NBSIR 76-1138

A Proposed Experiment to

Evaluate Environmental

Dependence of the Measurement of Noise from Medium

and Heavy Duty Trucks

Curtis I Holmer and Roger D. Kilmer

Applied Acoustics Section

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

September 1976

FINAL REPORT

Prepared for The Motor Vehicle Manufacturers Association

of the United States of America

320 New Center Building

Detroit, Michigan 48202

NBSIR 76-1138

A PROPOSED EXPERIMENT TO

EVALUATE ENVIRONMENTAL

DEPENDENCE OF THE MEASUREMENT OF NOISE FROM MEDIUM

AND HEAVY DUTY TRUCKS

Curtis I. Holmer and Roger D Kilmer

Applied Acoustics Section

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

September 1976

FINAL REPORT

Prepared for The Motor Vehicle Manufacturers Association

of the United States of America

320 New Center Building

Detroit, Michigan 48202

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

TABLE OF CONTENTS

Page

Abstract - 1 1. Introduction 1 2. Technical Background for Experiments 2

2.1. Proposed Test Procedures 3

a. SAE J366b Description 3

i. Test Site 3 ii. Vehicle Operation 3 iii. Instrumentation 6

iv. Measurement Procedure 6

b. DOT Interstate Motor Carrier Noise Emission Regulation 6

i. Test Site 6

ii. Vehicle Operation 7 iii. Instrumentation 8

iv. Measurement Procedure 8

2.2. Propagation of Sound over a Reflecting Plane 8

2.3. Potential Effects of Environment 10

a. Effects of Environment on Noise Emission 10

b. Effects of Environment on Noise Propagation 12

c. Variables to be Considered 14

2.4. Stationary Simulation versus Moving Tests 16

2.5. Near Field Sound Power Level 17

2.6. Statistical Aspects of the Experiment Design 22

a. Accuracy and Precision Considerations 22

b. Data Reduction and Data Analysis 22

c. Sensitivity of the Experiment 23

2.7. Objectives and Goals of a Proposed Experiment 24

3. Proposed Experiment 26

3.1. Overview of the Experiment 26

3.2. Hardware 29

a. Proposed Test Site 29

i. Moving Tests 29

ii. Stationary Tests 31
I

1

Page

iii. Environmental Conditions 32

b. Proposed Vehicles 34

c. Data Acquisition and Reduction Systems 35

3.3. Moving Tests 39

a. Test Parameters 39

b. Test Plan for Moving Tests 41

3.4. Stationary Vehicle Tests 42

a. Test Parameters 42

b. Test Plan for Stationary Tests 48

3.5. Variations from the Proposed Test Plan 50

a. Change in Scope from Evaluation of Preliminary Data 52

b. Changes in Experiment because of Vehicle Mounted

Near Field Microphones 52

4. Data Reduction and Reporting 52

4.1. Data Reduction 52

4.2. Presentation of Reduced Data 54

4.3. Reports 55

5. Data Analysis 56

6. Appendix A. Description of A Data Reduction Mathematical Model 59

7. References 62

ii
A PROPOSED EXPERIMENT TO EVALUATE ENVIRONMENTAL DEPENDENCE

OF THE MEASUREMENT OF NOISE FROM MEDIUM AND HEAVY DUTY TRUCKS

C. I. Holmer

R. D. KilmerIncreasing concern with the impact of transportation noise on communities

has led to an increase in the number of motor vehicle noise regulations at all

levels of government. As regulations become more widespread, the accuracy and

precision of vehicle noise measurements become more critical since each uncertainty in the measurement requires a corresponding increase in the margin that

manufacturers must allow between the regulated noise limit and vehicle design

levels. Although considerable uniformity has been achieved by existing

voluntary standards, there remain significant variations between noise

measurements made at different sites or at different times on the same site.

These variations are attributable to differences in the environment, including

site and meterological influences. Thus, there is a need for systematic

investigation of the various environmental and test site effects on noise

generation, radiation and/or propagation. This report presents the findings

of a feasibility study for an experiment to investigate environmental effects

on truck noise emission measurements. The result of this study is a proposed

experiment designed to examine the systematic and nonsystematic dependence of

observed truck driveby noise on the environmental effects present during the

measurement. The experiment is designed in such a manner as to permit

separation of emission and propagation effects. Although it may not be

feasible to determine "correction factors" for environmental effects within

the time and financial constraints of this experiment, it is fully expected

that the data obtained from this experiment will reveal the magnitude of

variations in the measurement and provide a broad enough information base for

study of the underlying physical mechanisms.

Key Words: Acoustics; environmental conditions; noise measurement; noise

(sound); transportation noise; truck.

1. INTRODUCTION

This report represents the findings of a feasibility study for an experiment to investigate environmental effects on truck noise emission

measurements. In the remainder of this introduction and in section 2, a non-technical review of the anticipated effects from the environment is

presented. The influence of these factors on experiment design are then

presented in sections 3 and 4, in the form of a detailed experiment plan,

which, if followed, is expected to provide an adequate data base for

subsequent analysis to determine physical mechanisms which produce the

observed changes. The general form of these analyses is discussed in section

5. The conclusion of this feasibility study is that the present

state-of-the-art of acoustic measurements permits the design of an experiment

which may be expected to reveal the magnitude of variations in the measurement

and to provide a broad enough data base for study of physical mechanisms,

within realistic time and financial constraints.
The purpose of this experiment is to provide a data base for the evalua- i

( tion of the effects of weather variables on truck noise measured in accordance

with the driveby test procedures of SAE Standard J366b and the stationary test

procedures of the Department of Transportation interstate motor carrier noise

emission regulations. The principal objectives of the experiment are to: 1)

Rank order, in terms of significance of variation induced in measured truck

sound level, the principal environmental variables associated with vehicle

noise measurement; and 2) Identify to what extent these variations are due to

changes in the propagation path, and conversely to what extent they represent

changes in the vehicle sound emission.

This experiment is designed to answer two significant questions: 1) Are

the data obtained in vehicle noise emission measurements systematically

dependent on ambient weather conditions at the time of the test?; and 2) Are

the data obtained in stationary vehicle simulations of a driveby test

statistically equivalent to moving driveby tests? The importance of these

questions lies in the consequences of positive answers.

In the event that the answer to question one is positive (i.e., there are systematic dependences) then the next logical step is to seek physical

mechanisms that explain these changes either through explanation of changes in

the vehicle emission or of characteristics of the sound propagation from the

vehicle to the measurement location. Once the region in which the predominant

changes that occur are identified, then physically or empirically based models

can be constructed for anticipating effects of environmental variations. These

models might then be used to design further studies of measured data on other

vehicles, in order to provide corrections for systematic changes; or to

produce more adequately documented environmental limitations for vehicle

measurements

.

The purpose of posing the second question is two-fold, and includes the

validation of a stationary test for use in assessing changes in vehicle

emission with environmental effects, plus the possibility that such a test

procedure may be attractive in its own right for assessing vehicle noise

emission.

Even in the event that the questions raised above are answered in the

negative, we expect that the developed data base will provide a means for

assessing the role of weather variables in determining the imprecision of

these measurements. This determination will permit those engaged in measurement of motor vehicle noise emissions to assess the probable uncertainty of

measurements, of vehicle noise on a more rational basis than at present, and

thus to choose environmental condition limits which are appropriate to the

test requirements.

2. TECHNICAL BACKGROUND FOR EXPERIMENTS

There are many variables which potentially could alter the sound level

measured during a truck driveby. In the following sections we provide brief

2
discussions of those aspects which influence the design of this experiment,

;| both in terms of factors to be studied, and those for which controls are to be

designed. These discussions should be viewed in terms of the net impact of

the particular item on the overall approach to the problem. These impacts

will then be summarized in terras of the specific goals of the experiment.

2.1. Proposed Test Procedures

The two procedures which will be utilized in this experiment are SAE

J366b for driveby and simulated driveby tests and the Department of

Transportation (DOT) interstate motor carrier noise emission regulation for

stationary run-up tests. The general provisions of these two procedures are discussed in this section.

a . SAE J366b Description

The test method predominantly used to measure the exterior noise produced

by medium and heavy trucks is the Society of Automotive Engineers (SAE)

Standard J366b — Exterior Sound Level for Heavy Trucks and Buses[l].— This

standard is designed to determine the maximum noise that can be produced by

the propulsion system without considering the influence of tire noise.

Because of the characteristics of the noise generating mechanisms of trucks,

i.e., propulsion system noise increases with engine speed while tire noise

increases with vehicle speed (see Figure 1), the tests are conducted at high

engine speed and fairly low vehicle speed. The noise level produced during

such a test probably represents the worst-case noise level for a truck

operating under urban driving conditions. Such a test would, in addition,

represent the worst-case propulsion system noise contribution to the overall

noise level produced by the vehicle at high speeds.

This standard specifies the test site topography, vehicle operation and

instrumentation required to determine the maximum (A-weighted) exterior sound

level generated by trucks and buses. These general specifications are discussed in subsections i through iv.

i . Test Site

The test site layout shown in Figure 2 consists of the vehicle path

including the acceleration point, microphone point, end zone and end point;

the triangular shaped measurement area formed by the acceleration point, end

point and microphone; and the surrounding terrain within 100 feet (30.5 m) of

either the microphone or vehicle path. The specification for the test site is

that it be a level open space free of large reflecting surfaces within 100

feet (30.5 meters) of the microphone or vehicle path. The vehicle path is to

be relatively smooth, dry concrete or asphalt free of any extraneous

materials. Similarly, the surface of the ground within the measurement area

is to be free from powdery snow, long grass, loose soil and ashes.

ii . Vehicle Operation

The test procedure calls for vehicle operation using both a full throttle

acceleration and a closed throttle deceleration. For the acceleration test,

1 /

' — Figures in brackets refer to references at the end of this report,

3
VEHICLE SPEED, km/hr

10 20 30 40 50 60 70 80 90

VEHICLE SPEED, mph

TIRE NOISE

STEERING AXLE DRIVE AXLE TRAILER AXLE

o

NEW RIBS Vi WORN

X BARS

NEW POCKET RETREAD

NEW RIBS NEW

X BARS

NEW RIB RETREAD

A NEW RIBS NEW RIBS NEW RIB RETREAD

Figure 1. Engine related and tire noise for an 18-wheel tractor-trailer as measured 50 feet (15.2 m) from the centerline of the lane of

travel of the vehicle [2].

4
NOTE DIMENSIONS ARE FT (m)

Figure 2. Test site specifications given by SAE J366b [1].

the highest rear axle and/or transmission gear and initial vehicle speed are chosen such that at wide-open throttle the vehicle will accelerate from the

acceleration point starting at no more than two-thirds of maximum rated or governed engine speed and will reach the maximum rated or governed engine

speed within the end zone without exceeding 35 mph (56.3 km/hr) before

reaching the end point. For the deceleration test, the vehicle is operated at

maximum rated or governed engine speed in the gear selected for the

acceleration test until reaching the microphone point where the throttle is

closed and the vehicle allowed to decelerate to one-half of maximum rated or governed engine speed. If the vehicle is equipped with an exhaust brake, the

deceleration test is to be repeated with the brake full on immediately

following closing of the throttle. [In the majority of cases the maximum

noise is generated during the acceleration mode of operation provided that the

vehicle is not equipped with an engine brake[2]. This will be checked during

the first week of the first field trip and, assuming this holds true for the

three vehicles being tested, only driveby acceleration tests will be performed

during subsequent field trips as discussed in section 3.]
iii. Instrumentation

The instrumentation to be used consists of a sound level meter which

meets the Type 1 requirements of ANSI SI -4— 1971[ 3] . As an alternative to

directly reading the sound level meter, a microphone or sound level meter may

be used with a magnetic tape recorder and/or graphic level recorder or indicating meter, providing the system meets the requirements of SAE J184[4].

iv . Measurement Procedure

The measurement procedure consists of determining the highest sound level

with the meter set for "fast" response using the A-weighting network for the

acceleration and deceleration of the vehicle as described above.

These measurements are to be made only when the ambient or background

noise is at least 10 dB lower than the level of the tested vehicle and the

wind velocity is below 12 mph (19.3 km/hr). The sound level for each side of

the vehicle is to be the average of the two highest readings which are within

2 dB of each other and the value to be reported is the sound level for the

side of the vehicle with the highest readings.

b . DOT Interstate Motor Carrier Noise Emission Regulation

To establish procedures for determining whether commercial motor vehicles

conform to the Interstate Motor Carrier Noise Emission Standards of the

Environmental Protection Agency[5], the DOT Bureau of Motor Carrier Safety

issued CFR325 — Compliance with Interstate Motor Carrier Noise Emission

Standards [6 ] . This regulation specifies the test site topography,

instrumentation and vehicle operation for determining the maximum A-weighted

sound level generated by motor vehicles with a gross combination weight rating

of 10,000 pounds (4536.0 kg) or more during driveby and stationary run-up

operations

.

For the purposes of this proposed experiment all driveby tests will be

performed according to SAE J366b procedures and only the stationary run-up

tests of CFR325 will be investigated to study the influence of environmental

factors on the measured sound levels. The remainder of this section provides

a discussion of the general specifications of this regulation for stationary

tests. Although variances in the specifications are given, in our proposed

experiment only the standard recommended test site and instrumentation

specifications will be utilized.

i. Test Site

The test site layout shown in Figure 3 consists of the microphone target

point, microphone location point, measurement area and the surrounding terrain

within 50 feet (15.2 m) of either the microphone target point or location

point. The microphone target point is established on the ground surface of

the site on the centerline of the lane in which the vehicle is parked at a point within 3 feet (0.9 m) of the vehicle’s exhaust outlet(s). The

microphone location point is established not less than 35 feet (10.7 m) and

not more than 83 feet (25.3 m) [50 feet (15.2 m) is the recommended distance]

from the microphone target point. The measurement area is the triangular area

6
COMBINATION

VEHICLE

MEASUREMENT^

AREA

MICROPHONE

LOCATION POINT

MICROPHONE

TARGET POINT

LONGITUDINAL

CENTERLINE

OF THE VEHICLE

STANDARD TEST SITE; STATIONARY TEST

Figure 3. Test site specifications given by DOT interstate motor carrier

noise emission regulation [6].

formed by the microphone location point and two points 50 feet (15.2 m) along

the centerline of the vehicle on either side of the microphone target point

(see Figure 3)

.

The test site specifications are similar to those of SAE J366b, but are

in general less stringent. It is recommended that the measurement area be a "hard" site (i.e., a ground surface covered with concrete, asphalt, packed

dirt, gravel or similar reflective material), but a "soft" site (i.e., a ground surface covered with grass or similar absorptive material) can also be

used if appropriate correction factors are applied to the measured sound

levels. The test site should be a level open space free of large reflecting

surfaces with 50 feet (15.2 m) of the microphone target point or location

point. However, certain objects specified by the regulation may be within the

test site boundaries and still permit testing.

ii. Vehicle Operation

For stationary measurements, the vehicle is parked on the test site and

7
all auxiliary equipment designed to operate at vehicle speeds of 5 mph (8.0

km/hr) or less are turned off. If the vehicle is equipped with a device which

automatically disengages the fan, the vehicle should be parked with the engine

running for a sufficient length of time (but not more than 10 minutes) to

permit the fan to disengage when the vehicle's noise emissions are measured.

With the transmission placed in neutral and the clutch engaged, the engine is

rapidly accelerated from idle to the maximum governed speed with wide open

throttle. After the maximum governed speed is reached the engine is returned

to idle.

iii . Instrumentation

The instrumentation to be used consists of a sound level meter which

meets the requirements of ANSI S 1 . 4— 19 71 [ 3 ] for a Type 1 or Type 2 sound level

meter. A Type S sound level meter is also permitted provided it has

A-weighting frequency response, fast dynamic characteristics of its indicating

instrument and a relative response level tolerance consistent with SI.

4

specifications for either Type 1 or Type 2 sound level meters.

iv. Measurement Procedure

The measurement procedure consists of determining the maximum reading of

the sound level meter for rapid engine accelerations as described above.

These measurements are to be made only when the ambient or background noise is

at least 10 dB lower than the level of the tested vehicle and the wind

velocity is less than 12 mph (19.3 km/hr). The reported sound level is the

numerical average of the two highest readings which are within 2 dB of each

other

.

2.2. Propagation of Sound over a Reflecting Plane

The presence of a reflecting plane influences the sound field around a source through two mechanisms, including reaction back on the source by

reflections from the surface, and the interference between direct and

reflected signals as they propagate away from the source[7,8,9,10] . Since the

effects of reflection usually are small (e.g., less than 1 dB) for sources more than 0.75 wavelengths from the reflecting surface, and the principal

truck sources which contribute to A-weighted noise are situated further than

this from the ground plane, we will only consider the propagation effects at

this time. Figure 4 schematically shows propagation paths from a source to a receiver. For source and receiver elevated above the ground plane, two

propagation paths (for the direct and reflected signals) occur. The path

length difference for these two paths depends on the height of the source and

receiver above the ground plane and the separation between them. The phase

difference along the two paths is the factor which is responsible for the

acoustic effects. This phase difference in radians is defined as the path

length difference in wavelengths of sound times 2 tt

.

When the path length

difference is an integral multiple of a wavelength, the phase difference of

the two signals is such that they add constructively. Since the wavelength is

determined from the speed of sound divided by frequency, the phase difference

for a particular geometry is frequency dependent. Figure 5 shows a plot of

8
Figure 4. Geometry for sound propagation over a reflecting plane.

pure tone

I /.'l octave ) a . , j

, ,, V pffertivr band width

full octave ^ "

Figure 5. Directivity of sound field of a point source above a perfectly

reflecting infinite plane (after Baade [3]).

9
the dimensionless frequency dependence of the interference pattern, while

Figures 6a, b and c give the apparent change in the frequency dependence of

the "transmission function" for an omnidirectional source of height 4 meters

for three different source positions simulating the first position of a driveby test (the second portion of the driveby test, i.e., from the

microphone point to the end point, is just the reverse order of these three

figures). As can be seen from these data, the effect of the presence of the

ground plane is the introduction of considerable frequency dependence.

Further, since the interference pattern depends on the geometry and the local

effective speed of sound, it may be expected that the full implications of

these effects cannot be fully understood from tractable analytic models of

simplified source configurations, but rather will require a substantial

experimental data base in order to evaluate the relative significance of the

many variables.

2.3. Potential Effects of Environment

Both steady and fluctuating changes in environment can induce variations

in the noise levels measured for truck drivebys. In investigating the effects

of changes in environmental parameters, the problem can be divided into three

parts

o Generation of the noise by the propulsion system and associated

auxiliary equipment,

o Radiation from the various vehicle noise sources into the

surrounding atmosphere, and

o Propagation of the noise from the immediate vicinity of the vehicle

to the measurement point.

Each of these processes may be affected independently by changes in the

environment; thus an investigation which treats generation, radiation and

propagation individually must be performed in order to separate these

dependences. A brief discussion of noise emission, i.e., generation and

radiation, is presented in section a. The succeeding section will deal with

the problem of the effects of environment on noise propagation.

a. Effects of Environment on Noise Emission

The effects of changes of environment on noise generation can be

qualitatively described by considering the individual truck noise sources,

i.e., engine structure vibration, exhaust outlet, air intake and fan. For

example, changes in the inlet air (i.e., temperature, humidity and/or barometric pressure) result in changes of the combustion pressure which in turn causes changes in the engine structure vibration. These environmental changes

would also affect the sound radiated because of changes to the radiation

resistance provided by the atmosphere, in particular changes in the

characteristic acoustic impedance of the atmosphere, pc.

10
Figure 6.

FREQUENCY, Hz

a) Source 4 m high, receiver 1.2 m (4 feet) high, separation 24.4 m

(start of establish full throttle zone)

b) as above but separation 19.0 m (end of establish full throttle

zone)

c) as above but separation 15.2 m (50 foot microphone point)

Transmission function for sound propagation from an omnidirectional source of height 4 m to the SAE J366b microphone

position for three points simulating a driveby test. The solid

curves correspond to a pure tone source and the dashed curves to a 1/3 octave band random noise source.

11
For the exhaust and inlet noise, radiation cannot be easily decoupled

from generation. In general, exhaust and inlet noise are related to mass flow

through the engine and to the ambient acoustic impedance. Changes in the

environment which would change the engine performance and therefore, the mass flow and the acoustic impedance, would result in variations in the noise

emission from the exhaust and inlet sources.

For fan noise, changes in the environment which result in a change of

engine performance, (i.e., a change of engine speed and thus fan speed) would

result in a change in noise emission. In addition such influences as steady

wind might also affect fan noise emission through a change in effective

loading on the fan.

In general, noise generation is a function of engine performance and

therefore some measure of engine performance is always required to characterize the circumstances under which noise emission was described. The engine

structure vibration is related to combustion pressure which is directly

related to engine power. Therefore the noise associated with the engine

vibration should also be related to engine power. Similarly, since power is

related to mass flow, exhaust and inlet noise should also be related to power,

although the functional relationship may not be simple. Although fan noise is

not directly related to engine power, it is a function of vehicle performance

in terms of engine speed. Thus, it appears that the changes in noise

emissions may be related to the changes in engine and vehicle performance in

terms of power and engine speed.

Other measures such as the inlet air temperature and pressure, engine

compartment temperature, and ambient temperature and barometric pressure can be used to estimate the changes of acoustic impedance for the various sources.

b . Effects of Environment qn Noise Propagation

The determinants of the propagation problem (see section 2.2) are geometry and local effective sound speed. The previously discussed effects of

the environment on vehicle noise emission produce their affects through

altering the vehicle performance. This change in vehicle performance can also

result in an alteration of the "geometry" of the test, in the specialized

sense that with altered vehicle performance (e.g. higher available

horsepower), the vehicle may have different operating conditions (e.g., higher

engine and fan speed) at each position along the test site. This could result

in changes in the position from which "maximum sound" is radiated, so that the

propagation paths may be different. The principal sources of measurement

variation are expected to arise from changes in the local effective sound

speed. These changes can arise from steady effects which change the speed of

sound uniformly everywhere, such as changes in mean temperature, mean

pressure, and steady uniform wind components in the direction of propagation.

In addition, and perhaps more significant for short distance propagation from

directional sources, are changes in the propagation path caused by gradients

of temperature and wind. These gradients have the effect of curving the ray

paths. Figure 7 provides a schematic illustration of the direction of these

12
b) Wind gradient

Figure 7. Schematic illustration of changes in propagation paths due to

temperature and wind gradients.

13
changes. For a number of conditions, the temperature and wind gradients may

be of comparable magnitude, with the result that their effects may roughly

cancel on the downwind side and add on the upwind side. Of importance on the

upwind side is the fact that the curved propagation path implies selection of

radiation from a different angular position from the source, which may be

highly significant for directional sources such as exhaust pipes at high

frequencies

.

Still a third form of change in the propagation path is possible due to

local fluctuations in phase along the propagation path resulting from wind or thermally induced turbulence. These effects may be viewed as producing fluctuations in the observed levels from a steady fixed source. Figure 8 shows some datafll] for a pure tone source. As can be seen from these figures, the

effect of turbulence is to reduce the depth of the transmission path

"attenuation" minima as shown in the upper portion of Figure 8. From the

middle and lower portions, it is suggested that the maximum level fluctuations occur near the minima in the transmission function, and for propagation on the

order of 50 to 100 feet (15.2 to 30.5 m) , fluctuation levels of 8-12 dB have

been observed for pure tones.

To summarize this discussion, it is seen that steady changes in the

environment can lead to steady changes in the "transmission path filter"

(e.g., changes in the frequencies of minimum transmission to a particular

microphone location) which may lead to spectrum shape dependent changes in the

measured level. Further, short term fluctuations in the environment can lead

to large scale fluctuations in measured level. Thus, the proposed experiment

should provide estimates on the magnitude of temperature and wind gradients over the propagation path, as well as some measure of environmental

fluctuations during the acoustic tests.

c. Variables to be Considered

In view of the above discussions, a broad experiment is required to

evaluate the relative significance of the various possible changes. In

particular, it is suggested, as a minimum, that the experiment include

monitoring the change in the propagation environment and the changes in the

vehicle environment (if different), as it relates to vehicle performance.

Thus, the minimum set of relevant parameters judged to be necessary to this

experiment include:

A. Vehicle parameters

a) Measures of vehicle performance such as engine speed and

horsepower vs. vehicle position.

b) Measures of vehicle environment such as, inlet air temperature

and pressure, fuel composition and engine compartment air

temperature

.

14
0

Figure 8.

S -io

-20

to -30

z -40

•50

-60

\T \ - V!

l

» \•V1 V

-

ll

i

\ /

\ 1

» MARCH 10, 1962

1 1

1 1

1 |

II

f * 1000 CPS

o MARCH 25, 1962

I! h- 4 FEET

• APRIL 8, 1962

1

J

1

1 Jj 1 1 L_L_

10 50 100 DISTANCE FROM SOURCE (FT) 500

Experimenta data and the calculated rms sound-pressure level for a 9mall source over a plane boundary (/ = 1000 cps).

DISTANCE FROM SOURCE (FT)

Measured and calculated fractional standard deviation

of p* as a function of distance.

Dependence of the fluctuation level (/.) on the fractional standard deviation of the fluctuations. i

Effect of turbulence along propagation path (after Ingard and

Maling [ 11] )

.

15
B. Environmental parameters

a) Steady environment measures including: mean air temperature

and temperature gradient, mean wind speed, direction and

gradient, pad surface temperature, relative humidity and

ambient pressure.

b) Measures of environmental fluctuation such as: short term,

temporal variations in wind speed and direction, and air

temperature, as well as estimates of spatial variation over

the test pad surface.

These vehicle and environmental measurements must be made concurrent

with the acoustic measurements. We have indicated that the propagation path

will be perturbed by local fluctuations which will lead to variations in the

measured level. These instantaneous fluctuations can be expected to be

different at different times and positions on the test surface. Because of

the scope of the measurement problem entailed in correlating instantaneous

measured sound level with instantaneous fluctuations in the local

environment, it is proposed that the scope of this experiment be limited to

dealing with the change in average measured levels associated with the. change

in short time average environment (average over the time period of one or several driveby or stationary measurements). It is presumed that a measure

of the fluctuation in measured sound level (e.g. the standard deviation of

the maximum sound level) can be correlated with measures of the magnitude of

environment fluctuations.

A last factor of the environment which has to this point gone

unmentioned is that of test site variation including test pad size, flatness,

composition, and changes of the latter two with time. Evaluation of this

effect is hampered by: a) the lack of a defined measurement procedure for any

of the above possible forms of variation, b) the confusion caused by the

large number of other environmental variables already included in this study,

and c) the relatively small number of test sites available for evaluation in

controlled experiments. This study will include some experiments directed at

assessing long term (e.g., between measurement trips) changes in the test pad

through study of changes in detailed transmission path measurements under

"steady" environment conditions. These studies will also hopefully provide

background information for future test pad evaluation measurements.

2.4. Stationary Simulation versus Moving Tests

The primary reason for including stationary simulation vehicle tests in

this study is to provide a basis for separating the effects of environment on vehicle noise emission from the effects on propagation. This separation is

obtained by using a near field microphone array to determine the vehicle

noise emission without the effects of propagation, which may then be compared

to the results determined using a far field microphone array.

16
Some questions have been rasied by a recent publication! 12 ] which

reports that the variations in noise level of light motor vehicles from day

to day and site to site may be primarily due to diffraction in the wake of

the moving vehicles. In that study measurements were made at 50 feet (15.2

m) for three types of vehicles operating on three different pavements

(denoted PA, GA and Cone) using three different procedures:

1) Full throttle acceleration (similar to SAE J986a [13]);

i

2) Coasting vehicle equipped with an electronic point source (mounted near the rear bumper to simulate the exhaust outlet) emitting five

continuous pure tones distributed through the significant part of

the exhaust noise spectrum; and

3) Stationary vehicle equipped with the electronic point source.

In addition to these acoustic measurements, the vertical profiles of

ambient wind and temperature were also monitored from the ground to a height

of either 4 or 5 feet (1.2 or 1.5 m) . The results of these measurements

showed that the sound level deviations for the three vehicles were similar

and therefore the deviations were averaged for the three vehicles. The data

measured for a full throttle acceleration are shown as functions of

temperature at 3 feet (0.9 m) in Figure 9. In this plot the dotted lines

correspond to the data for the three pavements and the solid line to the best

least square fit to all the data points. The slope of these curves indicates

that the sound level measured at 50 feet (15.2 m) for the moving vehicle

decreases by 1 dB for a temperature rise of about 14 to 20°F (7.8 to 11.1

°C) . The results for the electronic source on the coasting vehicle are similar though the data show more scatter. The data for the stationary

electronic source plotted in a similar fashion are presented in Figure 10.

These results show the measured sound levels increase with temperature. A

difference in behavior between stationary and moving sources is thus strongly

suggested.

These results may not apply to trucks since the noise source characteristics and locations are different from automobiles. Since there is

a possibility of opposite trends for stationary and moving tests, a practical

solution is to measure the noise and environmental conditions for both cases and from these results determine the suitability of stationary simulation of

driveby tests.

2.5. Near Field Sound Power Level

One method of minimizing the effects of propagation when measuring the

noise generated by a source located in an outdoor environment is to use a microphone array located close to the source to determine the near field

sound power level. The question then arises of how to determine the far

field sound power level from the near field measurements and given this, how

do the far field results relate to data which would be measured using the SAE

J366b procedure.

17
Figure 9. Influence of ambient temperature on measured sound level for

automobile drivebys (after Olson, Piercy and Embleton [12]).

TEMPERATURE - °F

Figure 10. Influence of ambient temperature on measured sound level for

simulated exhaust noise source (electronic point source) mounted

on a stationary vehicle (after Olson, Piercy and Embleton [10]).

18
Reference [14] presents the results of an extensive investigation of the

accuracy and precision of various measurement methodologies for determining

the estimated sound power output of 17 different portable air compressors,

and in particular, a comparison of the results of near field and far field

sound power measurements. The near field data consisted of measurements

every square meter on a rectangular surface one meter from the machine

surface and the far field consisted of measurements on an 84 point

hemispherical array of seven meter radius around the source. The sound power was estimated for each case from the product of the impedance of air, times

average mean square pressure over the measurement surface times the area of

the measurement surface. The deviation of the sound power level estimates

based on the near field measurements using a conformal measurement surface

from the estimates using the far field measurements are plotted in Figure 11.

As shown by these data the deviations are small, with an average deviation of

+0.5 dB and a standard deviation of the average deviation of +0.5 dB for the

A-weighted sound power level. The significance of the number of measurement

points is shown in Figure 12. These data indicate that an increase in the

number of measurement points does not significantly affect the accuracy of

the measurement over that for five measurement positions (for that particular

sample of data), although, there is a systematic dependence of precision of

the comparison for estimation of A-weighted sound power level. Based on this

information, the far field sound power level can be adequately estimated with

sufficient accuracy by a limited number of near field measurements (on the

order of eight)

.

The sound pressure level as measured using the SAE J366b procedure is

then related to the far field sound power level (or alternatively an average

rms sound pressure over some particular surface times a directivity function

for that particular source) and an interference function dependent on the

source/test site geometry. If it is assumed that the directivity and

interference functions change in predictable ways, the sound pressure level

measured by the SAE J366b microphone can be directly related to the far field

sound power level through this analytic model. Assuming the validity of this

model, independent of its actual existance, the effects of source variation

can then be studied independently of the changes in far field sound level

through direct evaluations of variations in the near field sound power level.

Recent work in underwater [15, 16] and ultrasonic transducer [ 17, 18, 19]

calibration have further indicated that directivity, and therefore changes in

directivity, can be estimated from near field measurements through

combination of near field measurements with appropriate attenuation and phase

shift

.

This discussion may be summarized by noting that the postulation of

separation of environmental dependence into two component problems associated

with source emission and noise propagation, is a useful analytic approach.

Near field estimates of sound power level provide a potentially powerful

experimental tool for empirically separating the problem and permitting the

direct study of source emission dependence on the environment. The only

potential concern is that raised in section 2.4, i.e., the question of

whether in fact the source performs in the same way during stationary and

19
2

o

o

“

Figure 11. Plot of average deviation of near field from far field sound power

level [ 14] .

20
AVERAGE

DEVIATION

OF

STANDARD

DEVIATION

NEAR-FIELD

PWl

FROM

OF

DEVIATION,

dB

FAR-FIELD

REFERENCE

PWL

Figure 12. "Bias" and "precision" of A-weighted sound power level (PWL) vs. number of measurement positions Cl 7 compressors) [14].

21
moving measurements. Technological problems associated with deriving a useful signal from microphones mounted on the vehicle preclude reliance on making near field measurements on the moving truck. Thus, investigating

source emissions on the basis of stationary measurements appears to be the

best approach.

2.6. Statistical Aspects of the Experiment Design

a. Accuracy and Precision Considerations

It is customary to distinguish between the accuracy of a measurement, as defined by the difference between the measurement result and some reference

value and the precision of the measurement which is a measure of the internal

consistency of the data. This distinction will be maintained through these

discussions

.

The principal objectives of the experiment involve the determination of

relative changes in measurement results to a high degree of accuracy. Since

the experiment involves a long time base, the only convenient way of

assessing the precision of the acoustic instrumentation is through a transfer

calibration to an accurately known standard (such as a pressure corrected

pistonphone source). However, since the objectives involve only relative

changes, and all data on specific vehicles should be reported in a normalized

form, as changes from an overall average level or an arbitrary zero. This

will eliminate from consideration the absolute comparison between specific

vehicles, but will retain the ability to compare relative magnitudes of

specific sources.

b . Data Reduction and Data Analysis

Because of the long time base of the experiment and the associated need

to show preliminary results during the experiment, both for evaluating the

progress of the experiment and making decisions concerning the overall test

plan, it is useful to make the distinction between data reduction and the

subsequent analysis of the pooled data.

In subsequent discussions these activities will be distinguished by

refering to "data reduction" as the process of putting raw data in the

form(s) suitable for later, detailed analysis and also applying preliminary

analysis techniques to identify the information content of the data acquired

to date. The term "data analysis" will then be used to refer to the

systematic evaluation of the data in the search for underlying physical

mechanisms

.

For preliminary analysis, multivariate statistical techniques

(principally partial correlation or path analysis [ 20] ) should be used to

identify the degree to which the observed variations of a selected principal

dependent variable are in fact correlated with other variables. It is

suggested that an appropriate dependent variable is the maximum A-weighted

sound level at the standard (i.e., J366b) measurement position. One possible

22
result from such analysis is the presentation of a regression equation and

associated confidence interval estimates. These confidence interval estimates

can then be used to determine when the data set is sufficiently broad so as to

permit reliable identification of dependence within some a priori defined

level of confidence. (A discussion of a possible form for developing such an analysis is included in Appendix A.)

A confidence interval for the actual physical coefficients in a regression equation can be estimated, and has the typical form of.

C. is within

1

C

i — t (l-a/2, n-m)

where t is the student’s t statistic for the lOO(l-a) percent level of

confidence, with n-m degrees of freedom, n is the total number of observations

of sound level; m is the total number of variables in the model; and S. and S. are the estimated standard deviations of the dependent and the ith independent

variables respectively.

c. Sensitivity of the Experiment

It is extremely difficult to obtain an estimate of the actual sensitivity

of the experiment to variations because of the lack of data. Thus, updates of

these estimates will be an important part of the data reduction scheme. As a rough estimate, for purposes of discussion, assume n-m > 30 and a 90 percent

level of confidence so t g,. > ^q gc

^ 2. Using data from passby measurements of automobiles [2]| tfie nominal ’spread is a 4 dB range in sound level

for a 50°C range in temperature. Thus, if the measurements were concentrated

near both ends of a 50°C band, a standard deviation of 1 dB in sound level and

a 10°C standard deviation of temperature might be expected. The resulting

sensitivity then is on the order of,

Present data on automobiles suggests a slope of about 0.04 dB/°C. In order to

detect this with 90 percent confidence would require,

2 ^ n = ( —) = 25 measurements .

Alternatively, with, say 3 trucks, 12 passbys/ truck/outing, and 6 outings the

90 percent confidence interval sensitivity for temperature dependence would be

for each truck,

S CD =

• - ^ 0.02 dB/°C

.

y/12 x 6

23
If the data for three trucks are statistically indistinguishable, so that the

results can be pooled, this improves to,

S. (1)

S (3) = — 'v 0.01 dB/°C.

C VT~

2.7. Objectives and Goals of a Proposed Experiment

In the previous sections we have reviewed a number of mechanisms by

which the environment can produce variations in the measured driveby noise.

It is the purpose of this section to pose the questions one might ask about

these variations and the degree to which answers to these questions may be

derived from the experiment. In so doing, we will identify the principal

objectives of the experimental program, and the goals of the study.

The principal question is what is the range of variation in measured

level of truck driveby noise and is it systematically dependent on observable

environmental variables. This question can only be answered definitively for

all trucks by an experiment on a sample representative of all trucks.

However, we can provide a hint at the answer through the study of a few

trucks and a selected group of environmental factors. To the extent that the

effects are similar on these trucks and that we can identify suitable

generalized physical mechanisms responsible for the observed variations, we

can infer the information required in order to find a general expression for

such variation. It is logical that a subsequent study of more general

applicability should concentrate on the more significant variables identified

in this experiment.

Thus, a first objective of this experiment should be:

1.

Rank order3 in terms of significance of variation induced in

measured truck sound level 3 the principal environmental variables

associated with vehicle noise measurement.

Since the rank ordering implies the effect of all potential sources of

variation are considered, this implies a study in which all sources of variation are considered, or alternatively, that only those variables which are included in the study can be so ordered. The initial proposed variable

catagories for study are as follows:

1. Mean temperature over a replication period.

2. Mean wind speed (component in the direction of propagation).

3. Atmospheric pressure.

4. Relative humidity.

5. Magnitude of temperature gradient.

6. Magnitude of wind gradient.

24
I

An experimental program should seek to maximize the range of variations

in these variables to the extent possible within financial limitations.

Since it is hypothesized that the above effects may be different for

different source types or geometries, several different source types should

be included. It is proposed that these sources should include:

1. whole truck.

2. truck fan noise (aerodynamic source).

3. truck exhaust noise (radiated from top of exhaust stack).

4. noise from radiating surfaces (presumed to remain when exhaust, fan

and, if necessary, inlet noise are removed).

A second objective of the experiment should be:

2. Identify to what extent these variations ave due to changes in the

propagation path3 and conversely to what extent they represent

changes in the vehicle sound emission.

This second objective implies that some analysis or measurement

procedure can be identified, which can separate these effects. It is

proposed that this can be accomplished by assuming that the change in the near field estimate of sound power is a direct measure of change in vehicle

emission. This test is most conveniently performed using a stationary

simulation of the driveby test. Since stationary simulations of driveby

tests have not previously been established as comparable with moving tests,

accomplishing this objective implies a two stage test. First, establish the

comparability of driveby testing with a stationary simulation of a driveby

and if comparability is established, use near field measurements of a stationary vehicle to directly determine the change in vehicle emission as a function of environmental variables.

Both of the above objectives can be accomplished with a relatively

limited set of acoustic and environmental measurements (e.g., A-weighted

sound level measurements at standard J366b measurement positions, and at an array of microphones in the near field of the vehicle during stationary

tests, plus typical measures of environmental variables such as mean wind

speed and temperature at various positions on and above the test pad)

.

A larger question than either of the above forms the principal goal of

the overall study. This question is simply what physical mechanisms are consistent with the observed variation in truck noise measurement. In order

to provide a data base adequate for testing various hypotheses in this

regard, it is necessary to have information of more considerable detail and

extent. In particular we note that these hypotheses should result in

25
experimentally verifiable predictions of changes in vehicle performance or changes in patterns (e.g., SPL vs. time, directivity or frequency spectrum)

of the radiated sound. In anticipation of this, the experiment performed

should provide additional data. The discussions in previous sections suggest

that as a minimum this additional data should include the following:

a) Vehicle position vs. time data, for correlation with sound level

vs . time

.

b) Vehicle performance measures such as engine speed and horsepower

vs. position.

c) Vehicle operational environment measures (e.g., typical temperatures and pressures at important locations in or near the engine)

.

d) Directivity of the vehicle (horizontal and vertical) at some position near the maximum noise point.

e) Spectrum of vehicle noise vs. time.

f) Spectrum of vehicle sound power vs. time.

g) Additional environmental data such as the detailed mean temperature

and wind gradients and a measure of spatial and temporal

fluctuations of these quantities.

This data base may then be used as the first testing ground for any

physically based models of environmental effects.

The study will thus proceed in three principal phases. This report

represents the culmination of the first phase of drawing together the

available information in order to prepare a logically comprehensive experiment. The second phase consists of obtaining a set of data according to the

comprehensive plan and performing a preliminary data reduction to show the

superficial dependence of truck noise data on the environment. The third

phase of the study consists of analyzing the resulting data in detail in

order to identify and test hypotheses which explain the results of the

measurements

.

3. PROPOSED EXPERIMENT

This section describes the experimental plan developed on the basis of

the technical background presented in section 2. Sections 3.1 and 3.2

contain a brief overview of the total experiment and a description of the

instrumentation and proposed test. Then in sections 3.3 and 3.4 the specific

details of the tests — moving and stationary — are discussed.

3.1. Overview of the Experiment

The proposed experiment will involve the measurement of acoustic,

vehicle and environmental data for a variety of vehicle and environmental

26
conditions. These data will be used to determine the systematic and

nonsystematic dependence of observed truck driveby noise on the environmental

effects present during the measurement.

Three trucks are proposed by MVMA to be' tested in the course of this

experiment: two diesel powered and one gasoline powered. These vehicles will

be provided by the appropriate member organizations of MVMA and will be

committed solely to this project until its completion. These vehicles will be

instrumented such that the necessary data to characterize their performance

and noise emission (i.e., generation and radiation) can be obtained during all

phases of testing. These vehicles will be operated and measurements taken for

three conditions: whole truck, truck without fan, and truck without exhaust or fan and exhaust.

At the field test site two basic types of test will be performed —

moving and stationary. The moving tests will be made according to the

standard procedures recommended by SAE Standard J366b. In addition to the

J366b microphones at 50 feet (15.2 m) on both sides of the vehicle, an array

of microphones will be used to determine the horizontal and vertical

directivity near the ground plane for truck drivebys . Two microphones will

also be mounted on the vehicle during the driveby to give some near field

information which may be useful in determining changes in noise emission.

Concurrent with the acoustic measurements, environmental data (wind,

temperature and their gradients, relative humidity and barometric pressure)

and vehicle parameter data (temperatures, pressures, drive shaft speed and

torque) will also be recorded.

For the stationary vehicle tests, measurements simulating the SAE J366b

test will be made using moving microphones on each side of the vehicle which

are traversed past the truck while the engine speed is controlled to simulate

a driveby. In addition, measurements will be made for stationary run-up tests

according to the procedures specified by the DOT Bureau of Motor Carrier

Safety regulations for compliance with interstate motor carrier noise emission

standards. For both types of tests a near field array consisting of eight

microphones will be used to determine an estimate of the sound power level.

Several stationary far field microphones will also be used in addition to the

moving microphones. As for the moving tests, vehicle and environmental

conditions will be monitored concurrently with the acoustic measurements.

To summarize, the overall test will involve three vehicles operated

under three conditions using moving and stationary tests. The proposed test

schedule and overall test plan are laid out in Figure 13. Five field trips,

during the period December 1976 to August 1977 (an optional trip is

tenatively scheduled for June), are presently planned. The first trip will

run for about two weeks — one week to become familiar with the test sites,

instrumentation and vehicle operation and one week for measurements — with

the subsequent four or five trips lasting one week each. During each trip

approximately 25 percent of the time will be spent on the moving tests and

27
Figure

13.

Overall

test

plan

for

vehicle

environmental

variables

study.

28
75 percent on the stationary tests. For both moving and stationary tests,

measurements will be made during cool night conditions and warm day

conditions so that tests will be performed with the largest possible spread

of environmental variables. After each trip a preliminary reduction of the

data will be performed and a trip report prepared. Phase II will then

culminate with a report summarizing the data obtained during the five field

trips and a proposal outlining the detailed analysis to be performed under

Phase III. More explicit details of this study are presented in the

remainder of section 3.

3.2. Hardware

a. Proposed Test Site

The proposed test site for this study is at the International Harvester

Truck Engineering Center in Fort Wayne, Indiana. This test site has the

facilities to perform both SAE J366b driveby tests and stationary tests with

moving microphones to simulate the SAE J366b procedure. These two test

facilities are shown schematically in Figure 14.

i. Moving Tests

The SAE J366b driveby test site, referred to as the Sound Pad facility,

consists of a single lane asphalt track approximately 1000 feet (304.8 m)

long with turn-arounds on both ends. The measurement area is located midway

along the track and consists of two triangular shaped asphalt pads extending

50 feet (15.2 m) from the centerline of vehicle travel to the microphone

locations and approximately 150 feet (45.7 m) along both sides of the track.

This particular arrangement thus allows measurements to be made for drivebys

in both directions.

In an effort to reduce the wind velocity at the test site, two wind

barriers have been erected near the measurement area as shown in Figure 14.

These barriers are constructed of wood arranged in a slotted design which

allows the wind to flow through the barrier resulting in a reduction of the

mean wind velocity. The barriers are approximately 20 feet (6.1 m) high and

are more than 100 feet (30.5 m) from the acceleration point, end point and

microphone as specified by SAE J366b.

This facility is fully automated in that only the drivers and test

vehicles are ordinarily at the site. The microphones and preamps used to

perform J366b measurements are designed specifically for outdoor environments

and remain permanently mounted at the test site year-round. To establish

truck position between the acceleration and end points, a system of two lines

of magnetically activated reed switches are used. Each line of switches is

200 feet (61.0 m) long with switches every 5 feet (1.5 m) and is installed

inside plastic tubing set in a shallow trough 15 inches (0.4 m) from the

roadway centerline. The reed switches are triggered by two electromagnets

suspended from the front of the vehicle. The magnets are constructed such

that their width defines the lateral error and if the truck is too far off

29
BUILDING

^

(—I CONTAINING

MINICOMPUTER

AND CONTROLS

Figure 14. Schematic diagram of proposed test sites at the International

Harvester Truck Engineering Center.

30
center, the switches are not actuated. One electromagnet is on full-time and

is used to sense vehicle position. An on-board logic system switches "on"

the other electromagnet when full throttle is established and then switches

it "off" when maximum rated speed is reached. In the International Harvester

system, the signals from the reed switches and microphones are relayed

through approximately one-half mile of cable to a central facility where the

signals are analyzed using a mini-computer . The mini-computer logic is also

used to send a signal to a set of "acceptability lights" at the test site

which tell the driver whether it was a good run or that the vehicle did not

meet the test parameters, i.e., the acceleration was before or after the

acceleration point, the maximum rated speed was reached too soon or too late,

the vehicle was too far off center and the reed switches were not activated

by the electromagnets, or the wind velocity was too high.

ii. Stationary Tests

The stationary test site, referred to as the Source Noise Analysis Pad

(SNAP), consists of a bow tie shaped asphalt and concrete test pad

approximately 100 feet (30.5 m) wide and 150 feet (45.7 m) long at 50 feet

(15.2 m) from the center point. This site is located in the center of a 300

foot (91.4 m) diameter circle cut of an existing woods. The purpose of

locating the test pad in these woods was to provide a natural barrier against

wind. The facility was designed to test a stationary vehicle (often one which isn’t drivable) under conditions which would correlate very closely

with the SAE J366b driveby test. To simulate this driveby test procedure,

two microphones (one on each side of the vehicle located at 50 feet (15.2 m)

from the center of the pad) are moved past the vehicle at a constant speed

while the engine speed (or fan speed) is servo controlled (all controls are located in the building approximately 200 feet (61.0 m) from the pad) to

correspond to the J366b engine speed/truck-microphone relative position

relationship determined from an actual driveby. The driveby simulation is

scaled in time such that the microphone traverse takes approximately 1 1/2

minutes as compared to 4 or 5 seconds for an actual driveby. This time

scaling is required to minimize the ambient noise of the moving microphone

due to wind and noise generated by the trolley system. At a test speed of

approximately 90 feet (27.4 m) per minute for the microphone traverse, the

ambient noise level is about 55 dB, which is more than 10 dB below the level

of truck noise to be measured.

This facility is equipped to test vehicles in various operational modes,

e.g., engine/exhaust only (no fan) to the fan only condition. The fan may be

driven by an external motor located in an acoustical enclosure. For the no fan case, engine cooling is provided by bypassing the radiator with a heat

exchanger of similar flow restriction and coolant pumped directly through the

engine. Tests associated with engine, transmission, exhaust and intake noise

utilize power absorption to provide a load for the vehicle. The power

absorbers are eddy current devices that are coupled directly with the

transmission. (These devices are approximately 52 inches (1.3 m) long and

are mounted to the vehicle frame rails between the transmission and

differential.) The absorbers are controlled by dc voltage and are capable of

31
I

absorbing 350 horsepower at 2100 rpm for approximately 20 minutes. The load

capability can be increased to 500 horsepower at 350 rpm but the test time is

limited to 6 minutes due to overheating of the bearings.

In general, International Harvester personnel will be utilized for both

moving and stationary tests. In particular IH will provide experienced

drivers for the J366b tests and technicians to operate the microphone/engine

speed servo controls at the stationary test pad. Two fuel tanks — gasoline

and diesel fuel — will be provided at the test site and maintained for use only by the vehicles in this study. Vehicle mechanics and any necessary

maintenance time will also be provided by IH. NBS will provide technicians

to maintain and repair test equipment which is brought to the test site and

also any photographic services necessary to document the test site layout,

equipment and vehicles.

iii. Environmental Conditions

As outlined in the overall test schedule, five or six field trips will

be made between December 1976 and August 1977. For this period, the weather

at the International Harvester test site will vary over a wide range of

environmental conditions. The meteorological data for 1974 and the normal

mean and extreme values based on records for the 1941 to 1970 period are given in Table 1.

On the average the daily temperatures range through an amplitude of

about 20°F (11.1°C) between low and high, and the seasonal range is from a daily winter minima of about 20°F (-6.7°C) to the daily summer maxima near

85°F (29.4°C). Since these temperatures are based on averages, an overall

temperature range of 10 to 90°F (-12.2 to 32.2°C) could be expected during

the planned field trips.

The prevailing wind directions are from the west and southwest and the

average annual velocity is just slightly more than 10 mph (16.1 km/hr)

[measured at 20 feet (6.1 m) above the ground surface]. For the period

December to August, the mean wind speeds range from a low of 7.7 mph (12.4

km/hr) in August to a high of 11.5 mph (18.5 km/hr) in January and February.

Precipitation, normally near 36 inches (914.4 mm) a year, is well

distributed from January to December. The average precipitation from

December to August is 3.08 inches (78.3 mm) per month. The mean number of

days of precipitation [0.01 inches (0.25 mm) or more] range from a low of 8

days in August to a high of 13 days in December. The mean number of days of snow or ice pellets [1.0 inch (25.4 mm) or more] is zero from May to August

and 1 to 2 days from December through April.

Based on these meteorological data, delays in testing due to

precipitation should be minimal (i.e., not more than an average of say 20

percent of field time) and the wide range of weather conditions should

provide a good basis for determining the effects of environment on truck

noise measurements.

32
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33
b . Proposed Vehicles

MVMA has previously arranged for vehicles to be supplied for use in this

program. It is expected that the three trucks to be used will include two

cab-over-engine turbocharged, diesel-powered trucks, and one gasoline powered

truck of undetermined cab configuration. In order to simplify later

analysis, and to reduce time involved in altering vehicle configuration

during the testing phase, there are a number of particular specifications

which would be highly desirable for incorporation into the test vehicles

prior to delivery. These specifications are listed below followed by a discussion of the rationale for these requests:

1. Cab-over-engine configuration.

2. Engine inlet air drawn from outside the vehicle through a side or snorkle configuration.

3. Single vertical exhaust above the cab.

4. Fan with positive clutch.

5. Adequate propeller shaft length and clearance between side rails to

accomodate the proposed dynamometer.

6. Vehicle parameter transducers installed.

The cab-over-engine configuration was selected not because of particular

virtue, but because it is believed that a similar layout of sources and

overall shape on all three vehicles would simplify the comparison between

vehicles for the purposes of this study.

The principal reason for requesting engine inlet air be drawn from

outside is to simplify the evaluation, and if necessary, the control of this

particular source in order to permit clear identification of fan, exhaust and

engine casing radiated noise. Further, an outside (i.e., not within the

engine compartment) air intake permits the assumption that the temperature

and pressure of inlet air are comparable with that of the ambient

environmental conditions measured for reference purposes.

For the same reasons that the cab-over-engine configuration is

requested, i.e., simplified analysis and comparability, the single vertical

exhaust is preferred. Further, however, the assymetry of the typical single

exhaust will provide a useful variation in the source geometry from the other

centrally located sources. Finally, since interaction with vehicle

turbulence has been hypothesized as a possible mechanism for difference

between stationary and moving tests, the fact that the vertical exhaust is

not in the slip-stream of the truck body should minimize this effect.

The use of a positive fan-clutch permits convenient engagement and dis34
engagement of the fan and positive speed control (i.e., some particular ratio

times engine speed) during "fan on" studies.

Figure 15 shows the space requirements for the dynamometer system to be

used in the stationary tests. In order to facilitate dynamometer

installation it is requested that the forward rear axle be positioned to

provide the clearance indicated in the figure plus some reasonable assembly

clearance. If necessary, on short frame, dual rear axle vehicles this can of

course, be accomplished by removing the forward axle of the pair, and

providing an appropriate length propeller shaft to reach the aft rear axle.

For each vehicle, it would also be preferrable to remove the fifth wheel and

intermediate side rail braces in order to provide space above and between the

frame rails for clear installation of the dynamometer. Since the vehicle

will not be required to carry or pull a load during these tests, and these

modifications are not expected to produce significant changes in low speed

noise, they are judged to have no influence on the measurements.

During a recent meeting of the MVMA Vehicle Sound Level Committee, a list of vehicle parameters was derived for which monitoring was believed

necessary in order to evaluate changes in the vehicle operation and

performance. This list includes:

1. Engine speed derived from a non-contacting drive shaft pickup.

2. Shaft torque derived from a strain guage bridge on the drive shaft

through slip rings.

3. Inlet air pressure (after turbocharger).

4. Exhaust gas static pressure before turbocharger.

5. Inlet air temperature (after turbocharger).

6. Exhaust gas temperature at top of exhaust stack.

7. Exhaust gas temperature at exhaust manifold.

8. Engine compartment air temperature.

9. Engine cooling water out temperature.

It is proposed that the vehicle supplier obtain and install transducers

for these variables, wired to a common junction box to be supplied by NBS

.

Recommended transducers are discussed in section 3.2.C.3 under vehicle

parameter data acquisition system.

c. Data Acquisition and Reduction Systems

The four major systems to be discussed here are the acquisition systems

for acoustic, environmental and vehicle parameter data and the data reduction

system to be used for all acquired data.

35
CENTER OF

TRANSMISSION

U-JOINT

TOP OF FRAME RAIL

:

i

VEHICLE FRAME RAIL < OF ABSORBERS ZV2

0 FROM LINE PARALLEL

• — 1 TO TOP OF FRAME RAIL

CENTER OF

U-JOINT AT

FRONT OF ABSORBERS

L = Total length of (2) absorbers = 51.5 inches (1.31 m)

.

X = 6 to 13.5 inches (0.15 m to 0.34 m) vertical range of centerline adjustment for propeller shaft - absorber alignment.

Y = Propeller shaft length (to be prepared at test site).

= 6 inches (0.15 m) minimum for 7-c propeller shaft.

= 12 inches (0.30 m) minimum for 1700 propeller shaft.

Figure 15. View of typical dynamometer mounting showing the necessary space

requirements

.

36
1. Acoustic data acquisition. The acoustic data acquisition system

will consist of a 14 channel instrument quality tape recorder configured with

one digital channel and 13 IRIG intermediate band FM channels for recording

time-varying data. Operated at 15 ips this system will provide 0-10 kHz

bandwidth with nominally 50 dB signal to noise ratio. The acoustic transducers will include 1/2 inch back vented condenser microphone cartridges with

dehumidifier sections, preamplifiers and battery operated power supplies.

For signal monitoring, an oscilloscope, real time analyzer and audio channel

will be provided. During moving tests, data from two microphone positions

fixed on the vehicle will be transmitted to the acoustic acquisition system

through a two channel FM telemetry system with 5 kHz bandwidth and 30 dB

dynamic range. The digital channel will encode a header incorporating test

identification information, time-of-day clock information for relating data

from other systems to this data, and vehicle or microphone positioning data

derived from existing transducers at the test site.

2. Environmental data acquisition. The environmental data to be determined include: ambient air temperature and wind speed and their respective

gradients in the region within about five meters of the test pad surface,

wind direction, pad surface temperature, relative humidity and barometric

pressure. To accomplish the wind and temperature measurements and provide

information on spatial variation of these data, two 5 meter towers will be

constructed on which hot wire thermoanemometers will be mounted at

appropriate intervals for determining estimates of temperature and wind

gradients. A total of six or seven thermoanemometers are anticipated for

each tower. The data from these transducers will be multiplexed and logged

together with time of day, wind direction and surface temperature on portable

digital data loggers. A complete set of these will be multiplexed and logged

every 5-15 seconds to provide nominally 50 sets of data during each complete

set of replications of driveby data. Relative humidity and pressure data

will be determined from analog instrumentation and logged on the acoustic

data reader.

3. Vehicle parameter data. The vehicle parameter data will be recorded

on IRIG standard intermediate band format using a 14 channel tape recorder

configured for 13 FM channels with nominal 100 Hz bandwidth using the 14th

channel in digital format to record time of day and vehicle identification.

The transducers for vehicle parameter data, to be provided and installed

by the vehicle suppliers, fall into two categories, including temperature and

other parameters.

2 / The temperature transducers consist of Type E— chromel-constantan

thermocouples (nominally 90 percent Ni, 10 percent Cr vs. 55 percent Cu, 45

percent Ni) in order to monitor the following temperatures:

—

2 / The letter designation is that recommended by ASTM Committee E-20.

37
1. Inlet air temperature (after turbocharger or prior to carburator)

.

2. Engine cooling water temperature out of the block.

3. Engine compartment air temperature.

4. Exhaust gas temperature at exhaust manifold (prior to

turbocharger)

.

5. Exhaust gas temperature at top of stack.

Leads from these thermocouples and other transducers should be

electrically insulated, mechanically secured, and run to a terminal strip in

a constant temperature "zone box" to be supplied by NBS. Cable connections

from this box, to the signal conditioning and recorder package, as well as a reference thermocouple, will be provided by NBS.

The remaining vehicle parameters to be sensed include:

1 . Engine speed.

2. Shaft torque

.

3. Engine air inlet pressure (turbocharged only)

.

4. Engine exhaust pressure.

The engine speed detector can be of any noncontacting type which produces a well defined number of pulses per engine revolution (consistent with a 100 Hz

bandwidth limitation). This transducer can operate on any shaft or gear

whose rotational speed is directly relatable to engine and propeller shafts

speeds

.

Propeller shaft torque can be most expeditiously determined with a bonded strain gage bridge mounted on the shaft with voltages carried through

a set of slip rings mounted on the transmission or differential. Electrical

connections should permit easy removal of the propeller shaft for stationary

tests. NBS will provide a controlled nominal 10 volt dc source for

energizing the strain gage bridge.

Transducers for the pressure measurements have not yet been selected,

since the question of differential vs. absolute pressures is as yet

unresolved. NBS will provide, in this system, stabilized dc amplifiers for

signal conditioning.

We note in conclusion that power for the tape recorder will also be

required from the vehicle electrical system during engine operation. Recommendations for wiring and electrical connection points for nominal 12 v dc,

400 watts will be required.

38
4. Data Reduction. All data collected from the above systems will be

converted where necessary to digital format and stored in a form directly

accessible by computer at NBS. Acoustic data will be reduced on a one-third

octave band real time analyzer in 100 ms time intervals as a first

approximation to fast response of a sound level meter. Position data will be

coded to give positions vs. time information. Environmental data will be

averaged to give mean and variance information at each measurement location

over a suitably short time interval. Vehicle parameter data will be speed

scaled and digitized (with an ADC) for use in generating plots of parameters

vs. time or position.

3.3. Moving Tests

In this section we review and discuss the specifics of the proposed

moving tests.

a. Test parameters

Figure 16 shows a detailed schematic view of the proposed test site and

microphone arrangements. It is expected that the site will be used in a bi-directional mode, which implies that the microphone arrays must be

symetrically disposed to the left and right of a common line, in order to

produce comparable data in each direction. The line selected for this

disposition is the line connecting the standard J366b microphone positions

(shown as solid circles on the figure).

Two microphone arrays for evaluation of directivity are planned, one in

the horizontal plane and one in the vertical plane through the line of

symmetry. The microphones will be arranged at +30° and +60° from the line

through the J366b microphones. Changes in horizontal directivity data are expected to provide information suitable for testing hypotheses about the

systematic effects of wind and wind gradient components perpendicular to the

direction of vehicle travel. The height to be used for this array is 1.2 m

(i.e., 4 feet, same as the J366b microphones). The vertical directivity

array, is intended to provide information with which to test analytic models

of the effect of temperature, temperature gradient, wind and wind gradient on

the transmission function from the vehicle to the J366b microphones. Thus

only the region within a few meters of the J366b microphones will be studied.

The proposed microphone heights for this array include 0 (ground plane), 0.6,

1.2 (J366b microphone), 2.0, and 4.0 meters. Data from this array can also

be used to directly evaluate environmental effects on alternate microphone

heights to the J366b position.

It is expected that comparability between stationary and moving tests

will be established through analyses of J366b microphone data. In order to

provide additional data points for such a comparison, and further, to provide

an alternative check for the study of changes in vehicle emission, two near field measurement positions are planned to be fixed with the vehicle. While

it is highly doubtful that these two positions will provide sufficient

39
MOVING VEHICLE TESTS

— - crow T

H ENVIRONMENTAL

MONITOR

LOCATIONS

Figure 16. Schematic diagram for moving vehicle test site showing transducer

array locations.

40
information to permit adequate study of this problem in themselves

(particularly when wind noise problems and dynamic range limitations of

state-of-the-art telemetry systems are considered) it is felt that the two

positions represent an acceptable compromise between information desired, and

financial constraints. These measurement positions will be selected to

correspond directly with measurement positions in the stationary test near

field array. It is expected that the positions will be those nearest the

vehicle fore-aft centerline in order to have positions which relate closely

with fan/engine noise and exhaust/ transmission noise sources, but it is

expected that this will not be firmly established until after preliminary

measurements

.

Vehicle positioning information and operational parameter data will be

determined from the existing position sensing equipment at the test site and

encoded on the digital channel. Control (i.e., acceptance or rejection of

the test for meeting J366b requirements with regard to vehicle acceleration

point, maximum engine speed point and lateral positioning error) will be

determined from the existing logic system. The normally acceptable range of

vehicle positioning with respect to acceleration and end zones will be used

in order to include their effect in the test variability determination.

It is proposed that prior to and at the end of a set of measurements,

environmental data will be recorded at a selected representative group of

positions to assess spatial variation of wind and temperature, but that

during a group of measurements on a vehicle the environmental transducer

towers will be operated at specific sites determined from prior surveys.

Test site acoustic evaluation measurements will be made during a time

when environmental conditions (notably wind and temperature gradients) are most conducive to measurement of test pad properties. These measurements (as

yet undefined, but probably consisting of determination of transmission

function from several fixed source heights to particular microphone

positions) will be made with the specific objective of determining that the

test pad flatness and acoustic impedance for near grazing incidence has not

altered significantly from previous evaluations. This investigation is

expected to provide a data base from which test procedures for site

qualification may also be studied.

b . Test Plan for Moving Tests

Based on recommendations from personnel in the Applied Mathematics

Division, Statistical Engineering Section at NBS, the schedule for testing

will be arranged in such a manner that the possibilities of systematic

biasing of the data will be minimized. To assure this, the order in which

the tests are performed will be randomized as much as feasible, considering

the limitations of setting up and calibrating instrumentation and changing

vehicle configurations. The controlled variables in the test are:

1. Vehicles: I, II, III

41
2. Vehicle configurations: A - whole truck

B - without fan

C - without exhaust or fan and exhaust

3. Microphone arrays: horizontal, vertical

4. Vehicle direction: east, west

5. Time of day: day. night

For the purposes of this study, day is defined as the time period when the

temperature gradient is a maximum due to the heating of the test pad (e.g.,

1:00 to 5:00 p.m.) and night as the time period when the temperature gradient

is a minimum (late evening or early morning)

.

A typical test plan, which would be completed in one calendar day and

repeated during each of the five (or six) field trips, is outlined in Table

2. Also given in this table is an estimate of the time required for each

test. For each test condition there are a minimum of 6 drivebys — 3 in the

east direction and 3 in the west direction alternating east, west, east, etc.

Using this test plan the order of testing the individual vehicles, vehicle

configurations and horizontal versus vertical microphone arrays will each be

randomized for a test day. Further randomization, such as intermixing the

vehicles or microphone array configurations, would be too time consuming and

thus is not considered feasible in this test plan. This test plan represents

the approximate schedule that will be followed during the second week of the

first trip and all subsequent trips.

During the first week of the first trip one day will be spent in setup

and checkout of equipment and site evaluation measurements. The second day

will be used to record drivebys of one vehicle using all three configurations

during both the day and night conditions to evaluate the test procedure. In

addition, measurements will be made to verify that all three vehicles

generate the maximum noise during driveby accelerations rather than

decelerations. The remainder of the week will be used to prepare for the

stationary tests.

3.4. Stationary Vehicle Tests

a. Test Parameters

Figure 17 shows a schematic diagram indicating the locations for transducers for these tests. As has already been mentioned, there are two phases

to the stationary tests — simulated drivebys and stationary run-ups. The

simulated driveby is to be operated as a time scaled simulation of the J366b

driveby, with engine speed controlled, at full throttle, with loading in

order to preserve the engine speed/ truck-microphone geometry relationship of

the driveby. This relationship will be derived from the IH driveby test

facility data, supported by the vehicle parameter data determined from NBS

recordings. One major unknown at this juncture is the accuracy achievable in

42
Table 2. Test Plan for J366b drlveby tests giving the order of vehicle testing.

Time of

Day

Microphone Array

Configuration

Test

Vehicle

Test Vehicle

Configuration

Estimated Time for Set

Up or for 6 Drivebys

(3 East and 3 West)

Initial Set Up and Calibration 1 hour

DAY HORIZONTAL I A 6 minutes

|

B 6 minutest 1/2 hour

C 6 minutes)

II B 6 minutes

)

C 6 minutes) 1/2 hour

A 6 minutes

)

III B 6 minutes \ A 6 minutes ) 1/2 hour

C 6 minutes ) Change Microphone Array and Calibration 1/2 hour

DAY VERTICAL III C 6 minutes

)

A 6 minutes) 1/2 hour

B 6 minutes

)

II C 6 minutes i B 6 minutes) 1/2 hour

A 6 minutes)

I B 6 minutes j A 6 minutes) 1/2 hour

C 6 minutes

)

Disassembly of Instrumentation to Protect Against Damage

During Time Allowed for Environmental Variables to Change Minimum 4 hours

Instrumentation Set Up and Calibration 1 hour

NIGHT VERTICAL II C 6 minutes ) A 6 minutes > 1/2 hour

B 6 minutes f III A 6 minutes

|

B 6 minutes) 1/2 hour

C 6 minutes ) I B 6 minutes J C 6 minutes) 1/2 hour

A 6 minutes

)

Change Microphone Array and Calibration 1/2 hour

NIGHT HORIZONTAL III B 6 minutes 1 C 6 minutes) 1/2 hour

A 6 minutes

)

I C 6 minutes I B 6 minutes) 1/2 hour

A 6 minutes

)

II A 6 minutes 1 B 6 minutes) 1/2 hour

C 6 minutes )

43
the control loop, and whether it is desirable or relevant to consider small

systematic changes in this speed control relationship which may occur during

the total period of the test. This question can only be resolved based on experience with the equipment.

For the stationary run-up tests the movable microphones will be fixed

either at the beginning of the trolley system (ready to start the next

driveby simulation) or at the midpoint on the trolley where they will be used

as the 50 foot (15.2 m) microphones if the far field microphones are not

available

.

As was previously noted, the stationary test site is surrounded on three

sides by trees at about a 150 foot (45.7 m) radius. It is expected that this

will produce relatively unusual wind conditions compared to the more common open driveby site. To the extent that wind is interrelated with temperature

and temperature gradient these factors will also be altered. The

significance of these variations is presently indeterminate since there is no comparable data for these effects on this or other sites. Thus, this

question also will not be resolvable until well into this experiment.

The J366b microphone positions used in these tests will use existing

traversing equipment and electronics. The microphone cartridges for these

channels as well as system electrical calibration (i.e., gain and frequency

response) will be supplied by NBS

.

The need for near field measurements is one of the principal reasons for

performing the stationary tests. It is thus not surprising that the majority

of the available acoustic channels are devoted to this function. The eight

microphone array shown schematically in Figure 17 is that studied during a previous program on measurements of portable air compressor noise[14] (see

section 2.5., Figures 11 and 12). This array apparently provides nominally

zero bias and measurement imprecision of on the order of 1 dB, for estimation

of far field sound power from large rectangular sources.

The coordinates of the array positions are given in a draft ISO

standard[21] , and are reproduced in Figure 18 and Table 3. The actual

positions depend on the physical size of the source. The positions are determined with respect to a reference parallelepiped which just encloses the source. For a cab-over-engine truck with a transmission that does not extend

substantially beyond the cab plan dimensions, this reference surface will be

the smallest rectangular box which just encloses the cab. Assuming the

overall dimensions of such a box would be say 3 m wide by 2.5 m long x 3.5 m

high, and a measurement radius of 1 meter the corresponding microphone

location dimensions would be (see Table 3)

a = 2.5/2 + 1 = 2.25 m

b = 3/2 + 1 = 2.5 m

h = 1/2 (2.25 + 2.5 - 1/2) = 2.125 m

44
STATIONARY VEHICLE TESTS

i a=dhL ~

:

Figure 17. Schematic diagram for stationary vehicle test site showing transducer array locations.

45
Figure 18. Location of measurement positions on the measurement surface 121].

46
Table 3. Microphone position coordinates [21].

Source reference surface dimensions are L, W, H.

L , , W u , ' a = — + r, b =

-^ + r, c = H + r

1 I* = ^-(a + b - y) , if h^ is greater than H then take h^ = H

1 r L

a

l =

2^ a + 2^ ’ ^ a

l is 8 reater than — then take a^ = L/2

1 c bf = —(b + —) , if b^ is greater than b then take b^ = b

Position Number X Y Z Distance from

reference surface

1 a 0 h

i r

2 0 b h

i

r

3 -a 0 h

i r

4 0 -b h

i r

5 a

l

b

i greater than H r

6 "a

l b

i

greater than H r

7 'a

l -b

i

greater than H r

8 a

l -bi

greater than H r

Origin for the coordinate system is the point on the ground plane under

the geometric center of the source.

47
a = 1/2 (2.25 + 1/2) = 1.375 in, > a

± = 1.25 m

b = 1/2 (2.5 + (3+l)/2) = 2.25 m

These locations are shown schematically in Figure 19. Note that minor

projections from the surface including bumpers, handles, fuel tanks, exhaust

pipes and air intake are ignored in determining the reference surface unless

there are known to be sources which penetrate the measurement surface.

It is expected that some experimental checks of measurement precision

(i.e., sampling error) will be made during the first measurement trip by

recording data at additional positions (see secondary measurement positions

Figure 18) . These data will be used to determine an estimate of probable

sampling error using the eight point array.

In addition to the J366b microphone positions, additional far field

fixed measurement positions are planned. Three data channels are available

for this purpose and transducers will be positioned to obtain horizontal and

vertical directivity information for comparison with driveby measurements.

In addition, it is expected that these data will be used to obtain

information for studies of the environmental effects on propagation without

the added complication of a moving source-receiver geometry.

Vehicle operational parameter data will be recorded as for the moving

vehicle tests, except that engine speed and horsepower data will be derived

from existing test facility transducers.

Environmental variables and test site measurements will be recorded as for the moving vehicle measurements

.

b . Test Plan for Stationary Tests

As discussed previously for the moving tests, the schedule for testing

will be arranged in such a manner that the possibilities of systematic

biasing of the data will be minimized. To assure this, the order in which

the tests are performed will be randomized as much as feasible, considering

the limitations of set up and calibration of instrumentation and changes of

vehicle configurations. The controlled variables in the stationary test are:

1. Vehicles: I, II, III

2. Vehicle configurations: A - whole truck

B - without fan

C - without exhaust or fan and exhaust

D - fan only

3. Time of day: day, night

48
1m « —9 k

- & tp2.25m 1.5m

1m RADIUS

1m

1m RADIUS

f"- 4

MEASUREMENT SURFACE OUTLINE

Figure 19. Plan and elevation of near field microphone array.

49
As defined in the previous section, day is the time period when the temperature gradient is a maximum due to the heating of the test pad (e.g., 1:00 to

5:00 p.m.) and night as the time period when the temperature gradient is a minimum (late evening or early morning)

.

A typical test plan, which would be completed in three calendar days and

repeated during each of the five (or six) field trips, is outlined in Table

A. Also given in this table is an estimate of the time required for each

test. For each test condition there are expected to be 5 simulated J366b

drivebys and 5 stationary run-ups. Using this test plan the order of testing

the individual vehicles and vehicle configurations will each be randomized

for a field trip. This test plan represents the approximate schedule that

will be followed during the second week of the first trip and all subsequent

trips. During the first week of the first trip approximately two days will

be spent in familiarization with the stationary test site and its operation.

One day will be spent in setting up equipment and making site evaluation

measurements. The second day will be used to record simulated drivebys and

stationary run-ups of one vehicle using all four configurations during both

the day and night conditions, to evaluate test procedures.

3.5. Variations from the Proposed Test Plan

This experiment breaks new ground in several areas with respect to

current technology on vehicle noise measurements, either in scale or in kind.

Several of these areas have been identified in previous discussions,

including, test site evaluation, comparability of stationary simulation vs. moving vehicle tests, on-board vehicle parameter and near field sound level

monitoring, vehicle near field sound power measurements, and environmental

variables monitoring. Because of inherent technological difficulties

associated with these several areas, the proposed measurement program should

be regarded as describing the intentions of a cautiously optimistic principal

investigator, with the realization that some modifications may be necessary

in view of state-of-the-art instrumentation limitations. In particular, the

structurebome noise problems associated with mounting a microphone on a moving vehicle are recognized as presenting serious design problems which may

not be solveable without a separate research program of substantial extent.

Further, the scale of the experiment in terms of the number of simultaneous

operating transducers and recorders is an invitation to interruption from

"bugs" and "glitches". As a result, the proposed test plans are to be viewed

as subject to minor modification to overcome these problems, at the

discretion of the principal investigator. Major changes, arising from

instrumentation limitations or new knowledge derived from this experiment or other investigators, will be made with the concurrence of the MVMA Vehicle

Sound Level Committee.

In this latter regard, it is appropriate to discuss in somewhat greater

detail, two possible major changes which might be anticipated, including

change in the scope of the tests motivated by inspection of preliminary data,

and changes due to evaluation of vehicle mounted microphone performance.

50
Table 4. Test plan for stationary tests giving the order of vehicle testing.

Time

of Day

Tes t VehiclesTest Vehicle

Configuration

Estimated Time for Set Up or for 5 Simulated Drivebys

and 5 Stationary Run-Ups

Instrumentation Set Up and Calibration 1 hour

DAY

( 1 )

20 minutes'

20 minutes

20 minutes

20 minutes.

21/2 hours

Disassembly of Instrumentation to Protect

Against Damage During Time Allowed for

Environmental Variables to Change Minimum 4 hours

Instrumentation Set Up and Calibration 1 hour

NIGHT

( 1 )

20 minutes")

20 minutes I 20 minutes

| 20 minutes

J

2 1/2 hours

Instrumentation Set Up and Calibration 1 hour

DAY

( 2 )

II 20 minutes -

"! 20 minutes I 20 minutes

| 20 minutes

21/2 hours

Disassembly of Instrumentation to Protect

Against Damage During Time Allowed for

Environmental Variables to Change Minimum 4 hours

Instrumentation Set Up and Calibration 1 hour

NIGHT

( 2 )

II 20 minutes"!

20 minutes I 20 minutes | 20 minutes_J

2 1/2 hours

Instrumentation Set Up and Calibration 1 hour

DAY III B 20 minutes"!

(3) A 20 minutes

> 2 1/2 hours

C 20 minutes

D 20 minutes^.

Disassembly of Instrumentation to Protect

Against Damage During Time Allowed for

Environmental Variables to Change Minimum 4 hours

Instrumentation Set Up and Calibration 1 hour

NIGHT

( 3 )

III 20 minutes"!

20 minutes I 20 minutes

[ 20 minutes

2 1/2 hours

51
a. Change in Scope from Evaluation of Preliminary Data

A decision point has been indicated in the experiment at the completion

of data reduction from the third measurement trip. At this point, it is

expected that it will be possible to project the sensitivity of the

experiment to systematic variations for all of the variables under study,

based on data to date and anticipated trips. Thus, depending on the outcome

of an evaluation of these data, the total additional data needs can be

reviewed and modified as indicated. In particular, the need for more or fewer measurement trips than the two subsequent planned trips can be

discussed

.

Unfortunately, the matrix of possible outcomes of the experiment at that

point is so large, that speculation on their variety, and the implied actions

which might be taken, is probably unjustified.

b . Changes in Experiment because of Vehicle Mounted Near Field

Microphones

The principal purpose of the stationary tests is to permit the

acquisition of near field sound power data during the equivalent of a J366b

driveby test. The objective of this data is to permit deduction of the

dependence of vehicle emission on environmental variables from direct

observations rather than through more complex analysis procedures. If an alternative procedure for deriving these data can be found, without

stationary tests, then these tests could be dispensed with. A vehicle

mounted microphone array would permit this, if the data so derived were of

adequate quality. Three, as yet incompletely understood, technological

problems are of significance to the quality of this data. These include: a)

the structureborne vibration question, already mentioned; b) the quality

necessary as compared with the bandwidth and dynamic range of telemetry

equipment; and c) the wind noise problem. Unfortunately, only testing

experience can reveal whether or not these problems can be adequately solved.

In the event that the problems can be solved to an acceptable degree, a proposal using this alternative can be expected. In this event the

stationary run-up tests will be performed with the vehicle parked in the

center of the measurement area of the driveby test site.

4. DATA REDUCTION AND REPORTING

This section outlines the proposed data reduction and reporting to be

performed in conjunction with this study.

4.1. Data Reduction

Acoustic data reduction implies dealing with several different types of

data, including J366b microphone data, horizontal directivity data, vertical

directivity data, moving source near field microphone data, near field sound

power data, stationary source far field data, all repeated for the number of

multiple sources, source configurations and replications. As has already

been noted this information will be reduced using a real time analyzer into

1/3 octave band spectra, plus A-weight and linear sound level integrated over

52
100 ms time periods. Since the vehicles are presumably traveling at speeds

of 35 mph (51.3 ft/s or 56.3 km/hr) or less and position information for

moving tests is determined every 5 feet (1.5 m) , the start of each 100 ms integration will be keyed on a position pulse and will thus represent the

average sound level while the vehicle traverses a particular 5 foot (1.5 m)

interval. Studies of initial data will be made to verify that this reduction

procedure does not yield spurious data due to the arbitrary choice of the

start of integration time and that the maximum values produced are compatible

with the fast response circuit of a sound level meter. The stationary

vehicle tests can similarly be handled, since relative position is

systematically related to time through the (constant) microphone traverse

speed. Additional study of the reduction procedure for these data will be

required, however, in order to verify that simply sampling for 100 ms at the

corresponding positions is an adequate representation. A first alternative

involves using an appropriately time scaled integration time, consistent with

the time scaling of the test (e.g., 1 second integration period for a nominal

10:1 time scale factor).

Data for a given microphone channel, reduced by the above procedure will

be corrected for instrumentation level calibration, frequency response and

background noise and will be stored as n x m dimensional matrices with n frequencies and m source positions and an associated column matrix of time

vs. position. These matrices will be interleaved sequentially with matrices

of the appropriate vehicle parameter and environmental data. Since it is

planned that the preliminary data analysis (except for horizontal plane

directivity) will concentrate on maximum A-weighted sound level at the J366b

microphone an additional separate data file will be written containing these

data, except that horizontal plane directivity data will be rewritten for the

vehicle position corresponding to the array center, rather than the position

of maximum noise.

Vehicle parameter data will be reduced by first transcribing position

code data onto the vehicle parameter tape, and then using the position code

to identify sample intervals. Data will be sampled at a rate consistent with

the rate of data change (typically 20 times per second) and corrected according to calibration tables, and also the reference temperature for

thermocouple data. (Calibration data on driveshaft torque bridges will be

derived from static tests made at NBS between field trips.) This data will

be stored in j x k dimensional matrices with j parameters and k time

intervals and position as an associated column.

Environmental parameter data will already be in digital format. Prior

to transfer to storage, this data will be corrected according to calibration

tables and then an average value and variance determined for the time period

of about 1 minute duration surrounding each data run. These data will be

stored in two (one for each tower) two-column by n parameter matrices

containing mean values and variances for each parameter. Data taken before

or after each major test period, to identify spatial variation, will be

reduced to means and variances versus position for evaluation as to spatial

homogeneity

.

53
4.2. Presentation of Reduced Data

The purpose of the preliminary data presentation is to reveal progress

of the experiment on three fronts, including: comparison of stationary

simulation and moving tests, validity and utility of vehicle mounted near

field microphone data, and evidence of systematic dependence of the acoustic

data on environmental variables.

The comparison of stationary simulation and moving tests will proceed

from two different areas of emphasis. The first might be called direct

comparison through study of obvious acoustic data under comparable

environmental conditions. The second, or indirect, method involves a comparison of the resulting "regression equations" for environmental

dependence. For direct comparison we may compare the following in order of

complex! ty

a) Position(s) for maximum A-weigh ted level.

b) A-weighted sound level vs. position.

c) Spectra at positions of maximum A-weighted level.

d) Spectra vs. position for all positions.

Data for comparisons a) through c) will be routinely reported. Statistical

analyses of these data for quantitative hypothesis testing are sufficiently

complex that they must await later, more detailed, analyses.

Comparison of the "regression equations" will be permitted at a preliminary level since these equations and their confidence limits will be

recomputed after each field trip.

Evaluation of the validity of vehicle mounted near field microphone data

will be a major question during and after the first field trip. Field

evaluation will emphasize identification of dynamic range and bandwidth of

the actual signal, and the ability of the telemetry system to handle these,

as well as estimates of s tructureborne noise and evaluation of the vibration

isolation provided by the microphone supports. Later data analysis will

provide information on comparability of spectra from vehicle mounted and

ground mounted near field microphones during stationary tests, as well as wind and tire noise data from coastby measurements.

For preliminary analysis of environmental dependence, a total of eight

sets of A-weighted maximum noise data are expected to be analyzed for each

vehicle according to a statistical model similar to that described in

Appendix A. The noise data to be analyzed are expected to include:

a) Moving vehicle test data:

54
1 . Whole truck.

2. Fan noise* (determined from whole truck less truck without

fan) .

3. Exhaust noise* (determined from truck without fan less truck

without fan and exhaust).

4. Radiated noise* (i.e., truck without fan and exhaust),

b) Stationary test data:

1 . Moving microphone, whole truck

.

2. Moving microphone*, fan only

.

3. Near field sound power*, whole truck.

4. Near field sound power*. fan only.

*Determined for the time interval or position producing maximum "whole

truck" noise.

The environmental variables expected to be included in this analysis are mean

temperature and the component of wind speed perpendicular to the vehicle axis

at a selected elevation, relative humidity and ambient pressure. These

results will be presented in the form of "regression" equations and selected

graphs

.

4.3. Reports

In order to convey the breadth of the findings in a timely manner, three

meetings and seven written reports are planned (see overall test plan,

section 3.1)

.

The meetings are planned to coincide with significant events in the

experimental study as follows:

1. During the second week of field trip one, at the test site.

Principal topic: test plan and near field data on moving vehicles.

2. After data reduction from field trip three, at NBS. Principal

topic: test plan and data review.

3. After submission of draft final report, at MVMA headquarters.

Principal topic: review of draft final report and proposal for

detailed data analysis.

At the option of the Vehicle Sound Level Committee an additional meeting

may be held at NBS at the conclusion of data analysis from the last trip, to

55
review projected contents for the final report.

The written reports include trip reports at the conclusion of analysis

of data from the previous trip, for the five planned trips (an additional

report will be provided in the event that an optional trip is taken) . The

contents of a typical field report are expected to include:

1. Documentation of test plans and procedures if different from

previous reports.

2. Ranges in environmental variables occuring during that trip and to

date in the study.

3. Documentation of analytic and statistical analyses used in the

preliminary data reduction, if different from previous reports.

4. Results of the preliminary statistical analyses including all data

to date.

5. Projections of experiment sensitivity to detect changes due to

variation in the environment according to projections of

environment to be encountered.

6. Estimates of the magnitude of nonsystematic variations.

The two remaining reports are the final report and the proposal for

detailed data analysis. The final report is expected to represent an integration of the several trip reports plus the results of the preliminary

data re-analysis. (The data re-analysis is intended to confirm or modify the

statistical model through a study of the data ensemble for missing variables

(e.g., variables with significant correlation which were omitted from the

primary statistical model) and correlation among the environmental variables.

Either of these factors could lead to serious misinterpretations of the

regression equations.

The proposal for data analysis will document in detail the types of

problems to be studied and the scope of required effort for detailed analysis

which is briefly discussed in the following section.

5. DATA ANALYSIS

Since an overall goal (restated) of this study is to provide information

on the physical mechanisms by which the environment influences vehicle

driveby noise measurements, it is appropriate to mention, at this point, some

of the data analysis procedures which might reasonbly be used in such a study

.

A rational model of the total physical process, subject to verification

in this experiment is a differentiation into a time varying source of

acoustic power ("the" vehicle) which injects a signal into a time varying

56
multipath "transmission line" ("the" propagation path) which has a two pole

terminal at each J366b microphone position. Both the "source" and the

"transmission line" are subject to variation in their properties as a function of the environment. The near field data can be viewed

alternatively, as is necessary, as a estimate of the envelope of power input

to the "transmission line" or, when summed with phase and weighting

information to predict directivity, as additional two pole terminals at

alternate points in the "transmission line". To the extent that changes in

the environment can be characterized, and their influence on the source or path can be physically modeled, the effects can be regarded as systematic.

Similarly, to the extent that the variables are not characterized or not

modeled, the effects in terms of "explaining" the data are the same, i.e.,

non-systerna tic variation. As an example of the analysis program applied to

these data, we pick one model component — "the" path — and one environmental variable — wind — leaving all other environmental factors

constant, and pursue a thought experiment to see where this form of analysis

might lead. If we look only at mean wind speed and its variance at some elevation and observe altered levels at the J366b microphones, study of the

data might reveal little systematic and large non-sys tematic changes during

repeated experiments. A physical model is introduced for changes in the

propagation path which predicts that for a given change in effective sound

speed and a particular source spectrum shape, the expected systematic changes

due to a change in effective local sound speed are on the same order as the

observed variations. Further, the magnitude of non-systematic changes is

roughly correlated with the magnitude of wind speed variance. More refined

analyses incorporating wind gradient are included in the model and

re-analysis reveals that the new systematic variation is larger, and

non-systematic variation is consequently reduced. Studies of the J366b

microphone data over various source configurations might reveal that the

dependence is different for fan sources than for exhaust or radiated noise,

and that fan noise controls the whole vehicle data in many of the higher

frequency bands. Further, the regression equations are found to be similar

in each frequency band for which the fan is the controlling source. On the

basis of this information, a physical model incorporating source variation

dependent on relative wind speed into the fan is postulated and data are re-analyzed. More significant systematic variation is found, but significant

non-systematic variation still exists. Analysis of near field vehicle

mounted sound level data confirms the model at high frequencies for both

whole vehicle and fan only data. Study of the horizontal plane directivity

data indicates a modified directionality in the presence of transverse wind

and an increase in relative level proportional to on axis wind speed at all

points compared with no wind. Examination of the vertical directivity data

confirms the predicted changes in the envelope of "transmission function" at

several microphone heights, when gradient data are incorporated in the model

as systematic variations in local sound speed with elevation. Finally the

magnitude of the remaining non-systematic variation in sound level is found

to be positively correlated with the average magnitude of wind speed

fluctuations with time during the observation period.

We have described the process of confirming experimentally an analytic

model of wind effect that has vehicle source dependence on magnitude and

57
fluctuation of relative on axis wind speed, and path dependence on source

spectrum, transverse wind speed, gradient and fluctuation with a residual

unaccounted component. Thus, several sources of systematic variations have

been identified for the purpose of this experiment

.

Whether all these

variables are monitored in subsequent experiments or not is not significant

since that will only determine whether or not they are classed as resulting

in systematic or non-systematic variation for that observation condition. In

either case, judicious estimates of their magnitude will provide, through the

resultant "regression" equation, a basis for anticipating the magnitude of

the effects.

The methodology for the analyses as outlined in the above thought

experiment is expected to involve iteration processes in which a variation is

noted and physical mechanisms hypothesized. Experiments which test the

hypotheses are then "performed" by analysis of the data bank created from the

experimental phase. The objective of the analyses is to reduce the observed

non-systematic variation to a minimum by incorporating as much of the

existing information in the data bank as is necessary to "explain" the

observed variations. The tools to be used in the analyses will probably

include examination of changes in apparent directivity and vehicle

performance, use of narrowband frequency analysis for better source characterization, development of analytic models for path characterization

and vehicle performance changes, as well as many of the multivariate

statistical analysis procedures developed to date. A more detailed

discussion of the data analyses to be pursued and emphasized will depend on

the outcome of the preliminary data reduction which will provide an ordering

of the apparent relative importance of the variables.

58
6. APPENDIX A

DESCRIPTION OF A DATA REDUCTION MATHEMATICAL MODEL

It is difficult to describe the data reduction process in detail,

without referring to a particular model. A simple model of this analysis

will then be postulated for this discussion with no assurances that this is

in fact the final model to be used for this purpose.

Figure A-l shows a path diagram for the model. The x_^ in this diagram

represent the measured variables, and the e^ represent exogenous variables

(e.g., error terms and other unmeasured or otherwise independent sources of

variation), while the arrows represent apparent "connections" between the

variables. When the variations of the variables are expressed in

standardized form (i.e., zero mean and unit variance) then the "connections"

can be numerically evaluated as path coefficients (b . .

)

which are found from

the partial correlation coefficients (r. .) between th^ variables. Using this

model the structural equation for variation in sound level is of the form,

x

5 “ b

15

x

l + b25

X

2 + b35

X

3 + b

45

X

4 + b

55

e

5

'

In this equation, since it is for standardized variables, the coefficient b

gives directly the fraction of the total variation in x,_ (sound level) which

is not correlated with variation in the four variables which have been

included. If this coefficient is too large relative to the other

coefficients, it may potentially be reduced by the inclusion of other

variables (implying an expansion of the model), or by the inclusion of

additional data on the variables already included (i.e., more tests). The

correlations between the other variables can be used to indicate to what

extent they are independent of each other so that the significance of the

final structural equation can be evaluated. Either (or preferably both) of

two criteria can be used to determine when sufficient data have been

collected. The first criterion already mentioned is the magnitude of the

term The second criterion is the stability of the coefficients b . to

the addition of new data, or equivalently, the cross-validation of the^

equations with a new data set.

The structural equation can be reformulated in physical units to

resemble a regression equation through the relation of the physical variable

to the standardized form, i.e..

y

.

- y

= s

.

J x. , 1 11

where y is the average value of the set of physical measurements y., and s. is the standard deviation of that set. Thus, the variation in sound level

equation, presented earlier, would be given by,

VSL = s, 51

y

l +

52

y

2 +

53

y

3 +

54

y

4 + b

55

e

5 + K

59
x-j - TEMPERATURE

x2 = WIND

x3

= PRESSURE

x« - RELATIVE HUMIDITY

x, = SAE SOUND LEVEL

Figure A-l. Hypothetical model for preliminary reduction of the data to illustrate process.

60
where s., „ , and e^ are defined in the previous equations, VSL is the

variation in^sound level (dB) , K is an appropriate constant, and y. are the

appropriate physical variables. In this example, y^ would represent the

temperature (degrees celsius for example) and the coefficient.

S

5b

51

= c.

would have the dimensions of dB/°C.
7. REFERENCES

[1] Society of Automotive Engineers, Inc., J366b - Exterior Sound Level for

Heavy Trucks and Buses, SAE Recommended Practice (Society of Automotive

Engineers, Inc., Warrendale , Pa., 1973).

[2] Leasure, Jr., W. A., and Quindry , T. L., Methodology and Supporting

Documentation for the Measurement of Noise from Medium and Heavy Trucks,

NBSIR 74-517 (National Bureau of Standards, Washington, D. C., June

1974) .

[3] American National Standard Specification for Sound Level Meters,

SI. 4-1971 (American National Standards Institute, New York, N. Y., April

1971) .

[4] Society of Automotive Engineers, Inc., J184 - Qualifying A Sound Data

Acquisition System, SAE Recommended Practice (Society of Automotive

Engineers, Inc., Warrendale, Pa., 1970).

[5] U. S. Environmental Protection Agency Final Noise Emission Standards,

Motor Carriers Engaged in Interstate Commerce, Title 40, Code of Federal

Regulations Chapter I, Part 202, U. S. Federal Register J39(209) , 38208 - 38216 (October 29, 1974).

[6] U. S. Department of Transportation, Federal Highway Administration Final

Regulation, Compliance with Interstate Motor Carrier Noise Emission

Standards, Title 49, Code of Federal Regulations Chapter III, Part 325,

U. S. Federal Register ^0 (178), 42432 - 42441 (September 12, 1975).

[7] Baade, P. K., Sound Radiation of Air-Conditioning Equipment; Measurement

in the Free Field Above a Reflecting Plane, ASHRAE Transactions 70 , 217-227 (1964).

[8] Baade, P. K., Standardization of Machinery Sound Measurement, ASME Paper

69-WA/FE-30, presented at the ASME Winter Annual Meeting, Los Angeles,

California (November 1969).

[9] Morse, P. M., and Ingard, U., Theoretical Acoustics (McGraw-Hill Book

Company, Inc., New York, N. Y., 1968).

[10] Junger, M. C., and Feit, D., Sound, Structures, and Their Interactions

(The MIT Press, Cambridge, Massachusetts, 1972).

[11] Ingard, U., and Maling, Jr., G. C., On the Effect of Atmospheric

Turbulence on Sound Propagated over Ground, J. Acoust. Soc. Am. _35 (7),

1056-1058 (July 1963).

[12] Olson, N., Piercy, J. E., and Embleton, T. F. W., Variability in the

Noise Testing of Light Motor Vehicles, presented at the 91st meeting of

the Acoustical Society of America, Washington, D. C. (April 1976).

62
[13] Society of Automotive Engineers, Inc., J986a - Sound Level for Passenger

Cars and Light Trucks, SAE Recommended Practice (Society of Automotive

Engineers, Inc., Warrendale, Pa., 1970).

[14] Holmer, C. I., Procedures for Estimating Sound Power from Measurements

of Sound Pressure, Joint NBS-EPA Report EPA-550/8-76-001 (NBSIR 75-652)

(National Bureau of Standards, Washington, D. C., July 1975).

[15] Trott, W. J., Underwater-Sound-Transducer Calibration from Nearfield

Data, J. Acoust. Soc. Am. J)6, 1557-1568 (1964).

[16] Ferris, H. C., Computation of Farfield Radiation Patterns by Use of a General Integral Solution to the Time-Dependent Scalar Wave Equation, J.

Acoust. Soc. Am. 4J., 394-400 (1967).

[17] Kerns, D. M., Plane-Wave Spectra and Spectral Coupling Products for

Arbitrary Radiating and Scattering Systems, NBS Report 10-719 (National

Bureau of Standards, Boulder, Colorado, March 1972).

[18] Kerns, D. M., Plane-Wave Scattering Matrix and Generalized Reciprocity

Relations for Antennas and Scatterers, NBS Report 10-720 (National

Bureau of Standards, Boulder, Colorado, June 1972).

[19] Kerns, D. M., Scattering-Matrix Description and Near-Field Measurements

of Electro-Acoustic Transducers, NBS Report 10-745 (National Bureau of

Standards, Boulder, Colorado, July 1972).

[20] Van de Geer, J. P., Introduction to Multivariate Analysis for the Social

Sciences (W. H. Freeman and Company, San Francisco, 1971).

[21] International Organization for Standardization Draft Proposal for

Determination of Sound Power Levels of Noise Sources — Engineering

Methods for Free Field Conditions over a Reflecting Plane, ISO/DIS 3744

(American National Standards Institute, New York, N.Y.).

63
NBS-l 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC A HON OR RI-: PORT NO. NBSIR 76-1138

2. (iov't Accession

No.

3. Recipient’s Accession No.

4 . title: and subtitle:

A Proposed Experiment to Evaluate Environmental Dependence

of the Measurement of Noise from Medium and Heavy Duty

Trucks

5. Publication Date September 1976

6. Performing Organization Code

7. AUTHOR(S)

Curtis I. Holmer and Roger D. Kilmer

8. Performing Organ. Report No. NBSIR 76-1138

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2004571

11. Contract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

The Motor Vehicle Manufacturers Association

of the United States of America

320 New Center Building

netroit. Michigan 48202

13. Type of Report & Period Covered

Final,.. Phase 1

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less lactual summary of most significant in formation. If document includes a significant

bibliography or literature survey, mention it here.) Increasing concern with the impact of transpor tatio

noise on communities has led to an increase in the number of motor vehicle noise

regulations at all levels of government. As regulations become more widespread, the

iccuracy and precision of vehicle noise measurements become more critical since each

mcertainty in the measurement requires a corresponding increase in the margin that

nanuf ac turers must allow between the regulated noise limit and vehicle design levels.

Although considerable uniformity has been achieved by existing voluntary standards, there

emain significant variations between noise measurements made at different sites or at

f erent times on the same site. These variations are attributable to differences in th:

environment , including site and me terological influences. Thus, there is a need for

ystematic investigation of the various environmental and test site effects on noise

generation, radiation and/or propagation. This report presents the findings of a easibility study for an experiment to investigate environmental effects on truck noise

emission measurements. The result of this study is a proposed experiment designed to examine tne systematic and nonsystematic dependence of observed truck driveby noise on th 2 environmental effects present during the measurement. The experiment is designed in such

a manner as to permit separation of emission and propagation effects. Although it may no:

lie feasible to determine "correction factors" for environmental effects within the time

and financial constraints of this experiment, it is fully expected that the data obtained

rom this experiment will reveal the magnitude of variations in the measurement and

I revide a broad enough information base for study of the underlying physical mechanisms.

1.'. KEY WORDS (six to twelve entries; alphabetical order, capitalize only the first letter of the first key word unless a proper name; separated by semicolons ) Acoustics; environmental conditions; noise measurement; noise (sound); transportation

noise; truck.

18. A V A

1

-ABILITY l Unlimited 19. SECURITY Cl .ASS 21. NO. OF PAGES

(THIS REPORT)

1 '• or Official Distribution. Do Not Release o N 1 IS IJNCI. ASS1EIF. I) 67

I rdcr E rom Sup. of Doc., U.S. Ciovernment 1 Vint Of f i cr 20. SET UKITY ( .ASS 22. Price

V\ .1 sli in^ton D.( . 20402. SD ( ,u. No. ('ll ( 11IIS PAGE)

[x'u

Sirdcr From

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Virginia 2 2 1 5 1 Service (N 1 IS) UNC LA SSI El 1 D $4.50

USCOMM-DC 2 904 2- P 7 4
NOTE: This report in July 1976 by S.G. Fattal was provided to OSHA and assigned NBS IR 76-1 139; it was a restricted document for many years.

In 1998 Gail Hixenbaugh—while assisting another requestor seeking this document—received a copy of this document from OSHA and gave it to Karma Beal/NIST Archivist who shelved it with the Archive’s IR’s Harry Brooks 15Jun2K

2. In consideration of the findings by the National Bureau of

Standards research study, Report Number NBSIR 76-1 139,

Investigation of Guardrails for the Protection of Employees from

Occupational Hazards, dated July 1976, this instruction permits the application of some

applicable determinations in advance of the eventual promulgation

of a revision to Subpart D of 29 CFR 1910.

Source: OSHA Instruction STD 1-1.10 June 30, 1981

Office of Compliance Programming

p. 2

athttp://www.osha-slc.gov/OshDoc/Directive data/STD l -l_10.html