Comparison of Acoustic Properties of Two USMC Helmets 

by Paula Henry, Jim A. Faughn, Timothy J. Mermagen 

ARL-TR-4383 February 2008 

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Army Research Laboratory 

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ARL-TR-4383 February 2008 

Comparison of Acoustic Properties of Two USMC Helmets 

Paula Henry, Jim A. Faughn, Timothy J. Mermagen 

Human Research and Engineering Directorate, ARL 

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14. ABSTRACT 

The United States Marine Corps (USMC) has recently replaced an older Combat Vehicle Crewman (CVC) communications 

headset (the MK1697/G) with a newer version (Enhanced CVC headset [ECVCH]) manufactured by the same company 

(Sonetronics). A communications headset consists of two earphones and a microphone with the capability to receive and 

transmit communications across a radio or intercom. The ECVCH headset provides improvements in situational awareness 

through the addition of a talk-through microphone, in hearing protection through thicker ear pads, and in greater field 

ruggedness. A set of evaluations was conducted to determine if the ECVCH showed improvements in attenuation and 

speech intelligibility over the MK1697/G headset. There was no improvement in sound attenuation with the ECVCH. The 

ECVCH showed a decrement in speech intelligibility, although this decrement was not large enough to impact operational 

performance. The difference in performance can be explained by the differences in frequency responses between the two 

headsets. 

15. SUBJECT TERMS

 hearing protection; helmets; speech intelligibility 

16. SECURITY CLASSIFICATION OF:

19a. NAME OF RESPONSIBLE PERSON

 Paula Henry 

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iii 

Contents 

List of Figures iv

List of Tables iv

1. Introduction 1

2. Methods 2

2.1 Measurement of Frequency Response of Headsets.........................................................3 

2.2 Sound Attenuation...........................................................................................................6 

2.2.1 Anthropometric Measures ...............................................................................................7 

2.3 Speech Intelligibility .......................................................................................................7 

3. Results 9

3.1 Sound Attenuation...........................................................................................................9 

3.2 Speech Intelligibility .....................................................................................................10 

4. Discussion 11

5. Conclusions 12

6. References 14

Appendix A. Raw Data From the Speech Intelligibility Portion of the Study 17

Distribution List 19 
iv 

List of Figures 

Figure 1. From left to right: CVC liner; CVC ballistic shell, assembled helmet with embedded 

headset..................................................................................................................................... 3 

Figure 2. KEMAR with CVC headset for frequency response measurements............................. 5 

Figure 3. Frequency responses of the two headsets....................................................................... 5 

Figure 4. Average speech intelligibility performance for each of the three test conditions ........ 11 

List of Tables 

Table 1. Anthropometric measures for the participants in the sound attenuation portion of 

the study.................................................................................................................................. 7 

Table 2. Real-ear attenuation (in dB) and standard deviations (SD) as a function of frequency 

obtained in the present study for the MK1697/G and ECVCH headsets.............................. 10 
1 

1. Introduction 

Communication in military settings must be clear and understandable to avoid possible fatal 

accidents and mistakes. Effective speech communication requires three components: clear 

speech by the talker, a non-distorting communications channel to transmit the speech from talker 

to listener, and good hearing and speech comprehension by the listener. 

Speech intelligibility is the overall quality of speech that makes it comprehensible. Speech 

intelligibility can be predicted with the use of various objective measures, but its ultimate 

indicator is the percentage of speech units that can be correctly identified during specific 

operational conditions. The speech units can range in size and complexity from phonemes, 

syllables, or words to phrases, sentences, or passages. The most common speech unit used for 

testing intelligibility is words. The transmission effectiveness of a system can be determined 

through calculation of the percentage of words correctly identified by the listener (Syrdal, 

Bennett, & Greenspan, 1994). 

Speech intelligibility depends on the properties of the three components of the communication 

system: the talker, the communications channel, and the listener. These components can be 

assessed individually or as a group. The objective of the current study is to evaluate the 

differences in speech intelligibility through the two communication headsets connecting a talker 

with a listener through an intercommunications system. Therefore, all three components of the 

system were included in the evaluations, with the headset as the only component changing. 

In order to measure the effects of communication headsets on speech intelligibility, the talkers 

and listeners must have normal speech and hearing and must be familiar with the test material. 

This study used the Modified Rhyme Test (MRT) as the speech material. The MRT is one of 

three American National Standards Institute (ANSI) standardized word tests authorized for 

measuring speech intelligibility over communication systems (ANSI, 1999). 

In addition to providing radio or intercommunications signals, communications headsets provide 

the user with hearing protection. The amount of hearing protection provided must be evaluated 

separately from speech intelligibility. The headsets in combat vehicle crewman (CVC) systems 

protect the wearer from potentially hazardous noise levels by reducing the amount of environmental sound that reaches the ears. Sound attenuation measurements determine how much the 

headsets reduce the listener’s exposure to the surrounding noise. One method of measuring 

sound attenuation is through real-ear attenuation at threshold (REAT) (ANSI, 2002) whereby 

hearing thresholds are measured with and without the hearing protection (in this case, the CVC 

helmets), and the calculated differences constitute the amount of attenuation. 

The purpose of this study was to conduct auditory measurements to compare the attenuation and 

speech intelligibility obtained through two of the United States Marine Corps (USMC) CVC 
2 

systems used for mounted operations. The older system was referred to by two names: the 

MK1697/G (referring to the headset-microphone kit) or the DH132B (referring to the shell). The 

older CVC system consists of a sized liner (in sizes small, medium, or large) with the embedded 

headset and the DH132B shell. The newer CVC system is called the enhanced CVC helmet 

(ECVCH). It consists of a universally sized liner with the embedded headset and either the 

DH132B or another CVC shell. The communications system used in this evaluation was the 

AN/VIC-3(V) vehicle intercommunications system (VIC-3). The AN/VIC-3(V) is the most 

common intercommunications system used on tracked vehicles. Both headsets are manufactured 

by Sonetronics Corporation based in New Jersey. For the purposes of this report, the old and 

new headsets are referred to as the MK1697/G and ECVCH, respectively. The USMC sponsored 

and provided funding for this effort. The voluntary, fully informed consent of the persons used 

in this research was obtained as required by 32 Code of Federal Regulations (CFR) 219 (OSD, 

1999) and Army Regulation (AR) 70-25 (HQDA, 1990). The investigator has adhered to the 

policies for the protection of human subjects as prescribed in AR 70-25 (HQDA, 1990). 

2. Methods 

The study was composed of an objective measures portion comprised of acoustic measures on a 

manikin and a subjective measures portion comprised of participant testing of speech 

intelligibility and attenuation. The objective manikin measures consisted of frequency response 

measures to determine the sound output from the headphones of the two headsets. The objective 

participant measures consisted of speech intelligibility and sound attenuation testing made with 

human participants. Some participants who completed the speech intelligibility portion of the 

study also completed the sound attenuation portion. 

A CVC system consists of a liner, a communications headset, and a hard protective shell (see 

figure 1). The headset is embedded in the liner and is comprised of headphones (which provide 

sound attenuation and communications from radios and intercoms), and a microphone (DA, 

1997). The Department of Defense has several technical manuals pertaining to the use and wear 

of the CVC systems. None of the documents reviewed by the investigators provides information 

about how to properly fit the system to the wearer’s head. The manuals state that the helmet 

should have a “tight fit” (DA, 1997, p. 1 through 8), that wearers should “adjust tension [using 

adjustable straps] so that the ear seal is tight against your head” (DA, 1997, p. 2-33), and that the 

wearer should “select the proper size for the individual’s use” and “adjust the chinstrap securely” 

(USACHPPM, 2006, p. 57). Written instructions provided by the USMC with the ECVC 

helmets included a statement that the helmet should be placed on the head so that the brow pad 

rests “two fingers above the nose”. 
3 

As stated earlier, the MK1697/G liner comes in three sizes: small, medium, and large. The 

ECVCH has a universal liner that is intended to be adjustable to fit all heads through the use of 

hook and loop straps on the sides and nape of the liner. The ballistic shells used for both helmets 

come in two sizes: small/medium and large. Two samples of the MK1697/G helmet were 

received for the evaluation, one sized small and one sized large. Five samples of the ECVCH 

were received for the evaluation. The determination of the appropriate size of the MK1697/G 

helmet for each participant was made, based on the participant’s reported comfort while wearing 

a particular size. The ECVCH was adjusted on each participant’s head for comfort. For both 

helmet models, the chin strap was secured and the helmet rested firmly on the participant’s head. 

Figure 1. From left to right: CVC liner; CVC ballistic shell, assembled helmet with embedded headset. 

2.1 Measurement of Frequency Response of Headsets 

Objective measures of the headsets were conducted to determine the frequency response of the 

output of the headphones for each of the CVC helmets. These measures were conducted to 

determine if differences existed between the different samples of the same helmet and between 

the old and new versions. For each measurement, a headset was placed on the Knowles 

Electronics Manikin for Acoustic Research (KEMAR) without the ballistic shell. The KEMAR 

is a manikin designed to represent the anatomical structure of the average adult male head and 

torso. It includes two pinnae, DB-100 Zwislocki couplers to simulate ear canals, and Etymotic 

Research, Inc., ER-11 microphones situated at the positions of the eardrums. Recording through 

the microphones of the KEMAR emulates recording at the eardrums of a human listener. The 

headsets were placed on the KEMAR with care to center the headphones over the manikin’s 

pinnae, and the chin strap was secured as it would be when worn by a human. Figure 2 shows a 

photograph of the KEMAR test apparatus with a CVC helmet. 

A maximum length sequence (MLS) signal was played out of the headphones of the CVC 

helmets and recorded through the microphones mounted inside the KEMAR. Both presentations 

of the MLS signal and recordings from the microphones were made through the 01dB1

 101dB is a trademark of Metravib. 
4 

Symphonie2 two-channel sound card and accompanying software. The results of the 

measurements revealed that the five samples of the ECVCH headset were nearly identical in 

frequency response. The two samples of the MK1697/G headset were also nearly identical in 

frequency response. Slight differences were noted in the frequency responses between the 

MK1697/G and the ECVCH headsets. Figure 3 shows the average frequency response from 

three of the ECVCH headsets along with the average frequency response from the two 

MK1697/G headsets. Recordings from the two headphones (left and right) were averaged to 

show a single response curve for the headset. As shown in the figure, the frequency responses of 

the two headsets were very similar except for the area between 800 and 1250 Hz where the 

ECVCH headset has a higher response than the MK1697/G headset. Since no large differences 

in frequency response were seen for the five samples of the ECVCH headset, three of the 

ECVCH headsets were arbitrarily selected for use in the study. Since the two samples of the 

MK1697/G were different sizes, both were used in the evaluation to accommodate participants 

with different head sizes. 

As mentioned previously, both of the headsets in the evaluation are manufactured by 

Sonetronics, Inc. In a telephone conversation with Sonetronics, an engineer indicated that 

several changes had been made in transitioning from the MK1697/G to the ECVCH. The 

changes that were relevant to this study included higher output sensitivity in the earcup, larger 

earcup volume, and thicker earcup padding. The increase in output sensitivity was designed to 

improve speech intelligibility in higher levels of background noise. The larger earcup volume 

and thicker earcup padding were designed to improve the amount of attenuation provided by the 

helmet. The microphones on the two helmets are identical. Testing was only conducted on the 

headphones of the systems rather than on both the microphone and headphones since no change 

was made in the microphone on the newer helmet. 

 2Symphonie is a trademark of 01dB. 
5 

Figure 2. KEMAR with CVC headset for frequency response 

measurements. 

-40

-30

-20

-10

0

10

20

30

40

50

0.4 0.5 0.63 0.8 1 1.25 1.6 2 2.5 3.15 4 5 6.3 8 10 12.5 16 20

Frequency (kHz)

Intensity (dB)

MK1697/G 3,4,5 ECVCH avg

Figure 3. Frequency responses of the two headsets. (Responses were averaged across ears within 

each headset. Open symbols represent the average of the two samples of the MK1697/G 

headset. The closed symbols represent the average of the three samples of the ECVCH 

headset used in the present study. Error bars indicate one standard deviation.) 
6 

2.2 Sound Attenuation 

Ten civilian volunteers between the ages of 27 and 56 (mean = 41.8) participated in the sound 

attenuation measurements through the REAT procedure (ANSI, 2002). Five of the volunteers 

were male and five were female. None of the participants had extensive prior experience using 

CVC helmets. All participants had normal hearing sensitivity defined as pure-tone hearing 

thresholds of ≤ 20 dB hearing level (HL) at audiometric frequencies from 125 through 8000 Hz 

(ANSI, 2004). Volunteers’ ears were examined with an otoscope at the time of audiometric 

testing and were found to be free of wax or abnormalities. None of the participants showed 

obvious signs of irritation or infection in the areas of the head or ears that would be contacted by 

the headphones and helmets (ANSI, 2002). Normal middle ear status was verified through use 

of a Grason-Stadler GSI-37 tympanometer. 

Instrumentation included the CVC helmets, an Interacoustics AC403 audiometer, three ElectroVoice4

 SX5 500+ loudspeakers, two Crown 605 amplifiers and a response button. The participant was seated in the center of a reverberant chamber. Pulsed one-third octave narrow bands of 

noise were presented at the seven octave frequencies 125 through 8000 Hz. The stimuli were 

presented over the loudspeakers through the audiometer. The loudspeakers were arranged in the 

room to create a diffuse stimulus presentation. Thresholds were obtained to the nearest 5-dB 

increment via an adaptive 10-down, 5-up methodology. Unoccluded (open ear with no helmet) 

and occluded (both helmet conditions) were tested with the ordering of systems and occluded/ 

unoccluded conditions counterbalanced across listeners. Each pair of unoccluded ear and headset trials was repeated once in accordance with the ANSI standard method of measurement 

(ANSI, 2002). Investigators manually recorded the participant’s threshold for each test stimulus. 

Participants were allowed to take breaks between helmet conditions. Participants depressed the 

response button to indicate when a sound was heard. The measurement of each pair of thresholds took approximately 20 minutes to complete for a total of 40 minutes for both CVC 

helmets. 

The helmets were fitted on the participants according to comfort since there are no specific 

sizing requirements (DA, 1997). Participants removed all earrings and eyeglasses before 

donning the helmet. The liner was fit to the person first, with adjustments made to the straps to 

ensure a tight fit. Based on information received from USMC, the brow pad of the helmet was 

aligned on the participant’s head to be “two fingers above the nose”. The ballistic shells for the 

CVC were then placed on top of the liners and pushed down into place by the participant. Each 

of the three ECVCH models was worn by 4 of the 12 participants. 

 3

Not an acronym. 

4Electro-Voice is a trademark of Bosch Communications Systems. 

5Not an acronym. 
7 

REAT was calculated for each of the seven third-octave frequency bands as outlined in ANSI 

S12.6. Attenuation was calculated as the difference between the unoccluded (no helmet) and 

occluded (with helmet) thresholds with the two attenuation values averaged for each CVC. 

Individual two-trial averages were averaged across participants, and the standard deviation of 

the average attenuation value for each CVC model was calculated for each frequency as the 

difference between the average attenuation of each participant’s two trials and the mean 

attenuation of the entire panel of participants. 

2.2.1 Anthropometric Measures 

In accordance with ANSI standard S12.6, two anthropometric measures were taken from 

participants in the attenuation portion of the study: bitragion width with a spreading caliper and 

head height with an anthropometer6. Table 1 shows the anthropometric measurements. 

Table 1. Anthropometric measures for the participants in the sound attenuation portion of the study. 

Participant Number Sex Bitragion Width (cm) Head Height (cm) 

1 F 12.6 12.3 

2 M 13.6 13.1 

3 M 14.8 13.5 

4 M 13.5 12.2 

5 F 12.6 12.2 

6 F 12.5 12.2 

7 F 14.0 12.3 

8 F 12.8 12.4 

9 M 14.3 12.9 

10 M 13.6 12.6 

 AVG 13.43 12.57 

 F 12.90 12.28 

 M 13.96 12.86 

2.3 Speech Intelligibility 

Twelve civilian volunteers between the ages of 26 and 51 (mean = 37.5) participated in the 

speech intelligibility portion of the study. Six of the volunteers were male and six were female. 

None of the participants had extensive prior experience using CVC helmets. All participants had 

normal hearing sensitivity defined as pure-tone hearing threshold levels ≤ 20 dB hearing level 

(HL) at audiometric frequencies from 125 through 8000 Hz. Volunteers’ ears were examined 

with an otoscope at the time of audiometric testing and were found to be free of ear wax or other 

abnormalities. None of the participants showed obvious signs of irritation or infection in the 

areas of the head or ears that would be contacted by the headphones and helmets (ANSI, 2002). 

Normal middle ear status was verified through use of a GSI-37 tympanometer. All participants 

had American English as their native language. 

 6The spreading caliper and anthropometer were made by GPM (not an acronym) of Zurich, Switzerland. 
8 

Instrumentation for the speech intelligibility measurements included the two CVC helmets, 

clipboards, writing implements, response sheets, chairs for the participants, four PSB7

International loudspeakers, a background noise file, a compact disk player to play the 

background noise file, a Crown8 Macro-Tech9 2400 amplifier, a Dell desktop computer that 

controlled the playing of the speech recordings, a Symetrix10 SX204 headphone amplifier, and a 

specially wired cable to interface the CVC helmets with the headphone amplifier. Background 

noise consisted of a 7-minute recording of the interior of a moving (10 mph) M113 armored 

personnel carrier played at 105 dB A-wtd. All practice and test trials were conducted in an 

acoustically treated room. 

Test material consisted of recordings of the 300 items of the MRT. The test consists of 50 sixword groups of monosyllabic English words (House, Williams, Heker, & Kryter, 1965). The six 

words in each group have the same vowel and differ by initial or final sound. An MRT list is a 

subset of the 300 words, consisting of 50 words, one from each group of six. Six lists, which 

between them contain all 300 words, comprise a full MRT set. Participants respond by marking 

on a paper form with a pen which word they heard. Participants completed one practice set 

along with two test sets, one for each of the two helmet models. 

Recordings of the MRT were made before testing. An expert male talker was used to create two 

recordings. For use in the practice trials, the talker recorded one ordering of the MRT in a quiet 

sound-treated chamber. For use in the test trials, the talker recorded a different ordering of the 

MRT item set in the presence of the M113 background noise played at 105 dB A-wtd. The same 

CVC microphone was used for both recordings. The carrier phrase used for the recordings was 

“Mark the _______ now” in which the MRT item was inserted into the blank within the phrase. 

The output of the microphone for the CVC headset was routed through the VIC-3 

intercommunications system and recorded to a Dell personal computer. The recordings were 

played back to the listeners through the same Dell personal computer routed through a Symetrix 

SX204 headphone amplifier to the headphones of the CVC headsets. In this way, the CVC 

microphone and the VIC-3 intercommunications system were constant components of the test 

paradigm without the need to have a live talker. 

Both helmets were properly fitted on each participant in the same manner as previously stated for 

the sound attenuation portion of the study. The volume control on the headphone amplifier was 

adjusted by all participants to be at a comfortable volume before the test portions of the study. 

For practice and test trials, listeners heard the MRT items played over the headsets and selected 

which word from the given six-word group was presented, using a paper and pen methodology. 

 7

Not an acronym. 

8

Crown is a registered trademark of Crown Audio Incorporated, a Harman International Company. 

9Macro-Tech is a registered trademark of Crown Audio, Incorporated, a Harman International Company. 

10Symetrix is a trademark of Symetrix, Inc. 
9 

Participants were familiarized with the MRT items before the start of data collection by 

completing a practice list of the MRT items recorded in quiet. Participants were seated in a 

sound-treated booth and wore the headset that would be worn for the first set of test trials. 

Participants listened to the items presented through the headset with no background noise 

presented. Four loudspeakers were positioned in the room to output background noise during the 

test trials. The loudspeakers were approximately 1 m from the position of the listener and were 

directed toward the listener’s chair. The background noise consisted of a 7-minute recording of 

an M113 armored personnel carrier played at a level of 105 dB A-wtd, which was identical to 

that used in the recordings. The order of the two helmets was counterbalanced across 

participants. Each of the three ECVCH models was worn by 4 of the 12 participants. 

During data collection, all 300 MRT words were used once for each set divided into six 50-word 

lists. A full set of 300 words took approximately 20 minutes to complete. After a full MRT set 

was completed, a short (about 5-minute) break was taken and helmets were changed as needed. 

Longer breaks were taken at the discretion of the participants. Total participant time was 

approximately 1.5 hours. 

The dependent variable of interest in this study was the listener’s performance in each condition. 

Data analysis includes descriptive statistics (means, standard deviations) and a paired samples 

t-test. Before data analysis, two transformations of the data were made. First, the adjusted 

percent correct scores were transformed to rationalized arcsine units (rau) (Sherbecoe & 

Studebaker, 2004; Studebaker, 1985). The rau transformation puts the proportion data into an 

interval form which is more appropriate for statistical analysis. Second, all individual 

transformed scores were adjusted for the probability of getting a correct response by chance. 

The adjustment for chance or guessing was calculated as 

Tc = (ATu −100)/(A −1) (1) 

in which Tc is the adjusted score, Tu is the uncorrected score in rau, and A is the number of 

alternate choices per item (6) (Sherbecoe & Studebaker, 2004). 

3. Results 

3.1 Sound Attenuation 

The Department of Defense has outlined attenuation requirements for the CVC helmet (DoD, 

1986). Table 2 shows the minimum attenuation values outlined in MIL-H-44117A along with 

the average REAT values and corresponding standard deviations obtained for each of the two 

headsets across the seven frequency bands in the present study. The values reported by the 

manufacturer are also included in the table. Comparisons between the MIL standard and the 

helmets show that both meet the minimum requirement, except at 2 kHz. Furthermore, 
10 

comparisons between the REAT values obtained in the present study with the helmets indicate 

that they provide very similar attenuation. The attenuation values obtained for the ECVCH are 3 

to 10 dB less than those reported by the manufacturer. 

Table 2. Real-ear attenuation (in dB) and standard deviations (SD) as a function of frequency obtained in the 

present study for the MK1697/G and ECVCH headsets. (The table also contains the criterion values from 

MIL-H-44117A and values reported by Sonetronics.) 

REAT Values for Each One-Third Octave Band (Hz) 

125 250 500 1k 2k 4k 8k 

MK1697/G 14.50 17.50 21.25 27.50 26.00 35.75 33.25

SD 4.56 4.44 5.59 5.26 3.08 2.94 3.35

ECVCH 17.00 18.50 20.75 25.00 23.25 38.00 34.25

SD 3.40 4.89 6.34 5.38 4.38 4.97 5.68

MIL-H-44117A 14 16 21 23 28 35 30

ECVCH data from Sonetronics 27 28 30 33 33 44 37

3.2 Speech Intelligibility 

Figure 4 shows the speech intelligibility scores (corrected for chance performance and converted 

to rau) averaged across participants for each of the three conditions: practice (in quiet), the 

MK1697/G, and the ECVCH headset. The raw data are provided in appendix A. As shown in 

the figure and as expected, performance was best in the practice condition. The introduction of 

noise in the test conditions resulted in reduced speech intelligibility performance, regardless of 

the headset worn. Between the two headsets, participants performed better on average with the 

MK1697/G than with the ECVCH. 

A paired samples t-test between the two headset conditions indicated a statistically significant 

difference in speech intelligibility t(11) = 2.484, p < .05, indicating that the performance with the 

MK1697/G headset was significantly better than with the ECVCH. 
11 

0

20

40

60

80

100

120

Headset Condition

Speech Intelligibility Performance (rau)

Practice in Quiet

MK1697/G

ECVCH

Figure 4. Average speech intelligibility performance for each of the three test conditions. (Speech 

intelligibility performance is shown in rationalized arcsine units corrected for chance. The gray 

bar indicates average performance in quiet [practice], the open bar indicates performance with the 

MK1697/G headset, and the solid bar indicates performance with the ECVCH headset. Error bars 

indicate +1 standard deviation.) 

4. Discussion 

Frequency response measurements of the MK1697/G and ECVCH showed essentially equivalent 

responses except in the region between 800 and 1250 Hz. In this area, the ECVCH showed a 

higher response (providing more information to the listener in this frequency region). 

As expected, speech intelligibility performance was shown to be best in quiet and poorer with the 

addition of background noise. Recall that the practice list was conducted with either headset, 

depending on which headset was the first to be tested in noise for that participant. It has been 

well documented that speech intelligibility is degraded in noise as compared to quiet 

environments. A comparison of the speech intelligibility between the two headsets shows that 

performance is poorer with the ECVCH than with the MK1697/G helmet. The differences seen 

in the frequency responses between the two headsets may have contributed to the differences 

seen in the speech intelligibility portion of the study. The differences in frequency response 

result in differences in audibility of the speech signal to the listener. One would think that 

providing a broader frequency range would improve speech intelligibility, but this is not always 

the case. When low frequency energy is increased in a broadband stimulus, its energy has the 
12 

potential to mask the energy at higher frequencies. This phenomenon is called “upward spread 

of masking” (Moore, 1997). Speech intelligibility, particularly in noise, relies on the listener’s 

ability to extract high frequency consonant information from the signal. The additional midfrequency energy that was present in the ECVCH headset that was not present in the MK1697/G 

may have contributed to poorer performance because of upward spread of masking (e.g., 

Stelmachowicz et al., 1990). 

Participants were allowed to adjust the volume control on the headphone amplifier separately for 

each test condition. It is possible that all participants set the output of the MK1697/G headset 

higher than the ECVCH headset, which would result in better speech intelligibility; however, 

there is no way to determine this since the volume control settings were not recorded for each 

participant. 

Note that although the statistical analysis revealed a significant difference between the 

performances with the two helmets, the actual speech intelligibility difference was very small 

(5%) and is within expected variability of such speech tests. Furthermore, this small difference 

would not likely be meaningful in a real-world scenario. The testing conducted in this study 

involved the speech intelligibility of individual words embedded in a carrier phrase. The 

contextual advantage of phrases and sentences typically allows for higher speech intelligibility 

than with individual words. Therefore, a 5% difference in intelligibility of individual words is 

not likely to impact operational performance with connected speech. 

Sound attenuation measured through REAT showed that the two headsets provide essentially the 

same attenuation in this test environment, but the measured sound attenuation values are less 

than those reported by the manufacturer. Both headsets meet the minimum sound attenuation 

criteria outlined in MIL-H-44117A except at the 2-kHz one-third octave band. The failure to 

meet the criteria at 2 kHz and the lower attenuation values seen in the present study as compared 

to those provided by the manufacturer are most likely attributable to suboptimal fitting of the 

helmet or other abnormality in the test environment rather than a true failure of the headsets. 

5. Conclusions 

Slight differences exist in the frequency responses of the headphones from the two headsets. 

Sound attenuation values as measured through REAT between the two headsets were essentially 

the same. Individual listeners were allowed to adjust the volume control on the headphone 

amplifier to a comfortable listening level and these settings were not recorded. Slight differences 

were seen in the speech intelligibility performance between the two headsets. These differences 

were likely attributable to the difference in the frequency responses between the two headsets 

and possibly differences in output through changes in the headphone amplifier volume control. 
13 

Although the differences in speech intelligibility were statistically significant, the difference was 

too small to be meaningful in real-world applications and falls within the expected variability for 

the test. The ECVCH headset showed an increase in the low frequency response when compared 

to the MK1697/G headset. The evaluation demonstrates that the ECVCH headset is 

operationally no different than the MK1697/G. 
14 

6. References 

American National Standards Institute. Methods for Measuring the Real-Ear Attenuation of 

Hearing Protectors; ANSI S12.6; ANSI: New York, NY, 1997, R2002. 

American National Standards Institute. Method for Measuring the Intelligibility of Speech Over 

Communication Systems; ANSI S3.2; ANSI: New York, NY, 1989, R1999. 

American National Standards Institute. Specification for Audiometers; ANSI S3.6; ANSI: New 

York, NY, 2004. 

Department of the Army. Operator’s Manual, Intercommunication Set, Vehicular AN/VIC-3(V); 

Technical Manual (TM) 11-5830-263-10; Washington, DC, 1997. 

Department of Defense. Military Specification: Helmet, Combat Vehicle Crewman, Ballistic 

Shell, DH-132A, MIL-11-44117, 22 December 1986. 

Headquarters, Department of the Army. Use of Volunteers as Subjects of Research; AR 70-25; 

Washington, DC, 1990. 

House, A.; Williams, C.; Heker, M.; Kryter, K. Articulation-Testing Methods: Consonantal 

Differentiation with a Close-Response Set. Journal of the Acoustical Society of America 

1965, 37, 158-166. 

Office of the Secretary of Defense (OSD). Protection of Human Subjects, 32 Code of Federal 

Regulations, Part 219, Washington, DC, 1999. 

Moore, B.C.J. Introduction to the Psychology of Hearing, 4th edition. Academic Press: 

London, 1997. 

Sherbecoe, R.; Studebaker, G. Supplementary Formulas and Tables for Calculating and 

Interconverting Speech Recognition Scores in Transformed Arcsine Units. International 

Journal of Audiology 2004, 43, 442-448. 

Sonetronics. Unpublished data, Belmar, NJ, 2006. 

Stelmachowicz, P.; Lewis, D.; Kelly, W.; Jesteadt, W. Speech Perception in Low-Pass Filtered 

Noise for Normal and Hearing-Impaired Listeners. Journal of Speech and Hearing Research

1990, 33, 290-297. 

Studebaker, G. A “Rationalized” Arcsine Transform. Journal of Speech and Hearing Research

1985, 28, 455-462. 

Syrdal, A.; Bennett, R.; Greenspan, S. Applied Speech Technology. CRC Press, LLC: 

Washington, DC, 1994. 
15 

United States Army Center for Health Promotion and Preventive Medicine. Personal Hearing 

Protective Devices: Their Fitting, Care and Use, TG 41, March 2006. 
16 

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17 

Appendix A. Raw Data From the Speech Intelligibility Portion of the Study 

Part # Practice MK 1697/G ECVCH Practice MK 1697/G ECVCH

1 272 259 240 28 41 60

2 250 235 230 50 65 70

3 299 250 242 1 50 58

4 299 252 225 1 48 75

5 297 237 219 3 63 81

6 296 211 235 4 89 65

7 294 250 201 6 50 99

8 299 262 249 1 38 51

9 295 233 243 5 67 57

10 295 257 239 5 43 61

11 296 251 232 4 49 68

12 297 219 213 3 81 87

Number of Items Correct Number of Items Incorrect

Part # Practice MK 1697/G ECVCH Practice MK 1697/G ECVCH Practice MK 1697/G ECVCH

1 88.80 83.60 76.00 93.93 87.64 79.79 92.72 85.17 75.75

2 80.00 74.00 72.00 83.78 77.89 76.05 80.54 73.47 71.26

3 99.60 80.00 76.80 116.53 83.78 80.57 119.84 80.54 76.68

4 99.60 80.80 70.00 116.53 84.61 74.24 119.84 81.53 69.09

5 98.80 74.80 67.60 112.98 78.64 72.13 115.58 74.37 66.56

6 98.40 64.40 74.00 111.62 69.39 77.89 113.94 63.27 73.47

7 97.60 80.00 60.40 109.3 83.78 66.07 111.16 80.54 59.28

8 99.60 84.80 79.60 116.53 89.01 83.36 119.84 86.81 80.03

9 98.00 73.20 77.20 110.41 77.15 80.96 112.49 72.58 77.15

10 98.00 82.80 75.60 110.41 86.75 79.41 112.49 84.10 75.29

11 98.40 80.40 72.80 111.62 84.19 76.78 113.94 81.03 72.14

12 98.80 67.60 65.20 112.98 72.13 70.07 115.58 66.56 64.08

avg 96.30 77.20 72.27 108.89 81.25 76.44 110.66 77.50 71.73

sd 5.90 6.45 5.60 9.92 6.20 5.04 11.90 7.44 6.05

Adjusted % Correct rau without chance corr rau corrected for chance
18 

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19 

NO. OF 

COPIES ORGANIZATION

 1 DEFENSE TECHNICAL 

 (PDF INFORMATION CTR 

 ONLY) DTIC OCA 

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 STE 0944 

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 SYSTEMS OF SYSTEMS INTEGRATION 

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 1 DIRECTOR 

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 2800 POWDER MILL RD 

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 2 DIRECTOR 

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 AMSRD ARL CS OK T 

 2800 POWDER MILL RD 

 ADELPHI MD 20783-1197 

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 ATTN AMSRD ARL HR ML J MARTIN 

 MYER CENTER RM 2D311 

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 AWC FIELD ELEMENT 

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 R POMRANKY 

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 S MIDDLEBROOKS 

 91012 STATION AVE ROOM 348 

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 2520 HEALY AVE STE 1172 BLDG 51005 

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 BATTLE CMD BATTLE LAB 

 415 SHERMAN AVE UNIT 3 

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20 

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 1 ARMY RSCH LABORATORY-HRED 

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 12423 RESEARCH PARKWAY 

 ORLANDO FL 32826 

 1 ARMY RSCH LABORATORY-HRED 

 ATTN AMSRD ARL HR MT C KORTENHAUS 

 12350 RESEARCH PARKWAY 

 ORLANDO FL 32826 

 1 ARMY RSCH LABORATORY - HRED 

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 1 ARMY RSCH LABORATORY - HRED 

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 1 ARMY RSCH LABORATORY - HRED 

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 BLDG 4 ROOM 332 

 FT BENNING GA 31905-5400 

 1 ARMY RSCH LABORATORY - HRED 

 ATTN AMSRD ARL HR MN R SPENCER 

 DCSFDI HF 

 HQ USASOC BLDG E2929 

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 1 ARMY G1 

 ATTN DAPE MR B KNAPP 

 300 ARMY PENTAGON ROOM 2C489 

 WASHINGTON DC 20310-0300 

 5 MARINE CORPS SYSTEMS CMD 

 ATTN A SCHWEIZER PM-ICE 

 2201A WILLIS STREET 

 QUANTICO VA 22134-6050 

NO. OF 

COPIES ORGANIZATION

 ABERDEEN PROVING GROUND

 1 DIRECTOR 

 US ARMY RSCH LABORATORY 

 ATTN AMSRD ARL CI OK (TECH LIB) 

 BLDG 4600 

 1 DIRECTOR 

 US ARMY RSCH LABORATORY 

 ATTN AMSRD ARL CI OK TP S FOPPIANO 

 BLDG 459 

 1 DIRECTOR 

 US ARMY RSCH LABORATORY 

 ATTN AMSRD ARL HR MR 

 F PARAGALLO 

 BLDG 459 

 10 DIRECTOR 

 US ARMY RSCH LABORATORY 

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 BLDG 459