 52.9 52.4 48.4 51.2 50.1 53.6 48.5 43.9 47.7 44.

5k 42.0 43.9 43.2 50.3 53.7 56.7 48.8 42.9 47.7 48.4 48.7 45.0 47.6 48.0 51.9 45.3 41.5 45.4 42.

6.3k 38.3 40.9 39.3 49.0 52.4 56.5 46.7 41.6 45.3 47.4 46.7 41.2 44.1 43.2 47.5 42.8 39.8 42.2 40.

8k 35.1 38.3 36.6 47.4 52.3 56.0 45.0 37.1 41.4 43.8 43.5 38.3 40.9 41.0 44.4 39.4 35.8 38.1 36.

10k 32.6 36.3 34.1 40.5 44.7 47.6 36.6 33.8 37.5 39.2 38.8 36.2 37.6 37.9 40.8 35.3 32.0 33.8 31.

12.5k 28.5 32.1 29.8 34.1 38.0 40.1 29.0 29.2 33.0 35.0 34.2 31.7 33.8 33.7 36.2 31.7 28.1 30.4 27.

16k 24.2 28.3 25.4 27.7 30.9 33.8 21.2 23.1 27.0 28.4 27.8 24.1 28.2 26.6 29.2 27.2 22.7 25.8 23.

20k 16.5 19.7 17.6 20.4 22.9 25.0 11.2 22.1 18.3 19.2 18.5 17.4 21.4 19.7 23.5 20.0 16.3 19.1 15.

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 42. Cottage Farm ICE Vehicles at Microphone Line - Event with Minimum LAeq1/2sec 

1/3 Octave 

Band Center 

Frequency, 

Hz 

6 mph, 

Reverse 

6 mph, 

Forward 

10 mph, 

Forward 

15 mph, 

Forward 

20 mph, 

Forward 

Acceleration Startup Idle 

100 to 20k 54.4 57.2 60.0 62.8 63.4 60.7 65.3 51.6 

100 32.0 32.7 31.3 38.4 32.3 NA 36.2 NA 

125 34.4 35.0 33.9 37.6 41.0 39.7 35.6 NA 

160 34.1 36.9 37.9 39.7 45.6 41.1 40.2 NA 

200 36.2 40.9 42.2 44.6 42.8 40.2 43.4 35.3 

250 38.2 43.6 44.1 49.6 44.4 41.1 47.4 35.1 

315 40.1 41.3 42.7 48.8 45.4 43.1 48.7 34.2 

400 40.9 42.6 45.1 46.1 46.5 47.0 50.8 37.5 

500 40.9 46.4 48.1 50.2 49.3 45.1 49.0 37.5 

630 44.7 46.0 47.4 50.7 53.0 49.7 49.9 39.3 

800 42.3 46.4 50.7 52.1 56.1 50.6 52.5 40.0 

1k 44.5 49.3 51.8 56.7 55.6 50.4 51.2 41.4 

1.25k 46.2 48.1 50.0 55.3 55.7 52.1 53.7 43.4 

1.6k 45.8 48.0 49.6 54.0 54.3 53.4 55.6 42.5 

2k 43.0 45.2 49.6 50.5 51.6 50.0 54.6 41.0 

2.5k 42.1 43.5 49.8 46.4 48.7 48.8 54.2 39.3 

3.15k 40.6 43.5 48.1 45.4 45.9 48.6 55.8 41.9 

4k 39.4 41.9 44.9 42.5 43.5 46.1 56.4 38.2 

5k 37.2 39.2 41.9 39.5 40.4 42.1 55.6 35.9 

6.3k 35.2 36.9 39.9 38.1 37.3 40.2 52.6 31.6 

8k 32.3 34.1 37.4 34.7 34.3 38.0 48.1 29.9 

10k 30.2 31.8 33.7 30.5 30.5 36.4 43.3 27.9 

12.5k 25.6 26.9 29.0 26.6 25.9 31.3 40.0 22.3 

16k 21.4 22.3 24.5 20.9 18.7 27.5 35.7 19.5 

20k 15.1 15.0 16.7 12.1 9.0 19.1 26.8 10.5 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 43. Cottage Farm ICE Vehicles at Microphone Line – Average* LAeq1/2sec 

1/3 Octave 

Band Center 

Frequency, 

Hz 

6 mph, 

Reverse 

6 mph, 

Forward 

10 mph, 

Forward 

15 mph, 

Forward 

20 mph, 

Forward 

Acceleration Startup Idle 

100 to 20k 58.4 61.1 63.6 68.1 70.2 66.7 70.7 55.2 

100 35.8 35.5 36.3 38.9 39.2 39.7 35.7 36.1 

125 36.6 38.4 40.1 41.3 41.5 44.3 36.0 36.3 

160 38.3 40.2 42.6 44.9 46.0 45.1 37.3 37.0 

200 40.4 41.3 44.3 48.1 48.6 46.7 40.3 36.6 

250 41.4 42.8 45.9 50.4 52.2 47.7 40.9 36.5 

315 42.1 43.9 46.9 50.2 52.5 49.8 44.2 37.3 

400 44.2 46.5 48.7 53.0 54.1 51.4 46.6 39.0 

500 45.4 47.9 51.2 55.6 57.1 53.4 51.8 42.1 

630 47.0 49.0 52.5 56.9 59.1 54.6 52.4 42.3 

800 48.3 51.1 54.6 59.5 62.3 55.1 55.2 43.2 

1k 48.7 51.4 55.2 60.2 63.2 55.6 57.8 44.9 

1.25k 49.3 52.2 54.6 59.6 62.2 57.2 60.5 46.3 

1.6k 48.7 52.0 54.3 58.8 61.3 57.0 61.1 45.4 

2k 46.9 50.3 52.0 56.1 57.9 55.7 60.5 44.6 

2.5k 46.2 49.1 50.3 53.9 54.9 55.1 61.1 43.8 

3.15k 45.6 48.6 49.2 52.4 52.1 54.9 61.6 44.1 

4k 44.2 46.9 47.5 50.5 49.5 53.2 60.9 42.4 

5k 41.9 44.1 45.0 47.8 46.4 50.8 59.2 40.3 

6.3k 39.6 41.8 42.9 45.7 44.0 48.7 56.9 37.6 

8k 36.5 39.0 40.0 42.8 40.9 45.9 53.8 34.6 

10k 32.6 35.6 36.5 38.9 36.9 42.4 49.4 31.0 

12.5k 28.3 31.1 31.9 34.2 32.3 38.0 44.6 26.6 

16k 23.1 25.9 26.7 29.1 26.8 32.5 38.8 21.7 

20k 15.6 18.2 19.1 21.1 19.2 24.9 30.9 13.9 

* Note: Averages in some cases are biased high because some quiet measurements were close or below the ambient level. Rather than 

overestimate their level, they were excluded from the computation. 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 44. Cottage Farm ICE Vehicles at Microphone Line - Event with Maximum LAeq1/2sec 

1/3 Octave Band 

Center Frequency, 

Hz 

6 mph, 

Reverse 

6 mph, 

Forward 

10 mph, 

Forward 

15 mph, 

Forward 

20 mph, 

Forward 

Acceleration Startup Idle 

Sum (100 to 20k) 64.8 63.8 66.3 71.1 73.8 72.9 77.0 59.9 

100 41.9 40.2 33.3 37.4 42.7 37.5 35.6 38.3 

125 39.9 35.9 40.7 42.2 44.5 41.0 36.3 -16.1 

160 41.7 41.0 46.9 45.2 46.9 47.5 36.1 36.7 

200 42.1 39.6 43.3 48.6 50.2 48.5 42.8 37.5 

250 43.3 41.2 45.9 53.8 55.5 48.0 46.6 38.6 

315 43.0 43.3 47.0 52.4 54.7 49.1 55.9 40.1 

400 45.2 44.8 47.6 55.9 58.1 51.4 54.3 40.5 

500 46.1 48.1 51.2 58.0 60.7 51.8 54.7 43.6 

630 49.9 48.8 52.6 59.8 63.1 52.9 59.9 44.1 

800 55.3 52.2 56.3 63.7 67.1 59.5 64.4 47.7 

1k 53.8 53.7 56.6 64.4 67.5 61.3 63.5 48.1 

1.25k 55.6 53.6 56.5 62.1 65.0 61.4 67.6 50.2 

1.6k 54.9 54.2 56.2 62.4 64.5 61.7 71.2 50.5 

2k 53.6 54.0 56.1 58.0 60.2 63.5 67.5 50.7 

2.5k 54.6 52.7 54.8 56.7 58.4 62.8 68.7 47.6 

3.15k 55.7 54.9 56.8 55.6 57.2 64.3 66.4 51.0 

4k 53.8 53.7 55.1 53.3 54.6 63.4 65.3 50.5 

5k 51.8 51.2 53.0 49.8 50.6 61.9 62.5 48.0 

6.3k 50.9 49.5 52.3 48.1 49.1 62.0 58.6 45.7 

8k 49.3 46.4 50.4 44.7 45.2 61.1 53.8 44.6 

10k 42.0 41.4 43.6 39.4 40.1 53.3 50.0 38.8 

12.5k 35.7 35.9 37.0 36.0 36.7 46.1 44.2 33.6 

16k 29.8 30.7 31.5 32.5 32.5 39.9 39.6 28.1 

20k 22.4 22.5 23.6 24.5 24.5 31.2 32.3 19.8 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 45. Sample Ambient Level During Countermeasure Testing on August 14th, 2010, AWeighted One-Third Octave Band Levels, dB(A) 

1/3 Octave Band Center 

Frequency, Hz Minimum Mean Maximum 

12.5 -12.9 -6.2 -1.3 

16 -4.2 0.6 6.2 

20 -0.4 6.3 9.8 

25 6.4 14.6 18.5 

31.5 13.6 22.7 26.9 

40 22.9 27.0 31.7 

50 25.6 30.5 33.7 

63 30.1 34.1 36.8 

80 33.4 37.0 42.5 

100 39.0 44.4 47.7 

125 42.4 46.0 47.9 

160 41.4 44.1 46.3 

200 43.2 45.5 47.8 

250 46.0 49.0 51.6 

315 43.9 45.5 46.8 

400 45.9 47.0 48.1 

500 46.2 47.8 49.3 

630 47.7 49.5 50.8 

800 47.3 48.7 49.8 

1k 46.4 47.8 49.4 

1.25k 44.4 45.7 46.9 

1.6k 43.1 44.2 46.2 

2k 41.9 43.6 45.7 

2.5k 36.1 37.1 39.1 

3.15k 32.5 33.5 36.5 

4k 28.4 29.4 32.4 

5k 23.0 24.2 28.3 

6.3k 19.7 20.3 23.6 

8k 18.1 18.5 20.4 

10k 16.6 16.9 18.8 

12.5k 15.1 15.4 17.6 

16k 14.0 14.4 16.6 

20k 12.8 13.0 14.3 

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Appendix A.2: Acoustic Measurements Results-Tabulated Data 

Table 46 Comparison of Ambient Level for Left and Right Sides of Binaural Head during 

Countermeasure Testing 

1/3 Octave Band Center 

Frequency, Hz SPL in Left Channel (dBA) SPL in Right Channel (dBA) 

12.5 -3.4 -3.7 

16 4.3 3.4 

20 10.2 10.1 

25 17.6 17.2 

31.5 21.1 21.4 

40 27.8 27.2 

50 32.3 32.4 

63 36.8 36.6 

80 37.7 38.0 

100 45.3 45.5 

125 47.7 47.4 

160 46.4 46.2 

200 47.4 46.6 

250 49.3 48.2 

315 48.2 48.0 

400 50.2 49.7 

500 49.8 49.1 

630 52.4 51.2 

800 54.4 51.9 

1k 53.7 51.2 

1.25k 50.7 48.6 

1.6k 48.3 46.1 

2k 44.9 43.1 

2.5k 38.0 35.9 

3.15k 33.9 31.3 

4k 32.1 28.9 

5k 31.1 25.5 

6.3k 26.1 19.5 

8k 22.7 16.4 

10k 23.7 16.2 

12.5k 19.2 16.0 

16k 28.6 29.3 

20k 28.0 28.9 

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Appendix A.3: Sample Experimental Script 

A.3 Sample Experimental Script 

Table 47. Sample Experimental Script: Block A for August 7th Session 

Trial # System Maneuver Sound ID Level File Name Direction Comments 

1 A Pass-by 5 High P5High B to A 

2 B Pass-by G High PGHigh A to B 

3 E Pass-by 1 Low P1Low A to B 

4 A Pass-by 1 High P1High A to B 

5 No-signal No-signal No-signal No-signal No-signal No-signal 40 sec 

6 R Pass-by n/a n/a n/a B to A 

7 A Pass-by 1 Low P1Low A to B 

8 E Pass-by 4 Low P4Low B to A 

9 D Pass-by Off n/a n/a A to B 

10 No-signal No-signal No-signal No-signal No-signal No-signal 35 sec 

11 A Pass-by 5 Low P5Low A to B 

12 E Pass-by 3 Low P3Low A to B 

13 C Pass-by n/a Low n/a B to A 

14 A Pass-by 2 High P2High B to A 

15 No-signal No-signal No-signal No-signal No-signal No-signal 30 sec 

16 E Pass-by 3 High P3High B to A 

17 C Pass-by n/a High n/a A to B 

18 B Pass-by G Low PGLow B to A 

19 D Pass-by Activated Max n/a A to B 

20 E Pass-by 4 High P4High A to B 

21 No-signal No-signal No-signal No-signal No-signal No-signal 30 sec 

22 A Pass-by 2 Low P2Low B to A 

23 R Pass-by n/a n/a n/a A to B 

24 E Pass-by 1 High P1High B to A 

Quieter Cars and the Safety of Blind Pedestrians Phase 2 – Final Report 

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DOT HS 811 496

October 2011