PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Effect of Head-Movement on Sound-Field Auditory Steady State
Response Measurement
Sreeram KAITHALI NARAYANAN 1; Søren LAUGESEN2; Valentina ZAPATA-RODRIGUEZ 2,3;
Jonas BRUNSKOG3; Cheol-Ho JEONG3
1

Department of Electronic Systems, Aalborg University*
Interacoustics Research Unit

2
3

Acoustic Technology Group, Department of Electrical Engineering, Technical University of Denmark

ABSTRACTSound-field auditory steady-state response (sound-field ASSR) measurement is an objective alternative forhearing aid fitting validation in infants. For including the hearing aid in the signal path, the stimulus ispresented via a loudspeaker. In this case, ASSR can be affected by the head orientation of the participants
and the measurement room resulting from the change in degree of modulation of the stimulus in themeasurement room. Eleven normal hearing participants were tested for three static head-orientations relative
to the loudspeaker in two different rooms. A speech modified NB CE-Chirps stimulus was used to eventually
force the hearing aid under test to provide the correct gain. The rooms chosen for measurement were an IEC
listening room (T30 of about 0.5 s) and an anechoic chamber (reference condition). A dynamic headorientation condition comparable to a real head-movement was simulated by randomly combining the
responses from the three static head-orientation measurements. The results show a limited influence of headorientation on ASSR level. However, at 4 kHz, a significant reduction in ASSR level was observed when thetest ear was oriented away from the loudspeaker. The overall mean ASSR level in the IEC room was reducedby 2.5 dB with reference to the anechoic condition.
Keywords: auditory steady-state response (ASSR), hearing-aid validation, head-movement

1. INTRODUCTION

The improvement in newborn hearing screening has resulted in infants being prescribed withhearing aids at a tender age down to two months. The standard validation tools like aided audiometryor questionnaires are ineffective in these circumstances. Hence, an objective electrophysiologicalmethod called steady-state auditory response (ASSR) measurement is considered (1). In sound-field
ASSR, the stimulus is presented through a loudspeaker to include the hearing aid in the signal path.
This approach comes with several challenges. The reverberation time and background noise of themeasurement room can affect the modulation depth of the signal resulting in a reduced ASSR level(2). The head-orientation of the infant during the recording may also affect the ASSR level. These twochallenges associated with the sound field ASSR were investigated in this study.
The effect of the room on the ASSR was first examined using simulated room environments (2).
This study instead measured sound-field ASSR in two real rooms to investigate the effect of the room.
The two rooms were an anechoic chamber and an IEC standard listening room (T 30 ~0.5 s) The ASSR
was recorded at three pre-defined static head-orientations to investigate the effect of head-movement.
The static head-orientations were considered as the analysis blocks (“epochs”) will be rejected during
the actual head-movement due to increase in EEG noise floor resulting from the muscle activity andstrain during the movement. Hence, the resultant ASSR in this condition would be an average acrossthe different static head orientations over the whole measurement period. A dynamic head-orientation
1*

skn@es.aau.dk

The research pertaining to this paper was undertaken during the first author’s tenure at Acoustic Technology Group,
Department of Electrical Engineering, Technical University of Denmark (DTU).

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condition was simulated from the static head-orientation responses for mimicking the response ofnatural head-movement during the measurement.
The recorded responses were post-processed and statistically analyzed for comparing the ASSR
level, detection time, EEG-noise levels, and detection rate for the defined measurement conditions.
For choosing a favorable position for the ASSR measurement in the room, modulation power analysis,
a useful tool in characterizing the efficacy of the stimuli for ASSR measurement (3), was performed.

2. METHODS AND MATERIAL

The measurements were carried out on 11 normal hearing test subjects (6 males and 5 females)with a mean age of 25 years. All the participants gave written informed consent and were compensatedfor their participation. The study was conducted under the approval of the Science-Ethics Committee
for the capital region of Denmark.
The stimulus used for the measurement was an ISTS-modified NB CE-Chirp (3). This stimulus
has speech like properties, which is critical in the validation of hearing aids in their normal mode ofoperation, to ensure that correct gain and signal processing features are activated. Each one-octave
band CE-chirp (4) was modified by applying the frequency-specific envelope of the International
Speech Test Signal (5). The response evoked by each octave-band is identified by the repetition rate
at which it was presented. Four-octave band CE-chirps were used, centered at 0.5, 1, 2, and 4 kHzwith repetition rates of 90.8, 94.7, 102.5 and 96.7 Hz, respectively. The basic NB-CE chirp was
created with a 32-kHz sampling frequency and 65536 samples per epoch, corresponding to a periodof 2.048 seconds.
The participants were lying down comfortably on a bed, and the room was darkened during themeasurement. Only one ear was stimulated at a time, while the other ear was blocked using an earplug.
Standard 4-electrode montage (high forehead reference, ipsi- and contra-lateral mastoids active, andcheek ground) were used for the ASSR recording. A custom-made MATLAB software loaded to alaptop controlled the playback and recording. The stimulus was routed to an in-house built two-way
coaxial loudspeaker, placed 1 meter above the test subject’s head, through an audio chain comprisingof an RME Fireface UC soundcard, a graphic equalizer, a custom-made crossover filter, twoattenuators, and an audio power amplifier. The Interacoustics Eclipse unit was used as the front-end
to deliver the line-level EEG signal to the RME Fireface UC soundcard.
To evaluate the effect of head-movement, three static head-orientation conditions were defined,
namely, No Head-Movement (NHM), Towards the Speaker (TS) and Away from the Speaker (AS).
These head-orientation conditions were characterized concerning the position of the test ear in relation
to the loudspeaker. In the NHM condition, the participants were instructed to not to move their head
and look straight at the speaker for the entire duration of the measurement. Then, the participants wereasked to move the test ear towards the speaker and hold that position for the TS condition. For AS,
the participants moved the test ear away from the speaker. This is illustrated in Figure 1. The headorientation of the participants for the TS and AS conditions were not constrained to a specific angle
but were left to the test subject’s comfort and convenience, to keep muscle artifacts in the EEG signals
to a minimum.

Figure 1: Static head-orientation conditions with reference to left ear (stimulated ear): (a) No HeadMovement (NHM), (b) Towards the Speaker (TS), and (c) Away from the speaker (AS)7856

Two room conditions were identified to understand the effect of the room on the ASSR
measurement. The first one was an anechoic chamber of dimension 4.8m x 4.1 m x 2.9 m (L x W x
H). This chamber is considered anechoic from 100 Hz to 10 kHz. The second one was IEC 60268-13
standard listening room henceforth referred to as the IEC room. The IEC room (7.52 m x 4.74 m x
2.76 m) with a T30 of about 0.5 seconds is comparable to a clinical room, albeit somewhat larger thana typical clinic. The anechoic chamber was considered the reference condition. The results from theIEC room were compared with those from the anechoic chamber to understand the effect of the room.
Before conducting the actual ASSR measurement, an un-normalized modulation power analysis of
the stimulus was performed. This analysis intends to predict the ASSR efficacy of the stimulus (3) ina given room and head-orientation condition. The stimulus was convolved with the impulse responses
recorded using a HATS and a DIRAC system for every defined room and head-movement condition.
This convolved stimulus was then passed through a gammatone filter bank having 24 filters 1/24 thoctave spaced apart representing the auditory filter with respect to the octave bands of interest (0.5
through 4 kHz). Edge effects were removed for the aperiodic speech-modified stimulus by multiplying
the sample epochs of each Hilbert envelope by a raised cosine window. Then a discrete Fouriertransform was applied on the gated envelopes, and the resulting modulation power was averaged
across the epochs and the 24 gammtone filter bands. The modulation power at each octave frequency
band was evaluated at the fundamental corresponding repetition rate (ignoring the higher harmonics).
The modulation power analysis was used to screen out un-favorable measurement positions.
The ASSR was recorded for all the participants at first in the anechoic chamber and then in theIEC room for the three defined static head-orientation conditions and for both ears. Special care wastaken to measure with low EEG noise levels (henceforth referred to as noise level), meeting therejection ratio of 40 µV. A simple F-detector (6) comparing the power at the repetition rate to theaverage power across 10 frequency bins on either side of the repetition rate was used to evaluate thedetection of the first harmonic at the repetition rate in question. A Bonferroni corrected 1% error rate
was used for this test. The measurement was concluded once the F-statistics for all four octave-bands
were above the F-critical value, or after a maximum of 20 minutes of recording time. Continuous
averaging of response and noise frequency bins improved the signal to noise ratio.
A simulated dynamic head-orientation condition comparable to real head-movement wasconstructed by combining one-third of each static head-orientation recording for each participant.
Initially, the responses were truncated to the length of the recording with the least number of epochs,
and these responses were divided into three parts. Then, one of the three parts of each static headorientation condition was chosen randomly and combined in a random order to simulate the dynamic
head-orientation condition. This is illustrated in Figure 2.

Figure 2: Illustration of simulated dynamic head-orientation condition.
Subsequently, the recorded responses were post-processed along with the simulated dynamic headmovement response. Three primary outcomes: noise-corrected ASSR level, detection time, and noiselevel were considered. In addition, detection rates were calculated. The noise corrected ASSR level(referred as ASSR level hereafter) was calculated as the difference between the estimated power at
the response harmonic and the noise power (6) converted to dB for a better assessment of relative7857

changes across conditions. The detection time was determined as the first time a response was detectedwith an F-detector using a 5% error rate in the successive weighted average. Any corrections for the
repeated measure was thus ignored. A log 10 transformation (relative to 1 s) of the determined detection
time was performed, and this log-transformed detection time is termed Tlog. The log transformation
was found to yield a more even distribution of residuals in the statistical analysis (3). The noise level
was estimated by averaging the noise power in the 10 bands on either side of the response harmonicwith respect to a specific repetition rate. The frequency bins close to 50 Hz line-noise, other knowinterferences, and any other repetition rate harmonics were excluded. The noise level thus determined
was later adjusted to the level that would have been found at a testing time of 100 s. This was achievedby adding 10 x log10 (t/100) to the noise level (in dB) of the whole recording, where t is the fullrecording time. This level is termed L100 . This adjustment was necessary as the noise is expected to
be lower with longer measurement time. As explained above, some measurements were concluded
after reaching the F-critical value, which means that the recording time of each measurement need not
be 20 minutes.
The three outcomes of the post-processing: ASSR level, Tlog , and L100 were analyzed using a linear
mixed model, whereas the detection rate was analyzed using non-parametric statistical tests. In themixed effects model, the participants (TP: 1, 2, 3..., 11) were considered a random effect. The fixed
effects considered were: test ear (TE: Left and Right), room conditions (RC: Anechoic and IEC), headorientation condition (HOC: NHM, TS, AS, DHM), and stimulus frequency (FREQ: 0.5, 1, 2, and
4 kHz). Estimated marginal means (7) were determined for the best-fit model, and a post-hoc analysis
comparing the means was performed using the Tukey method (8).

3. Results

The mixed model ANOVA results for the three primary outcomes are shown in Table 1. For the
ASSR level and L100 models, the test subject was found to be significant (p<0.001***).
Table 1: Summarized mixed model ANOVA results. Only the significant main effects and interactions are

Factor
FREQ
RC
HOC
HOC:FREQHOC:RC
HOC:TE
RC:HOC:TE

presented. ***p<0.001, **p<0.01, *p<0.05
ASSR level
Detection Time (Tlog )F statistics
Pr(>F)
F statistics
Pr(>F)
F(3,612)=35 <0.001***
F(3,662)=6 <0.001***
F(1,612)=83 <0.001***
F(1,662)=4
0.011**
F(3,612)=11 <0.001***
F(9,612)=6 <0.001***

Noise Level (L 100 )F statistics
Pr(>F)
F(1,678)=41<0.001***

F(3,678)=9
F(3,678)=4
F(3,678)=5<0.001***0.006**
0.002**

3.1 ASSR Level
The plot concerning the effect of stimulus frequency, head-orientation, and room conditions isshown in Figure 3. Even though the third order interaction is not significant, it helps to visualize all
the significant effects in a single plot, in combination with post-hoc testing (Tukey HSD). Regarding
the main effect of FREQ, the post-hoc analysis showed that levels at 0.5 and 1 kHz were significantly
smaller than those at 2 and 4 kHz (all p<0.001). The significant main effect of room (RC) indicateslower levels in the IEC room. The overall mean reduction in ASSR level in the IEC room wasapproximately 2.5dB. For the main effect of head orientation (HOC), the post-hoc analysis shows that
only the AS condition is significantly different from the other three (all p<0.05). Finally, regarding
the significant HOC:FREQ interaction, the post-hoc results show a significant difference only for the
AS condition at 4 kHz (p<0.001).

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Figure 3: Estimated marginal mean ASSR level plotted across stimulus frequency, room condition, andhead-orientation. Error bar indicates 95% confidence interval.
3.2 Detection Time
Figure 4 shows that the higher frequencies (2 and 4 kHz) had lower detection times than the lower
frequencies (0.5 and 1 kHz). Also, from Figure 5, it can be seen that detection times in the IEC room
were significantly higher than those from the anechoic chamber.

Figure 4: Estimated marginal mean detection time

Figure 5: Estimated marginal mean detection timecorresponding to stimulus frequency. Error bar

corresponding to room conditions. Error bar indicatesindicates 95% confidence interval

95% confidence interval7859

3.3 Noise Level
Figure 6 shows the plot concerning the effect of head-orientation, test ear, and room condition on
L100 . The most interesting result is the significant reduction in noise level in the IEC room. The posthoc results for the interaction RC:HOC showed that the major contributor to the overall decrease in
the noise level in the IEC room was the TS condition. For the significant HOC:TE interaction, thepost-hoc result showed a significant difference between the NHM and TS condition for the left ear.
The post-hoc analysis on the third order RC:HOC:TE interaction revealed that the difference in noiselevel at IEC and anechoic chamber were most significant for TS condition for both left and right ears
(both p<0.001).

Figure 6: Estimated marginal mean noise level L100 (dB re. 1nV) plotted across head-orientation, test ear,and room condition. Error bar indicates 95% confidence interval.
3.4 Detection Rate
An overall detection rate of 91% was obtained (63 non-detections out of 703 data points). The
results presented in Table 2 show higher detection at higher frequencies (2 and 4 kHz). The AS
condition had the lowest detection rates in both anechoic and IEC room. A Fisher exact test revealed
that the reduction in aggregate detection rate at IEC room (89%) compared to the anechoic chamber(93%) was significant (p=0.017).
Table 2: Detection rate in percentage across stimulus frequency, head-orientation conditions, and room
conditions.
FREQ

IEC Room

FREQ

NHM

TS

AS

DHM

NHM

TS

AS

DHM

(Aggregate %)0.5 kHz

95

95

86

91

82

91

68

82

86

1 kHz

95

91

86

91

82

95

91

82

89

2 kHz

100

100

100

95

95

100

100

95

98

4 kHz

100

95

77

100

100

100

64

86

90

HOC (Aggregate %)

98

95

87

94

90

97

81

86

RC (Aggregate %)7860

Anechoic Chamber

93

89

4. DISCUSSION

The general trend in the ASSR levels concerning the stimulus frequency is in good agreementwith the previous study (3), with higher ASSR levels towards higher frequencies (except for the AS
condition at 4 kHz). It is concluded that the effect of head-movement on ASSR level was limited
because the ASSR level for DHM condition corresponding to real head-movement was not
significantly different from NHM condition. However, if the head is oriented as in AS condition for
the entire time of recording, this could result in a significant reduction of ASSR level at 4 kHz. The
observed decrease in ASSR level at 4 kHz for AS condition can be attributed to the head-shadow
effect. The mean reduction of ASSR level of 2.5 dB in the IEC room compared to the anechoic
chamber is associated with the change in acoustics. The reflections from the surfaces and edges in theIEC room can alter the degree of modulation of the stimulus resulting in a reduced ASSR level.
The detection times generally showed an inverse pattern relative to the ASSR level, in agreementwith the previous study (3). It is expected that an increase in ASSR level is accompanied by a reduced
detection time, as the criterion signal-to-noise ratio is reached faster with a strong signal. The
detection rates also showed close relation to the ASSR level, with the higher detection rates at higher
frequencies and a significantly reduced detection rate in the IEC room.
The noise level results were peculiar; the reduction in noise level in the IEC room was unexpected.
It could be speculated that the participants were more relaxed and comfortable in the IEC room.
During the informal discussion after the measurement, some participant commented that the soundfrom the speaker in IEC room was more diffused and appealing than the sharp and uncomfortablesound in the anechoic chamber. However, a couple of participants felt the anechoic chamber to be
more relaxing. There can also be an order effect as the measurements were first performed in theanechoic chamber. Also, most of the participants did not have prior experience with participating inelectrophysiological experiments. All these could have resulted in an increased noise level in theanechoic chamber.

5. CONCLUSIONS

The aim of the study was two-fold: primarily to understand the effect of head-movement on soundfield ASSR and secondarily to understand the influence of the measurement room. The measurements
were carried out in an IEC standard listening room and anechoic chamber with three predefined static
head-orientation conditions.
The main findings from the study are:•

The comparison of results for NHM and DHM head-orientation condition indicates aminimal effect of head-movement on ASSR. DHM is considered comparable to a real headmovement. However, the AS condition at 4 kHz can influence the overall ASSR level andcan have serious implication in a clinical setting.

•

The change in acoustics in the IEC room resulted in an overall mean reduction of 2.5 dB
in the ASSR level compared to the anechoic chamber. This establishes the influence of themeasurement room on ASSR measurements.

ACKNOWLEDGEMENTS

The study was carried out as part of the master thesis project and further research assistantship ofthe first author at Acoustic Technology Group, Department of Electrical Engineering, Technical
University of Denmark. The study was co-funded by Technical University of Denmark and William
Demant Foundation.

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