Hearing protections: Effects on HRTFs and localization accuracy
Lorenz KROENER(1),(2) , Alexandre GARCIA(2) , Véronique ZIMPFER(1) , Christophe LANGRENNE(2)(1) Acoustic and Soldier Protection Group, French-German Research Institute of Saint-Louis, France, lorenz.kroener@isl.eu(2) Laboratoire de Méchanique des Structures et des Systèmes Couplés, Conservatoire national des arts et metiers, France

Abstract
Protecting the auditory system from dangerous sound pressure levels is essential for many professionals andpeople in daily life. Wearing hearing protections devices (HPDs) or tactical communication and protection
systems (TCAPS) modifies the geometry of the auricle. The user cannot adapt instantaneously to the modified
HRTF, leading to a less accurate sound source localization. Listening test showed that front-back confusions are
the major error type. Three different models of TCAPS were analysed to gain information about their influenceon sound source localization capabilities and on the modifications in the listener’s HRTF. HRTF measurementson different dummy heads were done during four scenarios: dummy head without TCAPS, dummy head wearing
two different in-ear TCAPS (earplugs) and one on-ear TCAPS (earmuffs). Sound source localization tests wereconducted with subjects under the same four scenarios. Based on those results concepts for prototype of new
generation TCAPS are presented.
Keywords: hearing protection, sound source localization, HRTF, front-back confusion, TCAPS

1

INTRODUCTION

Sound source localization is performed by the brain and the auditory system and helps humans in daily life
to orientate them and to analyze their environment. With the natural hearing sound source localization is aquite simple task, which is performed by the brain with high accuracy. It allows separating the individual
sound sources in a multi-source scene and recognizing dangers. Beside the interaural time difference (ITD)
and the interaural level difference (ILD), the individual Head-Related Transfer Function (HRTF) contributes in
identifying sound sources [1]. The HRTF filters the incoming sound depending on the angle of incidence andintroduces spectral cues, containing directional information. The HRTF is defined by the individual geometryof the listener’s anatomy (torso, head, and auricle). The listener’s brain is well trained to his own HRTF andhence sound source localization is performed with high accuracy [2].
In certain situations it is necessary to wear hearing protection to prevent hearing disorders. Either hearing
protection devices (HPDs) or tactical communication and protection systems (TCAPS) can be used, but independently of the model, they all change the geometry of the auricle. This change in geometry leads toinstantaneous changes in the HRTF and in the spectral cues. The listener cannot adapt instantaneously to themodified HRTF. This results in a less accurate sound source localization. Listening test showed that independently of the type of TCAPS the accuracy of sound localization decreases significantly and front-back confusion
is the major error type [3][4][5][6][7]. The loss of localization accuracy with hearing protection is reported bymany authors, but only few provide solutions to preserve the localization performances. Joubaud [9] and Rubak
and Johansen [8] propose to integrate an auricle like geometry on the outside of an earmuff shelf, such that
directional information is introduced to the sound that is captured by the TCAPS’s outside microphone.
Also hearing aids have to deal with the issue of modified directional cues and the degradation of the accuracyof sound localization by the user. It is reported by Biggins [10] and Bishop [11] that with reduced geometryof the hearing aid’s earplug improved sound source localization is obtained. The performance obtained with inthe-ear (ITE) hearing aids was quite poor and with in-the-canal (ITC) devices it was already acceptable. With
complete-in-the-canal (CTC) hearing aids the performance was nearly as good as with a healthy natural hearing.
Four different TCAPS are used in the presented work to identify their influence on the sound localization per-5807

formance and on the HRTF. Protection P1 (BANG by ISL) and protection P3 (QuietPro by Nacre) are earplugbased talk through TCAPS with dynamic talk through gain. Protection P2 (Z111 by Z-Tac) is a talk throughearmuff TCAPS with dynamic talk through gain and protection P4 (X5A by 3M) is a passive earmuff hearing
protection. These TCAPS are used for the localization tests with human subjects and the HRTF measurementson dummy heads. The HRTFs are measured with three different dummy head configurations: DH1 (ISL dummy
head with Type 3.4 ear simulators), DH2 (ISL dummy head with Type 3.3 ear simulators) and DH3 (B&K Headand Torso simulator with Type 3.3 ear simulators).

2

METHODS

2.1 Localization test
The localization test is intended to be conducted with subjects having a normal hearing. The hearing performance of the subjects is obtained by measuring the left and right ear audiogram, with the help of a Bekesy
test. The tested frequencies range from 125 Hz to 8 kHz in steps of octaves.
The listening test is performed in a listening room, which is optimized for speech listening test. The RT60
reverberation time of the listening room is 290 ms. The subject takes place on a seat at the listening positions.
A circular loudspeaker array of 16 equally spaced JBL Control 1 Pro speakers surrounds the listening positions.
The loudspeakers are located at φSP = 0◦ , 22.5◦ , 45◦ , ..., 337.5◦ . If not otherwise stated, the reference coordinate
system is a right hand coordinate system with its origin at the center of the circular array. The angle direction
φ = 0◦ is directly in front of the listener. The diameter of the array is 2.20 m. The seat is adjustable in height,
such that the interaural axis of the subject aligns the speaker’s plane. A curtain is mounted in front of the
speakers, such that the subject can’t see the speaker’s location.
Two groups are defined: the test group and the control group. The subjects are randomly attributed to thosegroups. Members of the test group complete a training and before they perform the actual test, those of the test
group perform directly the actual test. The subjects have to steer a cursor on a tablet computer in the direction
where he located the presented test sound and confirm his answer. The localization test is designed as unforced
choice test, thus the cursor can be positioned to point in any direction. The test sound consists of 200 ms ofwhite noise. Only real sound sources are tested. The sound level at the listening position is 72 dB SPL ± 3 dB.
For each test sound the sound level is chosen randomly within the limits of this interval. Each of the soundsource directions is presented twice during training and five times during testing. The order of presenting the
32 resp. 80 directions is chosen randomly. During training the subject gets a visual feedback on the tablet
computer of the actual direction after having confirmed his answer.
Each subject has to perform the localization test under multiple hearing conditions (HC). The first hearing
condition is always the natural hearing (HC0), followed by a random order of HC1 (hearing condition withprotection P1), HC2 (hearing condition with protection P2), HC3 (hearing condition with protection P3) andHC4 (hearing condition with protection P4). Prior to the localization test each subject gets 60 s to familiarize
with the test. They can freely set the direction of a music sound source by pointing the cursor on the table inthe desired direction.
The data which is collected for each tested sound is called data set. It includes the gain factor of the SPL
modulation, the true sound direction φt , the subject’s response φu and the response time tr . The metadata
contains information about the subject, including the group, the gender, the year of birth, the handedness andthe hearing condition.
2.2 Metrics for localization test
Based on the error ε, defined in equation 1, nine metrics for the evaluation of the localization test are defined:(+360 Φu − Φt < −180
ε = Φu − Φt(1)−360 Φu − Φt > 1805808

M1: Standard deviation error
sde = std (ε)(2)

M2: Root mean square error
s
err =

1 N
· ∑ ε(i)2
N i=1

(3)

M3: Accuracy
 

 
Φu
accu = mean abs
mod 1 − 0.5 · 2
22.5

(4)

M4: Front-Back Confusion Rate The Front-Back confusion rate is defined as the ration of the number ofsamples where the subject’s response and the true direction are front-back inverted to the number of overall
tested samples.
M5: Left-Right Confusion Rate The Left-Right confusion rate is defined as the ration of the number ofsamples where the subject’s response and the true direction are left-right inverted to the number of overall
tested samples.
M6: Mixed Confusion Rate The Mixed confusion rate is defined as the ration of the number of samples
where a front-back and left-right confusion appear to the number of overall tested samples.
M7: Left-Right Driftn+ = count (ε > 0) |Φt ∈ [0, 90[ ∪ ]270, 360]n− = count (ε < 0) |Φt ∈ [90, 270[n = count(Φu )|Φt ∈ Φs \ {90, 270}
dLR =

(5)

n+ + n−n

M8: Front-Back Driftn+ = count (ε > 0) |Φt ∈ ]180, 360[n− = count (ε < 0) |Φt ∈ ]0, 180[n = count(Φu )|Φt ∈ Φs \ {0, 180}
dFB =

(6)

n+ + n−n

M9: Frontal-Median Driftn+ = count (ε > 0) |Φt ∈ [0, 90[ ∪ ]180, 270]n− = count (ε < 0) |Φt ∈ [90, 180[ ∪ ]270, 360]n = count(Φu )|Φt ∈ Φs \ {0, 90, 180, 270}
dFM =

(7)

n+ + n−n5809

2.3 HRTF Measurement
The HRTF Measurements are performed in an anechoic chamber of a volume of 42.34 m3 and RT60 reverberation time of 240 ms. During the HRTF measurement the current dummy head is mounted on a turn tabled.
The position of the turn table can be controlled electronically and the HRTF is measured at an elevation angle
θ = 0◦ and azimuth angles φ = 0◦ , 22.5◦ , 45◦ , ..., 337.5◦ . A sine sweep signal ranging from 20 Hz to 20 kHz
is used for the HRTF measurement, generated by Standford Research Signal Analyzer SR780. The signal ispassed through a power amplifier DaytonAudio MA1260 and played back via a JBL Control 1 Pro loudspeaker.
The speaker is distanced by 3 m from the center of the dummy head. The signals of the dummy head’s left
and right microphone are preamplified by B&K Type 5935L and passed to the signal Analyzer. The signal
analyzer evaluates the transfer function HRTFRAW L (f, φ ), resp. HRTFRAW R (f, φ ) at 1600 frequency points. The
free field transfer functions HRTFFF L (f, φ ) and HRTFFF R (f, φ ) are obtained by dividing HRTFRAW L (f, φ ), resp.
HRTFRAW R (f, φ ) by the transfer function of the measurement system. This transfer function is obtained by replacing the dummy head and turntable by a reference microphone, while preserving the remaining measurement
setup and measuring the transfer function of the system.
The dummy heads are equipped with matched microphones and their geometries are perfectly symmetric along
the median plane, hence it can be assumed that the HRTFs, and consequently HRTFFF L (f, φ ) and HRTFFF r (f, φ ),are perfectly symmetric along the median plane. Accordingly to Jauboud [9] the front-back difference, averagefor each quadrant are calculated for the ipsi and contralateral side. This representation will be referred to as
Ipsi- & Contralateral Front-Back (ICFB) representation. ICFB contains two functions: one for the ipsilateralside and one of the contralateral side. Each of them is based on the difference between the frontal quadrant
HRTF and back quadrant HRTF.
ξThe root mean square (RMS) distance is used to evaluate measured HRTFs. The RMS distance RMSi,i+1 beξξ

tween two ICFB curves (ICFB( f )i and ICFB( f )i+1 ) can be calculated for the ipsilateral (ξ = I), resp. conξ

tralateral (ξ = C) side, as shown in equation 8. δi,i+1 is the distance between the i-th and i + 1-th ICFB curve.
v
u N
u1
ξ
2
(f)
RMSi,i+1 = t ∑ δi,i+1
N f =1(8)The RMS distance is calculated for the HC0 hearing conditions between the different dummy head configurations. Further, for each dummy head configuration the RMS distance between the HC0 hearing condition andthe hearing conditions HC1 to HC4 is calculated.

3

RESULTS

Note: Due to the number of subjects which has participated so far, the results of the listening test have to beconsidered as preliminary.
3.1 Listening test
5 subjects (3 male, 2 female) have completed the listening test. The average of their age is 30.6, their age
spanned the interval from 20 years to 57 years. 3 subjects were left handed, while 2 were left handed. The
Bekesy audiograms showed normal hearing characteristics for all subjects. The average answer time of all
subjects was 261 ms. The results of the Standard deviation error (M1), the RMS error (M2), the accuracy (M3),the front-back (M4), left-right (M5), and mixed (M6) confusion rate and the response time tr are listed in Table
1. The data is shown respectively for the test group and control groups and each hearing condition. The metricsof the left-right drift (M7), front-back drift (M8) and median-frontal drift (M9) are listed in Table 2 with respect
to the handedness of the subjects and the hearing condition, regardless of the subject’s group.

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Table 1. Metrics as defined in section 2.2 and minimum, average, and maximum answer time tr
Metric
M1
M2
M3
M4
M5
M6
min(tr )avg(tr )max(tr )

Unit
deg
deg
–
–
–
–
s
s
s

HC0
TG
CG
21.72 45.24
21.89 45.27
0.61
0.57
0.06
0.18
0.00
0.00
0.00
0.00
1.28
0.78
3.40
2.92
6.47 10.94

HC1
TG
CG
33.78 61.71
33.71 61.59
0.60
0.54
0.10
0.31
0.00
0.00
0.00
0.00
1.11
0.62
2.61
2.09
12.89 5.89

HC2
TG
CG
61.44 62.88
62.47 62.83
0.62
0.57
0.47
0.41
0.00
0.00
0.01
0.00
1.03
0.73
2.82
2.40
6.75
7.34

HC3
TG
CG
53.63 58.86
53.52 58.79
0.64
0.59
0.26
0.36
0.00
0.00
0.00
0.00
1.00
0.81
2.45
2.26
5.23
6.67

HC4
TG
CG
60.88 59.80
60.76 59.68
0.60
0.56
0.42
0.29
0.01
0.01
0.00
0.02
1.03
0.80
2.82
2.40
8.44 12.23

Table 2. Drift metrics M7 (left-right), M8 (font-back) and M9 (frontal-median) of the left (LH) and right handed(RH) subjects for hearing condition HC0 to HC4.
Drift
M7
M8
M9

HC0
LH
RH
0.56 0.53
0.79 0.53
0.60 0.75

HC1
LH
RH
0.56 0.54
0.7 0.48
0.68 0.76

HC2
LH
RH
0.56 0.56
0.74 0.50
0.69 0.90

HC3
LH
RH
0.61 0.54
0.70 0.47
0.72 0.82

HC4
LH
RH
0.46 0.57
0.71 0.48
0.67 0.86

3.2 HRTF Measurement
The upper part of Table 3 shows the RMS distance of the ICFB curves obtained with the dummy head configurations DH1, DH2, DH3 during hearing condition HC0. Lower part of the table shows the RMS distance
between different hearing conditions of the same dummy head configuration. The columns represent the dummy
head configurations in the HC0 hearing conditions.
Table 3. RMS distances between the ipsilateral and contralateral ICFB curves for different dummy head configurations.

DH1
DH2
DH3

DH1
–
3.94
3.47

HC1
HC2
HC3
HC4

–
–
1.93
–

ipsi
DH2
–
–
2.08
HC0
2.98
4.93
3.38
4.56

DH3
–
–
–

DH1
–
3.95
2.41

2.63
4.51
3.57
4.83

–
–
3.73
–

contraDH2 DH3
–
–
–
–
2.33
–
HC0
2.39 3.58
3.57 7.66
2.72 2.54
5.66 3.03

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4

DISCUSSION

4.1 Localization test
By its definition in equation 2 the standard deviation error can be interpreted as the precision of a subject.
From Table 1 it can be obtained that the standard deviation error and the RMS error are in average linear
dependent with a factor of 0.9987. For the hearing condition HC0 and HC1 an improvement of the precision
can be obtained by passing the training. The subjects of the test group localize the sound directions nearly
twice precise as those who haven’t performed the training. This does not hold for hearing condition HC2 to
HC4. There no particular improvement or degradation of the precision can be obtained.
Regardless of the hearing condition the accuracy gets improved by a least 0.04 when the subjects have completed the training. A learning process can be therefore obtained, which is due to the visual feedback thesubjects get during training. They get adapted to the direction of the sound sources and get trained to point
more frequently to the direction where they remember the true sound positions. Their responses are closer to a
loudspeaker position, than the responses of those subject without training.
The highest front-back confusion rates is obtained with HC2 which has an asymmetric distribution of microphones. Its microphone on each side is oriented towards the front. The least front-back confusion rates isobtained with the natural hearing (HC0). There appear no modifications in the HRTF and the subjects are best
trained to this hearing condition. Regarding the front-back confusion rate two trends can be obtained: With
HC0, HC1, and HC3 less front-back confusions are obtained for the test group than for the control group. WithHC2 and HC4 more front-back confusions are obtained for the test group than for the control group. P2 andP4 are earmuffs covering the entire outer ear in contrast to the in-ear devices P1 and P3 which don’t. Reducing
the size of the earplug and avoiding earmuff designs help to decrease the front-back confusion. This effect wasalso obtained by Zimpfer [5] and Biggins [10].
Mixed confusion rates and left-right confusion rates are nonzero in four hearing conditions. The nonzero values
reach a maximum of 0.02. Hence these hearing protections do not have any remarkable effect on this type of
localization error.
The average response time is the highest for the test group with hearing condition HC0. The reduced average
response time in the hearing conditions HC1 to HC4 might be due to adaption process, but also due to thesubject’s motivation. HC0 is the first round in the test, where the subjects might be highly motivated to achieve
good localization results. After having passed in the HC0 hearing conditions, the test does not provide new
motivation to the subject, consequently the subjects accelerate.
The results in Table 2 show that there is no particular drift to the left resp. right side as a function of thesubject’s handedness. For left and right handed subjects the M7 metric (left-right drift) varies between 0.46
and 0.61 without a link between the drift direction and the handedness of the subject. Same holds for the
M8 metric (front-back drift). For the M9 metric (frontal-median drift) a trend towards the median plane (values
greater than 0.5) can be observed for all hearing conditions. Sound sources are perceived more frequent towards
0◦ and 180◦ , than towards 90◦ and 270◦ . As this trend can be also observed for the hearing condition HC0,
this effect is not only due to the hearing protection, but also to the concept of the localization test.
4.2 HRTF measurement
The RMS distances of the ICFB curves, obtained with the different dummy head configurations with hearing
condition HC0, define the range of tolerance. HRTF whose RMS distances values lie within this interval areconsidered not to be modified seriously. The interval of tolerance range to 3.94. Applying this interval to thelower part of Table 3, it can be obtained that the HRTFs measured with P2 and P4 show important modifications. This holds for all of the measured ipsilateral HRTFs and dummy head configurations. P2 and P4 areearmuff based HPDs introducing high HRTF modifications. P1 and P3 which are in-ear devices introduce HRTF
modifications which lie in the tolerance interval. Hence the smaller the geometries of the hearing protection isand the less it covers the geometry of the helix, the less HRTF modifications are caused.

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5

CONCLUSIONS

A localization test was conducted with subjects under five hearing conditions. The subjects had to perform thelocalization with their natural hearing and with four different models of TCAPS. HRTF measurements were
performed on different types of dummy heads in an anechoic chamber. The HRTF measurements were donewith the same hearing conditions which were used for the localization test. The degradation of the HRTF
introduced by TCAPS was presented and discussed. The performance of the subjects during the localization
test was discussed and compared to the obtained HRTF degradation. It was obtained that a training can help toimprove the sound localization and reduce the front-back confusion for certain TCAPS.
The design of the new TCAPS either have to consist of a small dimensions of the in-ear earplugs to avoid
any major modifications of the geometry of the helix. Or, in the case of earmuff devices, they have to beequipped with additional signal processing elements, e.g. microphones and filters to reconstruct the original
directional information and to prevent any HRTF modifications. The design of signal processing chains mounted
on earmuffs based TCAPS is the task of current studies and this work will be presented in the future.

REFERENCES[1] Blauert, J. Blauert, J. Spatial hearing: the psychophysics of human sound localization. MIT press, 1997.
[2] Hofman, P; et al. relearning sound localization with new ears. Nature Neuroscience, 1 (5), 1998, 417 – 421.
[3] Joubaud, T. Prédiction objective de l’effet des systèmes tactiques de communication et protection sur lesperformances de localisation sonore, PhD Thesis, 2017.
[4] Joubaud, T.; et al. Degradation of front-back spectral cues induced by tactical communication and protective
systems. EuroNoise, 2015, 1943–1947.
[5] Zimpfer, V.; Sarafian, D. Impact of hearing protection devices on sound localization performance. Frontiersin Neuroscience, 8, 2014.
[6] Noble, W.; Russel, G. Theoretical and Practical Implications of the Effects of Hearing Protection Devices
on Localization Ability. Acta Oto-Laryngologica, 74, 1972, 29–36.
[7] Simpson, B; et al. Sound Localization with Hearing Protectors: Performance and head motion analysis in avisual search task. Human Factors and Ergonomics society, 46, 2002, 1618–1622.
[8] Rubak, P.; Johansen, L. G. Active hearing protector with improved localization performance. In INTERNOISE New Zealand Acoustical Society. 1, 1998, 627–632.
[9] Joubaud, T.; et al. Sound localization models as evaluation tools for tactical communication and protective
systems. Journal of the Acoustic Society of America, 141 (4), 2017, 2637–2649.
[10] Bishop, R.; et al. Biometric Calibration: improving directionality for Phonak VirtoTM B. Phonak, 2017.
[11] Biggins, A.; et al. LyricTM shows significant psychosocial benefits. Phonak, 2017.

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