ARL-TR-7914 ● DEC 2016

US Army Research Laboratory

Auditory Localization Performance with 

Gamma Integrated Eye and Ear Protection

by Angelique Scharine, Morgan Domanico, Ashley Foots, 

Timothy Mermagen, and Rachel Weatherless

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ARL-TR-7914 ● DEC 2016

US Army Research Laboratory

Auditory Localization Performance with 

Gamma Integrated Eye and Ear Protection

by Angelique Scharine, Ashley Foots, Timothy Mermagen, and 

Rachel Weatherless

Human Research and Engineering Directorate, ARL

Morgan Domanico

Oak Ridge Institute for Science and Education, Oak Ridge, Tennessee

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December 2016

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1 June 2016–30 September 2016

4. TITLE AND SUBTITLE

Auditory Localization Performance with Gamma Integrated Eye and Ear 

Protection 

5a. CONTRACT NUMBER

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6. AUTHOR(S)

Angelique A Scharine, Ashley N Foots, Timothy J Mermagen, and Rachel 

Weatherless

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A60A060FJ16389

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US Army Research Laboratory

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Aberdeen Proving Ground, MD 21005-5425

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ARL-TR-7914

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Natick Soldier Research, Development and Engineering Center

Attn: RDNS-OOR-B 

15 General Greene Avenue

Natick, MA 01760

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NSRDEC

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

Auditory localization performance was assessed for participants wearing the gamma integrated eye and ear protection (IEEP), 

a prototype tactical communications and protection system that also provides eye protection. Testing was conducted using one 

of the auditory localization measurement methods recently proposed as a standard by the Department of Defense Hearing 

Center of Excellence. Participants used a laser pointer to indicate the perceived location of a sound presented from 1 of 36 

loudspeakers. This task was completed both with ears unoccluded (no IEEP) and with the IEEP. Pink noise was used for the 

sound stimuli (either 250 ms or 4000 ms), randomly roved from 60 to 75 dBA. Localization accuracy was measured as the 

horizontal angular difference between the target loudspeaker location and the participant’s estimate. The data were analyzed 

taking into account individual ability, as well as the known effects of sound source azimuth and stimulus duration. After 

taking these known effects into account, we concluded the IEEP did not significantly change overall localization ability. 

However, it did have a small, but significant, effect on accuracy for the short-duration (250 ms) stimuli due to an increase in 

reversals (front-back confusions). 

15. SUBJECT TERMS

auditory localization, TCAPS, hearing protection, eye protection, auditory situation awareness

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19a. NAME OF RESPONSIBLE PERSON

Angelique Scharine

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410-278-5957

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Contents

List of Figures v

List of Tables vi

1. Introduction 1

1.1 Gamma Integrated Eye and Ear Protection (IEEP) 1

1.2 Auditory Localization 1

1.3 Localization with Hearing Protection and Other Headgear 4

1.4 Standardization of Localization Measurement 4

1.5 Auditory Localization Measurement Methods 1 and 2 4

1.5.1 Loudspeaker Array 4

1.5.2 Stimulus Duration 5

2. Method 6

2.1 Participants 6

2.2 Test Facility 6

2.3 Localization Test 7

2.3.1 Target Stimuli 7

2.3.2 IEEP Earphone Tip Fitting 8

2.3.3 Stimulus Blocks 8

3. Results 8

3.1 Independent Variables 8

3.2 Dependent Variables 9

3.3 Individual Differences 9

3.4 Signed Errors 10

3.5 Unsigned Errors 15

3.6 Reversals 21

4. Conclusion 24

5. References 26
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List of Symbols, Abbreviations, and Acronyms 28

Distribution List 29
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v

List of Figures

Fig. 1 IEEP device under test: shown with clear ballistic lens and Comply 

foam earphone tips.................................................................................1

Fig. 2 Schematic representation of the “cone of confusion,” described as the 

set of locations sharing the same set of binaural difference cues..........3

Fig. 3 Loudspeaker configuration in the dome room: method 2 testing 

incorporates 36 loudspeakers spaced at even intervals of 10°, 

beginning at 5° and continuing to 355° .................................................7

Fig. 4 Mean unsigned error for each participant, shown as a function of 

IEEP use and stimulus duration. (Error bars shown are the standard 

error of the mean)...................................................................................9

Fig. 5 Signed localization error as a function of stimulus duration ...............11

Fig. 6 Overall mean signed localization error shown as a function of sound 

source azimuth. (The gray circle represents 0° error.).........................12

Fig. 7 Mean signed error as a function of stimulus duration and sound 

source azimuth .....................................................................................13

Fig. 8 Mean signed error observed for participants as a function of IEEP 

use and stimulus duration.....................................................................14

Fig. 9 Mean signed error as a function of IEEP use, stimulus duration, and 

sound source azimuth...........................................................................15

Fig. 10 Mean unsigned error shown as a function of stimulus duration ..........17

Fig. 11 Overall mean unsigned error shown as a function of sound source 

azimuth.................................................................................................18

Fig. 12 Mean unsigned error shown as a function of stimulus duration and 

sound source azimuth...........................................................................19

Fig. 13 Mean unsigned error shown as a function of IEEP use and stimulus 

duration ................................................................................................20

Fig. 14 Mean unsigned error shown as a function of IEEP use, stimulus 

duration, and sound source azimuth.....................................................21

Fig. 15 Percent reversals shown as a function of sound source azimuth .........22

Fig. 16 Percent reversals observed as a function of stimulus duration and IEEP 

use ........................................................................................................23

Fig. 17 Percent reversals as a function of IEEP use, stimulus duration, and 

sound source azimuth...........................................................................24
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vi

List of Tables

Table 1 Summary of ANCOVA for signed localization error with subjects as a 

covariate...............................................................................................10

Table 2 Summary of ANCOVA for unsigned localization error with subjects as 

a covariate ............................................................................................16
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1. Introduction

This effort was funded by the US Army Natick Soldier Research, Development and

Engineering Center’s (NSRDEC’s) Warfighter Directorate. The objective was to 

measure the effect of a prototype designed to provide Soldiers with integrated eye 

and ear protection on auditory localization ability.

1.1 Gamma Integrated Eye and Ear Protection (IEEP)

The IEEP (Fig. 1) was manufactured for NSRDEC by Applied Research 

Associates, Inc. (ARA) and Revision Military Technologies under contract 

W911Q7-13-C0099. The Gamma-IEEP combines ARA’s hybrid hearing 

protection technology with Revision Military’s Sawfly Military Eyewear System. 

According to the manufacturer, in addition to ballistic eye protection and distortion 

free vision, the hearing protection portion of this technology provides passive sound 

attenuation capabilities and electronic limiters that suppress transmission of 

impulsive and high-level steady-state noise. In addition, the manufacturer indicates 

that the system’s active pass-through microphone technology restores normal levels 

of hearing when ambient noise levels are below 85 dBA.

* Testing was conducted 

with the system active.

Fig. 1 IEEP device under test: shown with clear ballistic lens and Comply foam earphone 

tips.

1.2 Auditory Localization

Localization of a sound source is determined using binaural, monaural, and 

movement-based cues. Binaural cues are the result of differences in arrival time 

and sound pressure level between the 2 ears, providing information about the 

right/left position of a sound. Monaural cues are created as the sound reflects off 

parts of the body and these reflections add into the original sound wave. Monaural 

cues give information about a sound’s elevation and placement along the front-back

 * Decibels A-weighted” is the sound pressure level adjusted for the sensitivity of the average human ear. 

The reference level is the loudness of a 1000-Hz tone presented at 40-dB sound pressure level. Humans are 

less sensitive to low frequencies and more sensitive to frequencies between 1 kHz and 10 kHz.
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axis. Movement of the head provides multiple sets of binaural cues, which in 

combination with information about the head’s movement reduces ambiguity in 

these binaural cues and increase accuracy of localization estimates.

Auditory localization errors stem from failure to fully resolve binaural and 

monaural cues. Imprecision with respect to binaural cues are the source of 

localization blur. Localization errors due to limitations in acuity are called 

localization blur, and this underlying variability is due to our limited ability to 

resolve binaural information. Humans can localize horizontally only within a 

certain range of error. Localization capability is further limited by the ability to 

discriminate between 2 sound sources: the minimum discriminable audible angle 

(MAA). In studies of the MAA, normal hearing listeners can, on average, reliably 

discriminate audible angles that differ by only 0.97° at 0°.

* However, this 

discriminability decreases to 3.65° at 90° (Saberi and Perrott 1990). This ability to 

discriminate between 2 source locations is thus reflected in measures of ability to 

localize a single sound source. If the response options are limited to the front 

hemisphere, measures of localization acuity range from approximately 4° at the 

midline (0°) to 9° at the interaural axis (90°) (Oldfield and Parker 1984).

The auditory spatial cues provided by brief transient sounds are vulnerable to 

distortion and masking. Binaural cues only specify a ring of potential locations 

known as the cone of confusion (Fig. 2), all sharing the same interaural time and 

intensity differences. Monaural cues can be used to resolve this ambiguity but they 

require that the listener have some prior knowledge of the sound source. The 

spectral changes that are the source of monaural cues can be altered by changes in 

hairstyle and the head’s profile, as well as masked by reflections and reverberation. 

In practice, these ambiguities are often resolved by movements of the head and 

visual confirmation (Wallach 1940). 

 * For the sake of discussion, 0° refers to directly in front of the listener. 90° refers to the angle directly to 

the right of the listener.
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Fig. 2 Schematic representation of the “cone of confusion,” described as the set of locations 

sharing the same set of binaural difference cues

Movement is often ignored in laboratory studies of localization; however, in more 

ecologically relevant conditions, it is a predominant cue. For longer sounds, head 

movements can provide the listener with multiple “samples” of auditory spatial 

information, greatly reducing the ambiguity (Wallach 1940). To make studies of 

auditory localization sensitive to other factors that affect localization, the sound 

signals are often kept brief in duration to prevent the listener from using movement 

cues, thus increasing the probability of errors due to altered monaural cues. Because 

the listener in these studies typically cannot use movement cues, data from these 

studies can be interpreted to represent the lower limit of localization ability. Given 

a longer stimulus duration, additional information is available in binaural, 

monaural, and movement cues that the listener can use to localize the sound source

(Thurlow et al. 1967). Therefore, testing with longer stimuli provides information 

about the upper boundary of individual localization ability.

When movement cues are absent, monaural cues are required to resolve binaural 

ambiguity. Failure to resolve this ambiguity manifests itself in errors that result 

from choosing another location within the cone of confusion that has similar 

binaural information, but different monaural information. Thus, if the potential 

response set is limited to the horizontal axis, the error for location “a” in Fig. 2 

results in choosing location “b” that is located in the same right/left position, but 

mirrored about the interaural axis in the reverse hemisphere.
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1.3 Localization with Hearing Protection and Other Headgear

Hearing protection devices (HPDs), tactical communications and protection 

systems (TCAPS), and other forms of headgear have been shown to alter the 

perception of monaural cues, resulting in both increased localization blur and frontback confusions. For example, physical changes to the region around the ears alter 

the monaural information in the head-related transfer function of the sound path 

and negatively affect the auditory localization performance of the listener (Riederer 

2003a, 2003b). Previous research has shown that helmets can increase unsigned 

localization errors on average as much as 13° (Scharine et al. 2014). Scharine and 

Weatherless (2014) measured increases to average unsigned localization error of 

approximately 18° for passive, level-dependent earplug style HPDs. Similar 

increases were measured for active earplug (~20°) and earmuff (~25°) style TCAPS

(Scharine and Weatherless 2013). In both cases, increases in front-back confusions 

near 0° and 180° were observed. These well-documented effects have led to the 

inclusion of auditory localization ability as a measure of auditory “situation 

awareness” for TCAPS, and it has been included as an element of the requirements 

drafted during the acquisition of Soldier equipment. 

1.4 Standardization of Localization Measurement

Recently there has been an effort to establish standard measurement methods for 

the assessment of hearing protection offered for use by the Department of Defense 

(DOD). In April of 2016, a DOD Hearing Center of Excellence (HCoE) working 

group meeting was held at Wright Patterson Air Force Base to discuss 

implementation of a uniform methodology for measurement of localization ability

(Buchanan 2016). Three methods were proposed, 2 that measure auditory 

localization, and a third that measures audio-visual target detection. Apart from a 

few minor details that will be described further, the IEEP was tested according to 

the proposed Method 2 described in the following sections. 

1.5 Auditory Localization Measurement Methods 1 and 2

1.5.1 Loudspeaker Array

Method 1 was developed as a simple and cost-effective measurement approach for 

manufacturers and others who do not have access to controlled acoustic facilities 

with large loudspeaker arrays. Method 1 has 2 listener orientations (0° and 45°) and 

specifies the use of 8 loudspeakers, strategically placed to allow the experimenter 

to detect differences in auditory localization blur and front-back localization errors.
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In contrast, Method 2 was developed for those who wish to more fully characterize 

the auditory spatial performance of listeners, either for research purposes or after 

down-selecting items tested with Method 1. Method 2 requires a hemi-anechoic 

facility, a horizontal array of 24–36 evenly spaced loudspeakers, and a position 

tracker that allows the listener to respond to the full 360° horizontal range.

1.5.2 Stimulus Duration

Real-world environments contain sounds of varying durations. Both methods 

specify the use of 2 different stimulus durations: short (250 ms) and long 

(4000 ms).

* The effects of stimulus duration on auditory localization have been 

established in research (Vliegen and Van Opstal 2004; Bernhard 2015). The short 

sound is used to prevent participants from turning their head to use movement cues 

to disambiguate the binaural information. The long sound provides time for the 

participants to move their head and gather more information that disambiguates the 

binaural information, providing an estimate of upper limit of the participants’

localization accuracy. Performance, as measured for the short and long stimuli, 

characterizes the range of localization ability demonstrated in the real world. To 

obtain reliable measurements in the minimum testing time, twice as many short 

stimuli are presented as long stimuli. More trials are needed for the short-duration 

stimuli due to greater variability in localization for these stimuli. In contrast, fewer 

presentations are required for the long-duration stimuli, as listeners are able to be 

both quite consistent and accurate. 

Currently no criteria exist for auditory localization performance with TCAPS; 

however, measures of auditory localization performance for a similar TCAPS 

device showed an average increase in error of approximately 25° for short-duration 

stimuli (Scharine and Weatherless 2013). It is likely that TCAPS more greatly 

impairs the localization of short stimuli because listeners are forced to rely only on 

monaural cues for their localization estimates. To the extent which the TCAPS 

preserves binaural information, they are predicted to minimally affect localization 

of long-duration stimuli since the user can gather more information through head 

movement. The IEEP is similar to other TCAPS devices in that there are small 

earphone tips used to provide passive protection from noise, as well as to transmit 

radio and ambient communications. Further, the protective lenses may reflect and 

 * The method proposed actually specifies that the longer stimuli should be at least 1000 ms, up to 

7000 ms. The stimulus is supposed to play until the listener responds. The testing apparatus did not permit us 

to stop the stimulus once triggered. Therefore, a workaround was to present a shorter stimulus of 4000 ms 

that was sufficiently long enough to allow the listener to respond, but not so long as to have them sitting and 

waiting for it to end.
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alter auditory information. Therefore, the IEEP should reduce localization ability. 

The objective of this study was to characterize these effects.

2. Method

2.1 Participants

Six participants participated in the testing. Study criteria required that they be age 

18 or older and have normal hearing. Normal hearing was defined as bilateral 

hearing thresholds no greater than 25 dB HL* at all audiometric frequencies from 

250 Hz to 8 kHz, including 3 kHz and 6 kHz. Further, bilateral threshold differences 

were not greater than 15 dB at a given frequency. Prior to testing, participants’ ear 

canals were visually inspected with an otoscope to ensure normal morphology and 

the absence of earwax and debris that might prevent the insertion of earphone tips. 

2.2 Test Facility

The study was conducted in the Dome Room of the Environment for Auditory 

Research, a large sound-treated room instrumented with a horizontal array of 180 

loudspeakers positioned at 2° increments (Henry et al. 2009). Thirty-six of the 180 

loudspeakers were used for Method 2; the participant was oriented so that the 

loudspeakers start at 5°, and there are 36 loudspeakers at 10° increments from 5° to 

355° (Fig. 3). The loudspeakers were marked during training by placing a pink 

ping-pong ball in a divot centered on the top of the loudspeaker. Participants were 

not provided with feedback during training or testing, so these markers were used 

to teach the listeners where the potential source locations would be and to increase 

the probability of reliable performance during training. During testing, these 

markers were removed to reduce the interaction of visual information with auditory 

spatial cues.

 *Decibels hearing level (dB HL) is the sound pressure level, in decibels, relative to the average human 

threshold level.
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Fig. 3 Loudspeaker configuration in the dome room: method 2 testing incorporates 

36 loudspeakers spaced at even intervals of 10°, beginning at 5° and continuing to 355°

All sound presentation was controlled using a custom MATLAB R2014b

(Mathworks 2014) script running on a computer operating Windows 7 and located 

in a separate control room. Research participants were seated in a rotating chair 

instrumented with a tracking system. Participants responded by rotating the chair 

and pointing a laser pointer at the perceived source location. The tracking system’s 

response coordinates were recorded by the control room computer.

2.3 Localization Test

2.3.1 Target Stimuli 

The target stimulus was a randomly generated pink broadband (200 Hz–14 kHz) 

noise signal, edited to 1 of 2 stimulus durations, short (250 ms) and long 

(4000 ms) with 10-ms cosine onset and offset ramps. To avoid the use of stimulus 

level as a source of localization information, stimulus level was randomly varied to 

1 of 4 levels (60, 65, 70, and 75 dBA). 
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2.3.2 IEEP Earphone Tip Fitting

A Comply Canal Tips Fitting Guide* measurement tool was used to identify the 

proper sized earphone tips for each participant. Participants were then instructed on 

proper insertion of the earphone tips and asked to practice inserting them until they 

felt comfortable doing so. The experimenter verified that the earphone tips were 

placed so as to be fully inserted in the ear canal, but did not assist with insertion 

after training.

2.3.3 Stimulus Blocks

Training: A block of trials consisted of 3 trials (1 long- and 2 short-duration signals)

presented from each of 10 test locations for a total of 30 trials, randomly selected

for each participant from the locations to be used in the test. The order of trials 

(duration and location) was random. Participants trained on the training blocks for 

at least 5 blocks. After the fifth block, if the average error for the last block was 

within ± 30% of the mean error for the last 5 blocks and no greater than 15°, the 

participant moved on to testing. If not, training continued until the criteria were 

met. All training was completed without the IEEP. All participants tested were able 

to meet the training criteria in fewer than 8 training blocks. 

Testing: A block of trials consisted of 3 trials (1 long- and 2 short-duration signals) 

from each of the 36 locations. Each block, therefore, had 72 short and 36 long trials. 

Participants completed a total of 8 blocks of trials, 4 blocks with the ears 

unoccluded (no IEEP) followed by 4 blocks with the IEEP. During the last 4 blocks, 

the IEEP was refitted after 2 blocks.

3. Results

3.1 Independent Variables 

The design of the experiment was a 4-factor, within-subjects design. The 4 

independent variables included in our analyses were IEEP use, stimulus duration, 

stimulus level, and sound source azimuth. The independent variable of primary 

interest was the effect of IEEP usage on localization error. However, a number of 

other factors were included in the analyses due to the known effects on error or to 

ensure that an irrelevant factor did not impact localization accuracy. Localization 

accuracy is known to vary as a function of sound source azimuth (horizontal 

location of sound source) and stimulus duration and therefore these factors were 

included in the analyses. Stimulus level was a random variable, and was not 

 *Comply canal tips are manufactured by SELEX communications.
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expected to affect localization ability as long as the target signal was sufficiently 

audible allowing participants the use of spatial information within the signal. It was 

included in the analyses to verify that that there are no differences in performance 

due to failure to hear the target signal. 

3.2 Dependent Variables

Horizontal localization error was measured as both the signed and unsigned angular 

difference between the source location and the location of the loudspeaker nearest 

the response location. Localization performance with the IEEP was compared to 

performance with the ears unoccluded (no IEEP). 

3.3 Individual Differences

Individual localization ability differed significantly across the participants. Figure 4 

shows the unsigned error (magnitude) for each participant as a function of IEEP 

use and stimulus duration. Note that there appears to have been a malfunction with 

the IEEP during the fourth subject’s testing. Because performance was clearly 

outside the range of performance found for the other participants, these data were

dropped from the analyses.

*

Fig. 4 Mean unsigned error for each participant, shown as a function of IEEP use and 

stimulus duration. (Error bars shown are the standard error of the mean)

 * Unfortunately, new equipment was installed in the laboratory, making it impossible to retest this 

participant.
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3.4 Signed Errors 

Signed error gives an estimate of bias if it exists in the data. Front-back reversals 

will become evident by positive errors in the front or negative errors in the back. 

Listeners can sometimes show a shift toward the interaural or the mid-sagittal axes. 

Sometimes the acoustics of the research space are asymmetric, or a loudspeaker is 

not well equalized resulting in asymmetric error patterns. Individuals can also show 

particular biases, resulting from cognitive or auditory factors. For a detailed 

discussion see these references (Letowski and Letowski 2012; Scharine et al. 2014). 

Thus, reporting signed error allows one to assess the strength of the localization 

data as well as to observe underlying biases. It should be noted that for the relatively 

small number of participants, perfect symmetry is unlikely.

The experiment was a factorial, within-subjects design. A 4-factor analysis of 

variance with subjects as a covariate was computed for signed localization error 

from the 5 participants. Table 1 summarizes the statistical significance of each of 

the factors. 

Table 1 Summary of ANCOVA for signed localization error with subjects as a covariate

Source df MS F p partial η2

Subject 1 652.84 1.98 0.160 0.001

IEEP 1 108.03 0.33 0.567 0.000

Stimulus durationa 1 38371.04 116.32 0.001 0.031

Stimulus level 3 335.05 1.02 0.385 0.001

Sound source azimutha 35 1319.70 4.00 0.001 0.037

IEEP × stimulus duration a 1 1736.91 5.27 0.022 0.001

IEEP × stimulus level 3 493.51 1.50 0.214 0.001

IEEP × sound source azimuth 35 443.38 1.34 0.085 0.013

Stimulus duration × stimulus 

level 3 520.25 1.58 0.193 0.001

Stimulus duration × sound 

source azimuth a 35 942.06 2.86 0.001 0.026

Stimulus level × sound 

source azimuth 105 296.27 0.90 0.761 0.025

IEEP × stimulus duration × 

stimulus level 3 296.95 0.90 0.440 0.001
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Table 1 Summary of ANCOVA for signed localization error with subjects as a covariate

(continued)

Source df MS F p partial η2

IEEP × stimulus duration × 

sound source azimuth a 35 616.12 1.87 0.001 0.017

IEEP × stimulus level × 

sound source azimuth 105 221.56 0.67 0.996 0.019

Stimulus duration × stimulus 

level × sound source azimuth 105 367.83 1.12 0.202 0.031

IEEP × stimulus duration × 

level × sound source azimuth 105 268.81 0.82 0.915 0.023

Error 3683 329.87 . . . . . . . . .

a = Significant at α < 0.05 level.

The main effect of stimulus duration was significant, F(1, 3683) = 116.32, p < 

0.001; performance was nearly perfect for the 4000-ms stimuli (Fig. 5). The main 

effect of sound source azimuth, F(35, 3683) = 4.00, p < 0.001, reflects a tendency 

to respond toward the front of the array thus making negative errors (Fig. 6). Both 

of these effects are consistent with previous auditory localization research

(Scharine et al. 2014).

Fig. 5 Signed localization error as a function of stimulus duration
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Fig. 6 Overall mean signed localization error shown as a function of sound source azimuth. 

(The gray circle represents 0° error.)

The interaction of stimulus duration and sound source azimuth is shown in Fig. 7. 

Errors were very near to zero for the 4000-ms stimuli. In contrast, participants 

showed a tendency to respond toward the front of the array, resulting in negative 

error values when presented with the 250-ms stimuli, and this bias increased for 

sounds originating from the rear hemisphere.
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Fig. 7 Mean signed error as a function of stimulus duration and sound source azimuth

There was no main effect of IEEP use; however, there was an interaction observed 

for IEEP use with stimulus duration, F(1, 3683) = 5.27, p < 0.022. The average 

signed localization error was –5.9°, suggesting a slight bias to respond to the front 

of the array. This bias increased an average of 0.8° for the 250-ms stimuli and 

decreased 1.8° for the 4000-ms stimuli when wearing the IEEP; however, neither 

simple effect was significant, p = 0.272 and p = 0.074, respectively (Fig. 8). 
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Fig. 8 Mean signed error observed for participants as a function of IEEP use and stimulus 

duration

The 3-way interaction of IEEP use with stimulus duration and sound source 

azimuth is shown in Fig. 9, F(35, 3683) = 1.87, p < 0.001. It should be noted that 

in many of the previous studies of auditory localization with TCAPS, the target 

stimulus duration was 250–400 ms (Scharine 2009; Scharine and Letowski 2013; 

Scharine and Weatherless 2013; Scharine et al. 2014). The effect of IEEP use is 

more apparent for the 250-ms stimuli and is seen as an increased bias to respond 

toward the front of the array for stimuli originating from the rear hemisphere, and 

a smaller bias to respond toward the back for stimuli originating near 0°. 

Conversely, for the 4000-ms stimuli, there is no significant effect of IEEP use. 
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Fig. 9 Mean signed error as a function of IEEP use, stimulus duration, and sound source 

azimuth

3.5 Unsigned Errors

Unsigned error is an estimate of the magnitude of an error. If the errors for 

individual estimates are large, it is not practically important that the average of 

those errors be near zero. Good auditory localization performance is that in which 

average unsigned errors are small. A summary of the 4-factor analysis of variance 

with subjects as a covariate computed for unsigned localization is given in Table 2. 
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Table 2 Summary of ANCOVA for unsigned localization error with subjects as a covariate

Source df MS F p partial η2

Subjecta 1 1641.51 5.53 0.019 0.001

IEEP 1 0.79 0.00 0.959 0.000

Stimulus durationa 1 34884.79 117.52 0.001 0.031

Stimulus level 3 67.17 0.23 0.878 0.000

Sound source azimutha 35 1545.31 5.21 0.001 0.047

IEEP × stimulus durationa 1 4990.83 16.81 0.001 0.005

IEEP × stimulus level 3 491.60 1.66 0.174 0.001

IEEP × sound source azimuth 35 206.93 0.90 0.909 0.007

Stimulus duration × stimulus 

level 3 258.15 0.87 0.456 0.001

Stimulus duration × sound 

source azimuth 35 529.76 1.79 0.003 0.017

Stimulus level × sound source 

azimuth 105 200.60 0.68 0.995 0.019

IEEP × stimulus duration × 

stimulus level 3 57.01 0.19 0.902 0.000

IEEP × stimulus duration × 

sound source azimuth* 35 457.66 1.54 0.022 0.014

IEEP × stimulus level × 

sound source azimuth 105 180.49 0.61 0.999 0.017

Stimulus duration × stimulus 

level × sound source azimuth 105 196.20 0.661 0.997 0.018

IEEP × stimulus duration × 

stimulus level × sound source 

azimuth

105 214.19 0.72 0.985 0.020

Error 3683 296.84 . . . . . . . . .

a = Significant at α < 0.05 level.
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Figures 10 and 11 show the main effects of stimulus duration, F(1, 3683) = 117.52, 

p < 0.001, and sound source azimuth, F(35, 3683) = 5.21, p < 0.001, respectively. 

On average, the longer 4000-ms stimulus duration reduced the size of errors by 

approximately 7°. Error sizes increased by an average of 6° in the rear hemisphere,

especially near 180°. 

Fig. 10 Mean unsigned error shown as a function of stimulus duration
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Fig. 11 Overall mean unsigned error shown as a function of sound source azimuth

Figure 12 shows the significant interaction of stimulus duration and sound source 

azimuth, F(35, 3683) = 2.86, p < 0.003. Overall, errors for the 4000-ms stimuli are 

very small at most horizontal azimuths. In contrast, there are large errors in the rear 

hemisphere for the 250-ms stimuli that are probably due to back-front reversals. 

This will be discussed in the following section.
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Fig. 12 Mean unsigned error shown as a function of stimulus duration and sound source 

azimuth

As with the signed error, there was no main effect for IEEP use for unsigned error 

(Fig. 13). There was a 2-way interaction of IEEP use with stimulus duration, F(1, 

3683) = 16.81, p < 0.001. Use of the IEEP increased average unsigned error by 2.2° 

for the 250-ms stimuli and decreased average unsigned error by 2.2° for the 

4000-ms stimuli. A simple effects analysis showed these differences to be 

significant for both the 250-ms, F(1, 4259) = 11.00, p = 0.001, and 4000-ms stimuli, 

F(1, 4259) = 5.58, p = 0.019.
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Fig. 13 Mean unsigned error shown as a function of IEEP use and stimulus duration

Figure 14 shows the 3-way interaction of IEEP use with stimulus duration and 

sound source azimuth, F(35, 3683) = 1.54, p < 0.022, appears to be the result of 

larger errors in the rear hemisphere for the 250-ms stimuli, the increase in these 

errors due to IEEP use, and for some larger errors at 135° and 175° for unoccluded 

ears (the no-IEEP condition). Humans are normally vulnerable to front-back 

confusions; it is interesting that use of the IEEP seems to reduce the occurrence of 

these errors for the 4000-ms stimuli. It may be that the participants were moving 

their head to balance the level arriving at each ear, and thus derived some benefit 

from their use. 
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Fig. 14 Mean unsigned error shown as a function of IEEP use, stimulus duration, and sound 

source azimuth

3.6 Reversals

The primary spatial auditory concern for users of TCAPS devices is whether the 

device will increase front-back confusions significantly. Localization blur usually 

results in a slight increase in the average magnitude of errors. However, visual 

feedback will allow the listener to refine the auditory estimate if the target is within 

the listener’s field of view. Conversely, front-back confusions, also known as 

reversals, can cause very large errors if the sound source azimuth is near 0° or 180°. 

Therefore, the data were coded for responses that were most likely due to reversals 

so that we could determine the degree to which reversals are the source of 

localization error. 

For each trial, the error was compared to what the error would have been if the 

sound source had been in the reverse hemisphere. If TT < 180°, the reversal TTrr was 

computed as follows:

TTrr = 180° − TT, (1)

else:

TTrr = 540° − TT. (2)
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The error for this reversed azimuth angle was computed by subtracting it from the 

estimated angle

EErr = EE − TTrr. (3)

A trial was coded as a reversal if the unsigned error was greater than 30° and if the 

unsigned error for the reversed azimuth angle was smaller than the unsigned error 

for the sound source azimuth. Reversals for sounds originating from near 0° or 180° 

are large. Conversely, if a sound originated from near 90° or 270°, it is difficult to 

determine whether an error is due to a front-back error or to localization blur. In 

practice, the effect is the same. Therefore, we include the limit that the original 

error must be greater than 30° for the trial to be coded as a reversal. Thus if

|EErr| < |EE| & |EE| ≥ 30° ≝ "reversal”. (4)

Overall, only 83 of a total 4260 trials (1.9%) were coded as a reversal. Yet, most of 

the significant effects reported in the previous section are the result of a higher 

percentage of reversals for certain factor values. For instance, in Fig. 15 we see that 

more reversals occurred near the horizontal azimuths of 105°, 185°, and 245°. This 

effect is predicted near 180°. It should be noted that the increases near 105° and 

245° might be an artifact of how we defined reversals. If we change the criterion 

for reversal to a minimum error of 20° or 45°, a slightly different pattern would 

emerge.

Fig. 15 Percent reversals shown as a function of sound source azimuth
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More reversals were observed for the short-duration stimuli, especially when using 

the IEEP (Fig. 16). Although reversals occurred for only a small percentage of 

trials, it is clear that they are the source of differences in error between the no-IEEP 

and IEEP conditions for the 250-ms stimuli. For the short-duration stimuli (250 

ms), IEEP use increased the percentage of reversals from 1.5% to 3.8%. For the 

long-duration trials, there were no reversals observed for the IEEP trials, but 1.4% 

of the no-IEEP trials were coded as reversals. Figure 17 shows the percent reversals 

as a function of IEEP use, stimulus duration, and sound source azimuth. 

It is not clear why participants derived no significant benefit from the longerduration stimuli for the ears unoccluded (no IEEP) condition. No feedback was 

given during training or testing. It may have been that the participants were overly 

confident in their ability to localize accurately when presented with the longer 

stimuli and made less of an effort to check their percept. Given feedback, 

participants might have adapted to the feedback and performed better for the ears 

unoccluded (no IEEP) condition. 

Fig. 16 Percent reversals observed as a function of stimulus duration and IEEP use
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Fig. 17 Percent reversals as a function of IEEP use, stimulus duration, and sound source 

azimuth

4. Conclusion

Auditory localization ability was assessed for participants wearing the IEEP, a 

prototype TCAPS that also provides eye protection. Testing was conducted using 

one of the methods recently proposed by the DOD HCoE for an auditory 

localization measurement standard. All participant-wise performance was reported 

in comparison to performance with their ears unoccluded.

Listeners using the IEEP showed good localization accuracy with the device. The 

IEEP, after taking into account the known effects of sound source azimuth and 

stimulus duration, did not significantly change overall localization ability. 

However, it did have a small, but significant effect on accuracy for the shortduration (250-ms) stimuli, increasing the average unsigned error from 10.3° to 

12.5°. This may be of particular relevance to Soldiers, as one sound that they might 

wish to localize is weapon fire, which is a short-duration sound. This is presumed 

to be due to the alteration of monaural cues caused by the insertion of the earphone 

tips as well as the fact that the short duration restricts the listener’s ability to use 

movement cues to localize. Conversely, given the time to move one’s head when 

responding to a longer stimulus (4000 ms), IEEP use decreased average unsigned 

error from 5.8° to 3.6°, perhaps because listeners learned to rotate until the input 

coming into both ears was equal.
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The data were coded to determine the proportion of trials for which reversals had 

occurred. For the short-duration stimuli (250 ms), IEEP use increased the 

percentage of reversals from 1.5% to 3.8%. For long-duration stimuli (4000 ms), 

no reversals were observed for the IEEP condition.

Auditory localization ability is one of several factors to be considered when 

determining the quality of auditory situation awareness provided by a TCAPS 

device (Clasing and Casali 2014). Other factors include one’s ability to detect, 

recognize, and identify sounds, including face-to-face and radio communications. 

Further acoustic testing is needed to ensure that hearing protection requirements are 

met. However, from the standpoint of auditory localization as tested in the current 

study, the IEEP has a minimal impact on localization performance, allowing the 

user to retain the majority of his or her auditory spatial capabilities.
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5. References

Bernhard SE. Auditory localization of steady state and impulse sounds in an urban,

relevant reverberant environment [PhD thesis]. [Towson (MD)]: Towson 

University; 2015.

Buchanan K. Interim standard for the measurement of auditory localization. 

working group meeting minutes. Dayton (OH): Hearing Center of Excellence;

2016 Apr.

Clasing JE, Casali J G. Warfighter auditory situation awareness: effects of 

augmented hearing protection/enhancement devices and TCAPS for military 

ground combat applications. Int J Audiol. 2014;53 Suppl 2:S43–S52.

Henry PP, Amrein BE, Ericson MA. The environment for auditory research. Acoust 

Tod. 2009;5(3):9–16.

Letowski TR, Letowski S. Localization error: accuracy and precision of auditory 

localization. Aberdeen Proving Ground (MD): Army Research Laboratory 

(US); 2012 May. Report No.: ARL-TR-6016. DTIC No.: ADA562292.

Mathworks. MATLAB Release 2014b. Natick (MA): The Mathworks, Inc.; 2014.

Oldfield SR, Parker SPA. Acuity of sound localisation: a topography of auditory 

space. I. Normal hearing conditions. Perception. 1984;13(5):581–600.

Riederer KAJ. Effects of eye-glasses, hair, headgear, and clothing on measured 

head-related transfer functions. Part 1B. J Acoust Soc Am. 2003a;114:2388. 

doi.org/10.1121/1.4777835.

Riederer KAJ. Effects of hair, clothing, and headgear on localization of threedimensional sounds. Part IIb. J Acoust Soc Am. 2003b;114:2389. 

doi.org/10.1121/1.4777865.

Saberi K, Perrott DR. Minimum audible movement angles as a function of sound 

source trajectory. J Acoust Soc Am. 1990;88:2639–2644.

Scharine AA, Letowski TR. The measurement of the effects of helmet form on 

sound source detection and localization using a portable four-loudspeaker test 

array. Aberdeen Proving Ground (MD): Army Research Laboratory (US);

2013 May. Report No.: ARL-TR-6444. DTIC No.: ADA588880.

Scharine AA, Weatherless RA. Helmet electronics and display system-upgradeable 

protection (heads-up) phase III assessment: headgear effects on auditory 
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perception. Aberdeen Proving Ground (MD): Army Research Laboratory 

(US), 2013 Nov. Report No.: ARL-TR-6723. DTIC No.: ADA592067.

Scharine AA, Weatherless RA. US Marine Corps level-dependent hearing protector 

assessment: objective measures of hearing protection devices. Aberdeen 

Proving Ground (MD): Army Research Laboratory (US); 2014 Jan. Report 

No.: ARL-TR-7203. DTIC No.: ADA597684.

Scharine AA. Degradation of auditory localization performance due to helmet ear 

coverage: the effects of normal acoustic reverberation. Aberdeen Proving 

Ground (MD): Army Research Laboratory (US); 2009 Jul. Report No.: ARLTR-4879. DTIC No.: ADA506629.

Scharine AA, Binseel MS; Mermagen TJ; Letowski TR. Sound localisation ability 

of soldiers wearing infantry ACH and PASGT helmets. Ergonomics.

2014;57(8):1–22.

Thurlow WR, Mangels JW, Runge PS. Head movements during sound localization. 

J Acoust Soc Am. 1967;42:489–493.

Vliegen J, Van Opstal AJ. The influence on duration and level on human sound 

localization. J Acoust Soc Am. 2004;115(4):1705–1713.

Wallach H. The role of head movements and vestibular and visual cues in sound 

localization. J Exp Psych. 1940;27(4):339–368.
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List of Symbols, Abbreviations, and Acronyms

ARA Applied Research Associates, Inc.

dBA A-weighted decibels 

dB HL Decibels relative to hearing level

dB SPL Decibels relative to absolute sound pressure level

DOD Department of Defense

HCoE Hearing Center of Excellence

HPD hearing protection device

IEEP integrated eye and ear protection

MAA Minimum Discriminable Audible Angle

NSRDEC Natick Soldier Research, Development and Engineering Center

TCAPS tactical communications and protection systems
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1 DEFENSE TECHNICAL

(PDF) INFORMATION CTR

DTIC OCA

2 DIRECTOR

(PDF) US ARMY RESEARCH LAB

RDRL CIO L

IMAL HRA MAIL & RECORDS

MGMT

1 GOVT PRINTG OFC

(PDF) A MALHOTRA

5 US ARMY NATICK SOLDIER

(PDF) RSRCH DEV & ENGR CTR

M MARKEY

D COLANTO

A CHISHOLM

S GERMAIN

J KRUSZEWSKI

1 US ARMY AEROMEDICAL

(PDF) RSRCH LAB

W AHROON

1 MARINE CORPS SYS CMD

(PDF) J O’DONNELL

1 WALTER REED NATIONAL

(PDF) MILITARY MEDICAL CTR

D BRUNGART

4 AFRL

(PDF) B SIMPSON

N IYER

E THOMPSON

G ROMIGH

1 AFRL RET

(PDF) R MCKINLEY

1 ARL RET

(PDF) T LETOWSKI

1 VIRGINIA TECH

(PDF) J CASALI

2 DOD HEARING CTR

(PDF) OF EXCELLENCE

T HAMMILL

K BUCHANAN

1 ARMY PUBLIC HEALTH

(PDF) CMND (PROVISIONAL)

M ROBINETTE

1 USAARL APPD

(PDF) E REEVES

1 HPW/RHCB

(PDF) H GALLAGHER

4 DIR USARL

(PDF) RDRL HRP

R WEATHERLESS

RDRL HRF D

A SCHARINE

M DOMANICO

A FOOTS
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