Naval Submarine Medical Research Laboratory

NSMRL Report #1222 23 October 2001

SENSOR-OPERATED HEADSET SELECTION FOR

VIRGINIA CLASS SUBMARINE CONSOLES

[3CI]

by

Joseph S. Russotti

and

Derek W. Schwaller

Released by:

M. D. Curley, CAPT, MSC, USN

Commanding Officer

Naval Submarine Medical Research Laboratory

______________________________________________________________________________

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23-10-2001

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technical

3. DATES COVERED (From - To)

01-01-2001 to 26-10-2001

4. TITLE AND SUBTITLE

Sensor-Operated Headset Selection for Virginia Class Submarine

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Russotti, Joseph S; Author

5d. PROJECT NUMBER

Schwaller, Derek W; Author 5e. TASK NUMBER

64559.50006

5f. WORK UNIT NUMBER

PMS450T2

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT

 NUMBER

Naval Submarine Medical

Research Laboratory

Naval Submarine Base New

London Box 900

Groton, CT 06349-5900

1221

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Naval Submarine Medical Resea NSMRL

rch Laboratory

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Groton, CT 06349-5900 NUMBER(S)

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A Approved for public release; distribution is unlimited.

13. SUPPLEMENTARY NOTES

This investigation was conducted for NAVSEA PMS 450T2 under Naval Sea Systems Command Task

Statement 64559.50006.

14. ABSTRACT

The current research evaluated various headsets for use on C3I (Command, Control, Communication and Intelligence

workstation) consoles installed in Virginia Class submarine. Of particular importance is their intended application in

accurately presenting not only communications but also passive acoustic sonar data and other future advanced auditory

displays that may use spatial coding. Of these applications accurate representation of broadband sonar data be-comes the

most challenging immediate task.

Headset evaluation of commercial off-the-shelf products confirmed our decision to press for development of high fidelity

ANC sensor operator headsets. This study reports evaluation of a prototype ANC high fidelity headset developed to our

specification for use in critical listening in a moderately noisy environment.

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Ronald Joe

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UNCLASSIFIED

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860-694-5157
Sensor-operator headset selection for

Virginia Class Submarine Consoles

[C3I]

by

Joseph S. Russotti and Derek W. Schwaller

NAVAL SUBMARINE MEDICAL RESEARCH LABORATORY

Report No. 1222

Approved and released by

M. D. CURLEY, CAPT, MSC, USN

Commanding Officer

NavSubMedRschLab
SUMMARY PAGE

Problem

With renewed interest in presenting relevant acoustic information to the 3CI (Command, Control

Communication and Intelligence workstation console) sensor operator including the broadband-search

sonar operator, headset selection continues to present a problem due to the degree of noise in the room

and its spectral content.

Findings

Conventional noise-occluding (closed shell) headsets were designed for communication and are

appropriately band-limited in frequency response to optimize speech intelligibility. Closed-shell earpieces

confound accurate sound reproduction due to cavity resonance and interaction between frequency

response and seal against the head. High-fidelity headsets, which extend frequency-response accuracy

to well beyond the speech range, avoid using noise-occluding closed shell designs. As a result they

provide little noise attenuation at necessary frequencies. Active noise cancellation (ANC) headsets can be

designed to solve both the noise reduction and bandwidth response, but, since currently marketed for

communication, have somewhat limited frequency response accuracy beyond the speech range. At-sea

measurements of the airborne noise at the location of the operator’s head reveal that its’ low-frequency

spectral content can severely interfere with operator detection performance. A high-fidelity ANC headset

is the least expensive solution to the noise interference problem and is also an immediate one. The best

solution would be to reduce the noise at its source. More efficient command and control designs, as

found on Virginia Class, further exacerbate the noise problem as common control areas increase the

concentration of hardware ventilation fans and console operators. Evaluation of commercial off-the-shelf

closed-shell high fidelity and studio monitor headset products confirmed our decision to press for

development of high fidelity ANC sensor operator headsets. This study reports evaluation of a prototype

ANC high fidelity headset developed to our frequency-response specification for use in critical extendedbandwidth listening in a moderately noisy environment. The prototype headset was developed by Bose

in interaction with the Naval Submarine Medical Research Laboratory. This extended-bandwidth

prototype is based upon design modifications to the Bose Series X commercial aviation ANC headset.

Based upon prior at-sea evaluations of similar earlier models, this advanced version is essential for use in

console spaces.

Application

Design of sonar signal-processing equipment for optimal human auditory discrimination.

Administrative Information

This investigation was conducted for NAVSEA PMS 450T2 under Naval Sea Systems Command

Task Statement 64559.50006. The views expressed in this report are those of the authors and do not

reflect the official policy or position of the Department of the Navy, Department of Defense, or U.S.

Government. This report was approved for publication on 23 October 2001, and designated at Naval

Submarine Medical Research Laboratory Report No. 1222.

ii
Sensor-operator headset selection for

Virginia Class Submarine

Consoles

[C3I]

by

Joseph S. Russotti and Derek W. Schwaller

NAVAL SUBMARINE MEDICAL RESEARCH LABORATORY

Report No.1222

Approved and released by

M.D. Curley, CAPT, MSC, USN

Commanding Officer

NavSubMedRschLab
ii

SUMMARY PAGE

Problem

With renewed interest in presenting relevant acoustic information to the C3I (Command,

Control Communication and Intelligence workstation console) sensor operator including the

broadband-search sonar operator, headset selection continues to present a problem due to the

degree of noise in the room and its spectral content.

Findings

Conventional noise-occluding (closed shell) headsets were designed for communication

and are appropriately band-limited in frequency response to optimize speech intelligibility. 

Closed-shell earpieces confound accurate sound reproduction due to cavity resonance and interaction between frequency response and seal against the head. High-fidelity headsets, which

extend frequency-response accuracy to well beyond the speech range, avoid using noiseoccluding closed shell designs. As a result they provide little noise attenuation at necessary

frequencies. Active noise cancellation (ANC) headsets can be designed to solve both the noise

reduction and bandwidth problem but, since currently marketed for communication, have

somewhat limited frequency response accuracy beyond the speech range. At-sea measurements of the airborne noise at the location of the operator’s head reveal that its' low-frequency

spectral content can severely interfere with operator detection performance. A high-fidelity

ANC headset is the least expensive solution to the noise interference problem and is also an

immediate one. The best solution would be to reduce the noise at its source. More efficient

command and control designs as found on Virginia Class further exacerbate the noise problem

as common control areas increase the concentration of hardware ventilation fans and console

operators. Evaluation of commercial off-the-shelf closed-shell high fidelity and studio monitor

headset products confirmed our decision to press for development of high fidelity ANC sensor

operator headsets. This study reports evaluation of a prototype ANC high fidelity headset developed to our frequency-response specification for use in critical extended-bandwidth listening

in a moderately noisy environment. The prototype headset was developed by Bose in interaction with Naval Submarine Medical Research Laboratory. This extended-bandwidth prototype is

based upon design modifications to the Bose Series X commercial aviation ANC headset. 

Based upon prior at-sea evaluations of similar earlier models, this advanced version is essential

for use in console spaces.

Application

Design of sonar signal-processing equipment for optimal human auditory discrimination.

Administrative Information

This investigation was conducted for NAVSEA PMS 450T2 under Naval Ships Systems

Command, Task Statement 64559.50006. The views expressed in this report are those of the

authors and do not reflect the official policy or position of the Department of the Navy, the Department of Defense, or the US Government. This report was approved for publication on October 23 2001, and designated as Naval Submarine Medical Research Laboratory Report No

1222
iii

OPERATIONAL ABSTRACT

In trying to select an accurate method of presenting acoustic information to submarine console-operators, and in particular the broadband-search operator, we are faced with a problem. 

The room is noisy. From in-situ tests of the acoustic output of headphones of all types, it is

clear that any noise-occluding headset currently manufactured has a band-limited frequency response. Such band-limited headsets are specifically designed for communications. Because a

noise-occluding headset requires a complete seal around the ear, headband pressures required

for such a seal are extremely uncomfortable in extended wear. Once the headset has been

specifically designed to work with a seal, the seal must be maintained or the low-frequency output inside the headshell is degraded. The inability to consistently maintain that seal with each

repeated placement over the ears, creates a major critical loss of low frequency signal output.

Since conventional passive noise attenuation used in headsets is less effective at low frequencies, that critical loss of low frequency information in the received signal, caused when proper

seal is lost, is even more deleterious to detection. With reduced low-frequency output from the

earphone, the airborne low-frequency components, which pass through the headshell, mask the

relevant signal to be detected.

Commercial off the shelf (COTS) circumaural (around the ear) closed-shell headsets of excellent fidelity have been selected for the BSY-2 sonar system. But, high fidelity headsets have

not been designed to attenuate noisy environments, therefore none attempt to tackle the formidable problem of maintaining a noise occluding complete seal against the head. Active noise

cancellation (ANC) headsets circumvent the earcushon seal and headshell attenuation problem

by acoustically monitoring the non-signal related noise inside the headshell and creating its inverse to actively cancel it. Also, because of their active electronics, ANC headsets can be

equalized to reproduce accurately. ANC headsets solve this noise problem and dramatically

improve detectability.

Evaluation of COTS closed shell headset products confirmed our decision to press for development of high fidelity ANC sensor operator headsets. This study reports evaluation of a

prototype ANC high fidelity headset developed to our frequency-response specification for use

in critical extended-bandwidth listening in a moderately noisy environment. This extendedbandwidth prototype is based upon design modifications to the Bose Series X commercial aviation ANC headset. The modified Series X provided the best combination of frequency response

accuracy and sound attenuation of sonar shack noise. Based upon prior at-sea evaluations of

similar earlier models, this advanced version is essential for use in console workstation spaces.
iv

ABSTRACT

The current research evaluated various headsets for use on C3I (Command, Control,

Communication and Intelligence workstation) consoles installed in Virginia Class submarine. Of

particular importance is their intended application in accurately presenting not only communications but also passive acoustic sonar data and other future advanced auditory displays that may

use spatial coding. Of these applications accurate representation of broadband sonar data becomes the most challenging immediate task.

With renewed interest in presenting relevant acoustic information to the broadband-search,

and workload share operators as well as to the sonar supervisor, headset selection presents a

problem. The listening environment is somewhat noisy, especially in lower listening frequencies. From in-situ tests of the acoustic output of headphones of all types, it is clear that any

passive noise-occluding headsets are band-limited. Characteristically noise-occluding headsets

are designed for use in communications where limited bandwidth (less than the full hearing

range) is tolerable and cost effective. Noise-occluding headsets, which seal against the head to

reduce noise, must maintain their seal or suffer a critical loss in low-frequency output. High fidelity headsets, on the other hand, are of open or vented design to avoid this seal problem, but

as a result suffer from poor noise-attenuation. Active noise cancellation (ANC) headsets circumvent the headset-seal problem by electronically canceling unwanted noise and can also be

internally equalized for accurate frequency response. These headsets are more expensive

than conventional designs, but they achieve a dramatic improvement in at-sea detection performance.

Headset evaluation of commercial off-the-shelf products confirmed our decision to press

for development of high fidelity ANC sensor operator headsets. This study reports evaluation of

a prototype ANC high fidelity headset developed to our specification for use in critical listening

in a moderately noisy environment.

.
DRAFT COPY

1

INTRODUCTION

Accurate representation of an acoustical signal has been a long desired goal of both acoustic signal analysts and audio engineers engaged in sound reproduction. For the former group the

task is simpler. An accurate transfer of acoustic to electrical energy is the requirement for signal

storage and signal analysis. But, for sound reproduction, the process of signal storage must be

followed by a far more difficult task. Namely, the recreation of the original acoustical signal into

an acoustic space that is most likely substantially different from the one in which the original signal was generated. The simplest way to avoid the confounding of room acoustics with sound reproduction is through the use of headsets. In the case of passive acoustic information gathered

from the ocean it is essential that all of the available energy, that can be received, be monitored

so that any man made energy generated can be detected. Broadband monitoring is necessary,

since this energy to be detected is of unknown frequency. Its radiated energy is not only speed

related but is modified by many physical characteristics such as frequency dependent propagation loss, multiple paths from source to receiver caused by reflection, effects of salinity, temperature, depth and other factors. However well the acoustic signal is gathered and converted to

an electrical signal, accurate reproduction of that received acoustic signal is equally critical.

For our particular application in accurately presenting acoustic information, especially sonar

target information, in the confined and hardware-cluttered space of a military vessel, headsets

are ideal. Although the task of accurate sound reproduction now becomes simpler, it is still a

formidable one. The first major obstacle now becomes the evaluation of headset frequency response so that we can predict the sound pressure level for a given voltage at any specified frequency. Given the added requirement of listening in a noisy environment, an around-the-ear

(circumaural) headshell is necessary. Shaw and Thiessen (1962), Shaw (1966), and others

found that standard headphone coupler measurements did not represent sound pressure measurements taken inside headphone headshells using calibrated probe-tube microphones. Based

on these findings, a report by the United States of America Standards Institute Writing Group S3-

1-W-37 on the coupler calibration of earphones (Benson et al 1967) concluded it could not justifiably write a standard for the coupler calibration of circumaural headphones.

Russotti et al (1998) devised a technique which accurately measures headphone response

when acoustically loaded by an ear-simulator and referenced to the diffuse free-field. The ideal

headset reproduces the information without imparting any alteration in the original signal. Using

this technique, measurements of both military and commercial off-the-shelf (COTS) high-fidelity

headsets were taken and recommendations were made for headsets for application in passive

aural sonar. In experience gained over 15 years of testing headsets using this measurement

technique, open-air circumaural headsets and closed headsets having a controlled pressure-leak

were found to produce the most consistent and most accurate frequency response. Headsets

designed to completely seal around the listener’s ear produce large variations in their low frequency response due to a less than perfect seal. Differences in head size, gaps between the

mandible and neck, and presence of hair and eyeglass temple-pieces all can contribute to a less

than perfect seal. As a consequence, passive noise-occluding headsets, which require a good

seal for noise attenuation, are not a desirable design for accurate sound reproduction. Tests of

military and commercial headsets have found that noise-occluding headsets are also characteristically band-limited (Russotti, 1995). Such headsets were originally designed for communica-
DRAFT COPY

2

tions and are cost effectively band-limited to the speech range for that application. Despite the

superior response accuracy of commercially available high-fidelity headsets, in selecting headsets for use in the noisy environment on all but the newest quieter sonar suites such as BSY-2

(Commanding Officer, Naval Submarine Medical Research Laboratory 1990) such high fidelity

headsets could not be recommended. Because of their poor noise attenuation characteristics,

ambient noise levels masked the signal in the headset. Instead, custom modified Active Noise

Cancellation (ANC) headsets were designed to produce more accurate frequency response. 

These frequency-enhanced versions were recommended because of their demonstrated ability,

during at-sea tests, to enhance detectability of contacts (Commanding Officer, Naval Submarine

Medical Research Laboratory, 1993).

Noise measurements (Russotti, 1998; Heller et al, 2000) clearly document the spectral content and intensity of airborne-noise in BQQ-5 and BSY-1 sonar spaces. The measurement system, used in place of a traditional sound level meter, complies with ANSI S1.4-1983 and S1.6-

1984. Figure 1 presents a graphic comparison of differences between BQQ-5 and BSY-1 sonar

control room noise. Root mean square (RMS) averages of the energy measured in 1/3-octave

bands are depicted. Time-averaged sample-duration was approximately 30 seconds. All measurements are depicted re 20 μ Pascal. Comparisons are shown in 1/3-octave bands from 25 Hz

to 20 kHz. At far right, dBA and overall dB SPL are represented as A and SPL respectively. The

overall RMS, A-weighted, and 1/3-octave band data were averaged across data gathered at the

operator’s Command/Display Consoles (CDCs) and at the position of the sonar supervisor.

Although within safe noise levels for hearing conservation, BSY-1 and BQQ-5 systems have

excessive noise in sonar operator work-areas where critical listening is required. There is significant low-frequency energy below 1 kHz. As seen in Figure 1, bands with center frequencies of

Sonar Control Room Noise

0

10

20

30

40

50

60

70

80

90

25

40

63

100

160

250

400

630

1K

1.6K

2.5K

4K

6.3K

10K

16K

A

1/3 Octave-Band Center-Frequency (Hz)

dB SPL

BSY-1 BQQ-5

SPL

Figure 1. Spectral representation of sonar compartment on BQQ-5 and BSY-1 systems
DRAFT COPY

3

400, 630 and 800 Hz are above 60 dB SPL on both systems. For BQQ-5 there is especially unacceptable low-frequency noise in the region below 160 Hz. At those frequencies, and in fact at

all frequencies below 500 Hz, attenuation available from conventional passive noise reducing

headsets is greatly reduced and inadequate.

The presence of this high level of airborne low-frequency noise in the listener’s ear, when

wearing a passive noise reducing headset, means that there is inadequate dynamic range above

the noise floor at normal listening levels. Since the overall level of the sea noise signal in the

headset must be kept at a reasonably comfortable and safe level, these constraints result in airborne noise masking the low-frequency component of the target signal to be detected within the

sea noise. This explains why previous reports (Russotti, 1993; Benedetto et al, 1995; Russotti,

1995) found improved sonar operator performance with ANC headsets. Should the target of interest have most of its radiated energy in these lower frequencies, the decrement in performance

caused by the interference of this low-frequency airborne noise becomes even more detrimental.

ANC communications headsets use real-time techniques to remove unwanted acoustic signals that have passed through the headshell. A microphone inside each headshell provides a

monitor signal, which is electrically compared against the headphone input-signal. The difference is inverted and added to the electrical input to cancel the unwanted energy inside the headshell. Inherent in such unique design is the potential to correct for diminished output due to a

poor seal and the capability of enhanced frequency response through active equalization. Interactive work between Bose Corporation and NSMRL has produced a modified version of their

ANC commercial aviation headset with enhanced frequency response for use in passive sonar. 

These headsets (Bose Series I commercial aviation headset-[nsmrl prototype]) have been tested

at-sea with highly favorable results and commensurate acceptance by the sonar community

(Russotti 1993, Russotti 1995, Benedetto, et. al. 1995, Commanding Officer USS San Juan

1993, Commanding Officer USS Albuquerque 1995). Subsequently the Bose Series II Aviation

headset in standard form exhibited even more accurate response than our custom Series I

model. As a result we recommended the Bose Series II COTS version. Unfortunately, the Series I and Series II models have been supplanted by the Series X Aviation headset, a superior

design that is a more comfortable, more durable headset, but of lesser broadband fidelity. In

COTS version this headset did not exhibit the necessary fidelity, exhibiting a 19 dB variation over

the 40Hz to 10kHz frequency range. The earlier Bose Series II model exhibited a 9 dB variation

in this range.

Since our needs for a replacement headset were immediate, we renewed our search for

newer headsets, both ANC and conventional, and simultaneously began work with Bose to pursue modifying their ANC electronics to enhance broadband fidelity of the Series X model. Fidelity criterion was: lowest amplitude variation measured across the 40 Hz to 10 kHz frequency

range.

The present research task was undertaken to identify ANC headsets of appropriate fidelity to

effectively reduce low-frequency interfering noise, which cannot be reduced using conventional

headsets. Given the concentration of console operators and hardware, noise reduction was essential for critical sonar listening. However, meeting hearing protection requirements was not of

concern. Relative measures of low frequency headset attenuation would be adequate.
DRAFT COPY

4

HEADSET RESPONSE

METHOD

The measurement technique we devised in 1985 and proposed for use in earphone calibration in 1986 uses a laboratory type Zwislocki ear simulator which includes multiple cavities to

model the acoustic load that an average human wearer would place on the earphone element. 

Impedance measurements of human ears by Zwizlocki (1957), Ithell (1963a, 1963b) and Delaney

(1964) lead to development of several ear simulators. Zwislocki's (1970, 1971) easily replicated

device successfully simulated the complex impedances found in average human ears. In standard form, this coupler uses a machined surface and fifth resonant cavity to simulate the external

ear (or pinna). In developing a test and evaluation tool for hearing aid performance, Burkhard

and Sacks (1975) incorporated the eardrum simulator portion of the Zwislocki coupler into the

anthropometrically average manikin KEMAR. They accurately substituted flexible pinnae and

metal ear canals for the corresponding portions of the Zwislocki coupler. Acoustic measurements on this version of the KEMAR manikin are in close agreement with similar measurements

on human subjects (Burkhard, 1975), and the KEMAR manikin now conforms to ANSI (1985)

standards intended for airborne sound measurement.

Figure 2. Headphone under test on KEMAR manikin
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5

Figure 2 shows the KEMAR manikin as used in our application. The measurement procedure is outlined in detail in Russotti et al., 1988. The Zwislocki coupler, modified in the KEMAR

manikin, has decided advantages for headset evaluation over a hard-surfaced machined plate, in

that any wearable headset can be tested. Headset design should be free from coupler imposed

constraints. For a real ear, and also for the simulator, the airborne acoustic signal that arrives at

the eardrum has had its frequency response modified by the external ear structure, by resonance

created by the pinna, and by the complex loading of the ear canal and eardrum with its ossicular

chain. For practical use a conversion function is necessary to relate the received signal at the

eardrum back to the airborne sound environment. This converted response should correctly reference the signal measured in the coupler back to the external sound field. If the original airborne signal had equal sound pressure across frequency then the properly converted transformation of the signal measured at the "eardrum" should produce this same flat response. If instead of airborne presentation a headset is used, then it too should re-create this flat response. 

By referencing the signal back to the airborne sound field we can evaluate how well the headset

recreates the original airborne sound the ear would have heard. In practical terms if a constant

voltage signal is supplied to the headset over the frequency range appropriate for the ear simulator, the transform-corrected response measured by the ear simulator should ideally be a

straight line.

The required conversion function shown in Figure 3 references the earphone element response back to the diffuse field. This transformation is the response of the human ear, or in this

case the manikin-mounted ear simulator, without regard to any one direction. Diffuse field

measurement removes directionality created by the head and pinna from the transformation. 

Should directionality need to be coded into the presentation then head related transform functions (HRTFs) can be imposed onto the electrical signal presented to each ear. Earmuff shape,

size, seal, headband effectiveness and placement of headset on the head and against the ear

are all major contributors to the variability one finds in earphone response measurements. Our

technique samples these variables taking 5 measurements each, of 4 earphone elements. The

headphone is removed and repositioned for each of the 20 measurements. All of the x y plots of

sound pressure as a function of frequency are stored using an A/D converter. They are averaged and the diffuse-field conversion function applied

Figure 3. Conversion function necessary for diffuse-field transformation
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6

As a comparison, the upper left curve in Figure 4 shows a prototype supra-aural (on the ear)

earphone tailored to have flat response on a standard 6cc ANSI volumetric coupler. Below it is

the averaged response of the same earphone element measured on the ear simulator. This

lower curve is the diffuse-field corrected response of the earphone measured with appropriate

acoustic impedance coupled to the earphone output. Note the huge difference in measured response with proper acoustic load placed on the headphone.

Figure 5. Averaged diffuse-field response of Bose Series II Aviation headset.

Figure 4. Earphone response on ANSI 6cc coupler and as measured loaded by an ear simulator.

Bose Series II

Aviation Headset
DRAFT COPY

7

Figure 5 plots the diffuse field response for the Bose series II Aviation headset. These data were

collected in previous research (Russotti, 1998; Heller et al, 2000). Frequency response measurements are relative to 80 dB SPL headphone output at 1kHz. Using the previously outlined

measurement procedure, the specimen model had a total response variation of 9 dB over the 40

Hz to 10 kHz frequency range. In fact, in terms of frequency response variation in the 40 Hz to

10 kHz range, it is the most accurate headset of any type that we have ever tested.

Figure 6 plots the diffuse field response of the COTS Bose Series X headset which replaced

the Series II model. Despite major improvements in headset and earcushon design to improve

comfort durability and weight, the diffuse field response, while adequate for its intended application of communication, fell far short of its predecessor for our application. Following the standard

procedure, the specimen model had a total response variation of 19 dB over the 40 Hz to 10 kHz

frequency range.

Previous distortion measurements using ear simulators have shown that headsets of appropriate frequency response accuracy have extremely low distortion that is close to the limits of the

necessary measurement hardware (Russotti et al 1985). In that study, at 95 dB SPL nine of the

11 top models had distortion levels of less than 0.1%. As a criterion for headset accuracy then,

smallest variation in acoustic output in the 40 Hz to 10 kHz frequency range was the design goal,

given the unknown and potentially changing spectral composition of the signal to be detected. 

Accurate reproduction of all energy in the 40 Hz to 10 kHz bandwidth would allow accurate representation of the signal to the ear. As a result of the unacceptably reduced frequency response

accuracy found on the Series X replacement for the Series II ANC aviation headset, it was necessary to reassess COTS available models to determine if a substitute was available. Simultaneously Bose was contacted to determine feasibility of improving frequency response of the Series X replacement for the discontinued Series II.

Figure 6. Averaged diffuse-field response of Bose Series X Aviation headset
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8

FREQUENCY RESPONSE RESULTS

ANC and high fidelity closed headset models from various manufacturers were evaluated for

their diffuse field response using the methodology described earlier. Only three previously untested closed-shell audio-quality models could be found.

Figure 7. Averaged diffuse-field response of Sennheiser closed circumaural headset eH2200

Figure 8. Averaged diffuse-field response of Sennheiser closed circumaural headset eH2270

Sennheiser eH2270

Sennheiser eH2200
DRAFT COPY

9

Figures 7, 8 and 9 represent the diffuse field response characteristics of 3 current closedshell audio headsets that were evaluated for potential use in less critical listening applications or

in quieter areas. Of the three, only the Beyer DT 770 exhibited an appropriate frequency response. Total response variation for this model was 14 dB from 40 Hz to 10 kHz. Noise attenuation measurements would determine potential suitability for passive sonar broadband listening applications.

Beyer DT770 Pro

Figure 9. Averaged diffuse-field response of Beyer DT770 Headset

Figure 10. Averaged diffuse-field response of Sennheiser MDR NC-20 ANC headset

Sony MDR-NC 20
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10

Figure 11. Averaged diffuse-field response of Sennheiser HMEC 300 ANC headset.

Figure 12. Averaged diffuse-field response of Bose OuietComfort Consumer ANC headset

Bose QuietComfort

Consumer ANC

Sennheiser HMEC 300
DRAFT COPY

11

Other than the Bose Series X, only three appropriate non-communications ANC headsets

could be found. Figure 10 plots the diffuse-field response of an ANC headset, the Sony MDR

NC-20. Response in the 1 to 10 kHz range is severely deficient and not appropriate for

broadband signal reproduction. Figure 11 depicts the averaged diffuse field response of the

Sennheiser HMEC 300 ANC headset. In the 500 Hz to 3 kHz region it varies by 21 dB.

Figure 12 graphs the averaged diffuse -field response of a new Bose consumer ANC headset. This model had a total variation of 21 dB in the 40 Hz to 10 kHz range. Physically, the

COTS headset designed for consumer use would not withstand the rigors of 24 hour daily use in

a shipboard environment.

None of these COTS ANC models, including the Bose Series X Aviation headset (Figure 6),

exhibited a response that would allow recommendation as an appropriate headset for use on Virginia class C3I consoles. Of the available models, the Bose Series X headset was clearly a far

more rugged model with decided improvements over its predecessors in weight and overall design. As a consequence, it alone had potential application, provided that internal equalization

could produce an acceptable response. Previous Bose models had been required to use greater

headband tension for noise reduction and depended upon gel filled earcushions to achieve

sound isolation and comfort. These Series I and Series II headshells were heavily laden with

electronic components, whose added weight reduced comfort in extended wear despite the superior fit and comfort of the ear cushions on the wearer. Further, the gel cushions required care to

maintain their integrity and periodic replacement was necessary. The Series X had successfully

eliminated all of these drawbacks. Once worn on the head, reverting back to older models was

not a desirable option for the wearer. Following our measurements, Bose was contacted and in

response to our request for a flatter frequency response, board level modifications were attempted. As an outcome of this interaction, a frequency-enhanced Series X prototype was developed and two samples were tested at NSMRL. Averaged results of diffuse-field response

testing are presented in Figure 13. 

Figure 13. Averaged Diffuse-field response of Bose Series X Submarine Virginia [SVX] prototype.
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As seen in the figure, the total variation of the Bose/NSMRL prototype is 13 dB from 40 Hz to

10 kHz, and 9.5 dB from 100 Hz to 10 kHz. This is a vast improvement from the COTS model.

HEADSET NOISE ATTENUATION

METHOD

Because anticipated noise levels in sonar spaces would be well below damage risk criteria,

standards for hearing protectors (ANSI S12.6-1997) were not required. In addition ANSI measurements do not assess the attenuation characteristics below 125Hz. Objective 1/3 octave band

measurements of the intrinsic ability of the noise-attenuating headsets to reduce noise were conducted using a specially modified KEMAR manikin. Pliant bags of lead shot placed within the

manikin were used to block sound transmission to the calibrated microphone “eardrum” from

pathways other than the ear canals. Electroacoustic test results confirmed the ability to attenuate such transmission at 40 dB or greater from 160 Hz to 500 Hz, 50 dB or better from 500 Hz to

2 kHz, and 60 dB or more from 2 to 15 kHz. Manikin measurements do not take into account the

range of real-ear bone conduction pathways that reduce noise attenuation. However, they allow

accurate evaluation of the relative attenuation characteristics available from a particular headset

design over the entire 40Hz to 10 kHz measurement range afforded by the ear simulator. 

A high-intensity sound system was instrumented to operate within a 30 x 16.5 x 11 foot high

cement-block-walled reverberant room. The system consisted of four speaker arrays, each

containing two 18 inch drivers, a 15 inch driver, and one 4 inch diameter titanium horn-driver. 

Each transducer was independently powered, by a configuration of Crown amplifiers capable of

producing 1,310 watts RMS in each of 4 channels with Total Harmonic Distortion (THD) at less

than .02% at rated power. Each channel of the system was fed by a separate channel of a

Digidesign Pro Tools III digital hard disk recorder/editor. Four analog output channels of a

Digidesign 888 interface were distributed to the amplifiers of each array through a pair of Rane

AC-23 active crossovers. Just ahead of the AC-23 inputs, each of the channels was digitally

controlled by a separate Wilsonics model PATT attenuator. Using the Digidesign system, each

of four pink noise signals were fed through the separate speaker arrays to produce a

homogeneous sound field of 94 dB SPL re 20 micro (μ) Pascal. Sound level measurements

were taken without the manikin present at the location that would be occupied by the manikin

head.

Ear simulator output was spectrally analyzed using a Data Physics ACE signal analyzer. 

Headset noise attenuation was calculated as the relative difference between open ear and ear

occluded by the headset under test. Root mean square (RMS) averages of the energy measured in 1/3-octave bands were calculated. These represent the relative attenuation afforded by

the headset device under test.

NOISE ATTENUATION RESULTS

Figure 14 represents the noise attenuation capabilities of the Bose Series X ANC headset

compared to the Beyer DT770 headset. As seen in the figure, the passive attenuation available

from the Beyer DT770 closed circumaural headset is insufficient for our application. In fact, in

1/3 octave bands below 500 Hz the headshell actually magnifies the external sound. In comparison, the Bose series X headset has far greater capability in reducing the transmission of external

noise into the headshell. At 40 Hz center frequency, there is a reduction of external noise of 18
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dB, and with the exception of the 1/3 octave band centered at 63 Hz, there is a 20 dB or greater

attenuation at all other bands measured. From the 160 Hz to 10 kHz band, the noise attenuation

afforded by the ANC is approximately 24 dB or better. 

For reference purposes similar measurements were made on the David Clark 12507G-20

federal stock headset used in passive sonar systems. Because they have become popular in the

sonar community, results are also presented for the COTS Sony MDR-V600. Both closedheadset models use only passive noise reduction. Results of noise attenuation measurements of

Noise Attenuation in a Diffuse Sound-field

-10

-5

0

5

10

15

20

25

30

35

40

45

40506380

100125160200250315400500630800

1.0k

1.25k

1.6k2.0k2.5k

3.15k

4.0k5.0k6.3k8.0k

10.0k

1/3 Octave Band Center Frequency [Hz]

Attenuation in dB

Beyer DT770

Bose Series-X

Noise Attenuation In a Diffuse Sound- field

-10

-5

0

5

10

15

20

25

30

35

40

45

405063801001251602002503154005006308001.0k1.25k1.6k2.0k2.5k3.15k4.0k5.0k6.3k8.0k10.0k

1/3 Octave Band Center Frequency [Hz]

Attenuation in dB 

Bose Series-X

David Clark

Sony MDR-V600

Figure 14. Headset noise attenuation of Bose Series X vs. Beyer DT 770.

Figure 15. Noise attenuation of Bose Series X vs. David Clark 12507G-20 and Sony MDR V600 headset.
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both of these headset models are presented in figure 15. Given the spectral content of the airborne noise in console spaces, these headsets clearly place the listener at a great disadvantage.

 The high levels of lower frequency airborne noise, which pass through the headshell, effectively

mask the signal presented in the headset.

SUMMARY

Newer extended bandwidth afforded by modern A/D converters has enhanced the utility of

broadband search and aural tracking operations. While headsets with appropriate bandwidth are

essential they are of limited use unless they can effectively present the entire signal to the listener. Given the spectral content of airborne noise levels measured on current sonar suites, only

ANC headsets can effectively mask the lower frequencies. The presence of such low-frequency

energy interferes with critical listening rendering the best closed-headshell professional studio

monitor headsets useless. While most ANC headsets have been cost-consciously designed for

the limited bandwidth of communications, the active electronics available in ANC designs allows

for electronic enhancement of frequency response characteristics. Part of NSMRL's research

effort has been, in interaction with Bose, to provide an extended bandwidth or high-fidelity ANC

headset for use in critical listening. Early Series I NSMRL prototype and Series II production

models met that goal but were substantially heavier than conventional headsets. In addition specialized silicone gel filled ear cushions required periodic replacement. Tests of other ANC headsets conducted in the present study found none with appropriate frequency response. Current

tests of passive attenuation headsets revealed adequate though not exceptional frequency response. Previously available COTS models had exhibited lower variability. However none of the

current or previously manufactured passive attenuation models were effective in reducing the intrusion of low frequency noise into the headshell. The prototype Submarine Virginia Series X

ANC headset, due to its noise attenuation and frequency response bandwidth meets the needs

of critical listening in spaces containing low frequency equipment noise. Given the concentration

of operators and console hardware anticipated on future systems, these headsets can also serve

to reduce speech interference from adjacent operators. These specialized headsets are also essential for Advanced Rapid COTS Insertion (ARCI) and should be incorporated into the procurement system. Application of these headsets for surface naval operations is also strongly recommended.
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REFERENCES

American National Standard Method for Coupler Calibration of Earphones." ANSI

S3.7-1973 (R1986), American National Standards Institute, New York, 1973.

"American National Standard Specification for a Manikin for Simulated In-Situ Airborne Acoustic Measurements." ANSI S3.36-1985 (R1990). American National

Standards Institute, New York, 1985.

"American National Standard Methods for Measuring the Real-Ear Attenuation of

Hearing Protectors" ANSI S12.6-1997, American National Standards Institute,

New York, 1997.

Benedetto, J. L., Russotti, J. S., Harvey, J. T., and Hanna, T. E. (1995). At-sea tests

of headphones and active filters (NSMRL Report 1195). Groton, CT: Naval Submarine Medical Research Laboratory.

Benson, R. W., Charan, K. K., Day, J. W., Harris, J. D., Niemoeller, A. F., Rudmose,

W., Shaw, E. A., and Weissler, P. G. (1967) Limitations on the Use of Circumaural Earphones. Journal of the Acoustical Society of America 41, 713-714.

Burkhard, M. D., and Sachs, R. M. (1975). Anthropometric Manikin for Acoustic Research. J. Acoust. Soc. Am., 58, 214-222.

Commanding Officer, NSMRL (1990). Sonar Headset Evaluation and Recommendation. Naval Submarine Medical Research Laboratory ltr. NSMRL 3960, Ser

50/459 dtd 13 April 1990.

Commanding Officer, NSMRL (1993). Sonar Headset Evaluation and Recommendation for ACINT use. Naval Submarine Medical Research Laboratory ltr.

NSMRL 3960, Ser 40/418 dtd 12 October 1993.

Commanding Officer, USS Albuquerque SSN 706 (1995). Testing of Bose Aviation

Acoustic Headphones. USS SAN ALBUQUERQUE 9400, Ser WEPS/2 dtd 31

December 1995.

Commanding Officer, USS San Juan, SSN 751 (1993). Evaluation of Noise Canceling Headset for AN/BSY-1 Sonar Suite. USS SAN JUAN 3900, Ser 622 dtd 14

December 1993.

Delaney, M. E. (1964). The Acoustical Impedance of Human Ears. J. Sound Vibration, 1, 455-467.

Heller, L., Russotti J.S., Carr, W., Santoro T.P. (2000). Current NSMRL Information

Processing and Display Research, Presented at Technical Workshop Poster

session on Submarine Acoustic Superiority Proceedings Naval Undersea Warfare Center Report, May 2000.

Ithell, A. H. (1963). The Measurement of the Acoustical Input Impedance of Human

Ears. Acustica, 13, 140-145.
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16

Ithell, A. H. (1963). A Determination of the Acoustical Input Impedance Characteristics of

Human Ears. Acustica, 13, 311-314.

Paakkonen, Rauno and Kuronen, Pentti (1998) Low-Frequency Aviation Noise and

Noise Attenuation of Aviation Headsets and Helmets Using Active Noise Cancellation. Journal of Low Frequency Noise, Vibration and Active Control, 17,

191-197.

Russotti, J. S., Haskell, G. B., Neal, R, Santoro, T. P., and Carpenter (1985) Frequency response and distortion measurement of headsets for use in auditory sonar displays (NSMRL Report 1066). Groton, CT: Naval Submarine Medical Research Laboratory.

Russotti, J. S., Santoro, T. P., and Haskell, G. B. (1988) Proposed Technique for

Earphone Calibration. Audio Engineering Society Journal, 36, 643-650.

Russotti, J. S. (1993) At-Sea Evaluation of a Prototype Active-Noise Canceling

Headset. Naval Submarine Medical Research Laboratory Memorandum for the

Record, June 1993.

Russotti, J.S. (1995). Sonar headphone selection for optimum performance: an

overview (NSMRL Report 1197). Groton, CT: Naval Submarine Medical Research Laboratory.

Russotti, J.S. (1998). Active Noise cancellation Headsets for ARCI and NSSN, Presented at NSSN System Integration Team Brief, Electric Boat, Groton, CT, April,

1998

Shaw, E. A. G., and Thiessen, G. J. (1962). Acoustics of Circumaural Earphones.

Journal of the Acoustical Society of America, 34, 1233-1246.

Shaw, E.A.G. (1966) Ear Canal Pressure Generated by Circumaural and SupraAural Earphones. Journal of the Acoustical Society of America, 39, 471-479.

Zwislocki, J. J. (1957). Some Measurements of the Impedance at the Eardrum. J.

Acoust. Soc. Am., 29, 349-356.

Zwislocki, J. J. (1970). Acoustic Coupler for Earphone Calibration. Spec. Rep. LSCS-7, Laboratory of Sensory Communication, Syracuse, NY.

Zwislocki, J. J. (1971). An Ear-Like Coupler for Earphone Calibration. Spec. Rep.

LSC-S-9, Laboratory of Sensory Communication, Syracuse, NY.