Morikawa et al.

Proceedings of Meetings on Acoustics
Volume 19, 2013

http://acousticalsociety.org/

ICA 2013 Montreal
Montreal, Canada
2 - 7 June 2013
Psychological and Physiological Acoustics
Session 4aPPb: Binaural Hearing (Poster Session)
4aPPb3. Head movement during horizontal and median sound localization
experiments in which head-rotation is allowed
Daisuke Morikawa*, Yuki Toyoda and Tatsuya Hirahara​​ *Corresponding author's address: Toyama Prefectural University, Kurokawa 5180, Imizu, 939-0398, Toyama, Japan,
t074001@st.pu-toyama.ac.jp
We measured subjects' head movements during horizontal and median sound localization experiments in which head-rotation was allowed inorder to know how they move their heads to localize sound in a head rotation condition. The head movements in a head-rotation condition were
measured while localizing 500-Hz low-pass noise (LPN), 12-kHz high-pass noise (HPN), and white noise (WN). With regard to horizontal
plane, sound localization became easier with head-rotation than head-still condition. All subjects turned their heads toward the presented
sounds, yet they did not necessarily turn their heads to face the sound. The amount of head rotation was small for WN compared to that for
LPN and HPN. Sound localization also became easier with head-rotation than head-still condition for the median plane. All subjects swung
their heads right and left centering on 0°, no matter what the stimulus elevation angle was.
Published by the Acoustical Society of America through the American Institute of Physics© 2013 Acoustical Society of America [DOI: 10.1121/1.4800148]
Received 22 Jan 2013; published 2 Jun 2013
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INTRODUCTION
We move our heads when we listen to sounds, because we subconsciously know that sound localization is moreaccurate when we move our head. The effect of head movement on sound localization has been known since its
report by Wallach [1−11]. However, it is not clear how we move our heads when we localize sound.
Thurlow and Runge examined head movement during sound localization using a small number of sound sources:
center, right front, lower right front, lower right, and upper left loudspeakers. By analyzing head movement duringsound-localization experiments [2], they detected many horizontal head rotations when locating the sound direction.
Based on Thurlow and Runge’s result, Blauert concluded that people face the sound source because the minimum
audible angle is smallest [3]. Iwaya et al. examined the effect of head movement on the front–back error in soundlocalization using 12 loudspeakers, which were placed along a horizontal circle at 30° intervals. They reported that
listeners tended to move their heads more dynamically when the sound was presented away from the front (0°),
trying to capture it directly in front of them [7]. By contrast, Nojima et al. examined head movement during soundlocalization using 5 or 6 loudspeakers which were placed along horizontal right half-circles at 45° intervals [12] oron a horizontal circle at 0°, 30°, 135°, 180°, 215°, and 315° [13]. They reported that listeners moved their heads
toward the sound image in the lateral direction and could localize the sound even when they were not facing soundsource. Toshima et al. examined the effectiveness of TeleHead, which is a steerable dummy head that tracks humanhead movement in real time. They carried out sound-localization experiments using TeleHead with 12 loudspeakers,
which were placed on a horizontal circle at 30° intervals. They reported that listeners did not necessarily turn theirheads to capture the sound in front of them [8].
In the experiments mentioned above, Thurlow and Runge, and Nojima et al. only clarified the rough tendency ofhead movement during sound localization, as they measured head movement during sound localization using a small
number of loudspeakers. By contrast, Iwaya et al. and Toshima et al. measured head movement during soundlocalization with a large number of loudspeakers. However, the purpose of their experiments was not to clarify how
we move our heads, but to clarify the effect of head movement on the front–back error in sound localization or theeffectiveness of TeleHead. Therefore, they did not obtain a detailed analysis of head movement during soundlocalization.
In our study, to clarify how we move our heads when we localize sound, we measured the listener’s head
movements during horizontal- and median-plane sound-localization experiments with real sound sources underhead-rotation conditions. We quantified the listeners’ head movement when they localized sound when head
movement was allowed for each stimulus. In addition, we discuss the head movement strategy for sound localization.

METHODS
Three-dimensional positions and Euler angles of the listener’s head were measured using a motion sensor (Flockof Birds, Ascension Technology) placed on top of the listener’s head. The motion sensor has static positional
accuracy of 1.8 mm RMS and static angular accuracy of 0.5° RMS. Figure 1 depicts a block diagram of theexperimental system. Two PCs were used: one for controlling the sound-localization experiment and the other for
simultaneously recording the stimuli sound and head movement data to confirm their synchronization. The sampling
frequency of the sound was 48 kHz and that of the motion sensor was 128 Hz.
The loudspeakers were placed around a chair in a horizontal circle as well as along the median plane of 1 m
radius in the upper hemisphere at 30° intervals. The horizontal loudspeakers were mounted at a height of 1.1 m.
Listeners sat on a chair placed at the center of the speaker array without using any head fixing device. Stimuli
were presented from one of the loudspeakers. In the horizontal-sound-localization experiment, each session
consisted of 60 trials, and the stimuli were presented in random order from 12 loudspeakers. In the median-soundlocalization experiment, each session consisted of 35 trials, and the stimuli were presented in random order from 7
loudspeakers. Each experiment consisted of four sessions. The listener’s head movements were measured during thehorizontal- and median-sound-localization experiments under head-rotation conditions.
White noise (WN), i.e., Gaussian-distributed random noise, 500-Hz low-pass noise (LPN), and 12-kHz high-passnoise (HPN) were used as stimuli for sound localization. WN can be easily localized while LPN and HPN are
difficult to localize because many front–back errors occur [14]. The durations of both the stimulus and the interstimulus intervals were 3 s. A 30-ms linear taper window was applied at the beginning and end of the stimuli. The
sound pressure level of WN was 70 dB, whereas those of HPN and LPN were lower in accordance with the amountof filtering.

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Morikawa et al.

The listeners were instructed to close their eyes and were encouraged to freely rotate their heads while eachstimulus was presented. Six and four males in their 20s participated in the horizontal and median experimentrespectively.
Roland UA-101

Fs = 48 kHz
DAC1
ADC

PC2

PC1

monitor

1m

30㼻

1m

Flock of
Birds

30㼻

Fs = 128 Hz

DAC2

Fs = 48 kHz

FIGURE 1. Block diagram of experimental system and photograph of the experiment room. PC1 was used for stimuli
presentation for the sound-localization experiment. PC2 was used to simultaneously record the sound stimuli and the listener’shead movement data.

RESULTS
Horizontal-plane sound localization
Figure 2 shows the typical head-yaw angle trajectory of the listener’s head for LPN. The gray line represents thestimulus waveform and the red line represents the head-yaw angle trajectory. The blue and green circles represent
the presented and perceived azimuthal angles of the stimulus, respectively. All the stimuli were localized correctly.
In most cases, the listener turned his head toward the presented stimulus direction, and then swung it back andforth. For the first stimulus, presented at the front right (60°), he turned his head right and swung it to right. For thethird stimulus, presented at the right lateral direction (90°), he turned his head right once. For the 5th stimulus,
presented at the rear left (–120°), he turned his head left and swung it left and right. For the 7th stimulus, presented
at the front left (–30°), he turned his head left and swung it to left. For the 8th stimulus, presented directly in thefront (0°), he turned his head right once. Usually, the maximum yaw angles at which he turned his head were
smaller than the stimuli angles.
These head movements were classified into two patterns: single-side head swing and both-side head swing. The
single-side head swing pattern is defined as the listener turning and swinging his head one or two times to either theright or left. The both-side head swing pattern is when the listener swings his head both to the right and left. Stimuli
number 1, 3, 4, 6, 7, 8, 9, and 10 resulted in the single-side head swing pattern, and stimuli number 2 and 5 resulted
in the both-side head swing pattern.
The MEANi , j (T ) , MAXi , j (T ) , and MINi , j (T ) define the mean, maximum, and minimum head-yaw angle
during one stimulus that was recorded for each stimulus and angle. T is the stimuli angle, i is the stimuli number of
each angle, and j is the listener number. n represents the number of single-side or both-side head swings for eachangle. The average values of the MEANi , j (T ) , MAXi , j (T ) , and MINi , j (T ) for each listener are given by Eqs. (1)–(3), respectively.
1 n
MEAN j (T )(1)
¦ MEANi, j (T )ni1

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Morikawa et al.

1 n(2)¦ MAXi, j (T )ni1
1 n
MIN j (T )(3)
¦ MINi, j (T )ni1
Figure 3 shows the MEAN j (T ) , MAX j (T ) , and MIN j (T ) for the single-side head swing (upper panel) and bothside head swing (lower panel) patterns for each angle of the presented stimuli, where N is the number of single-sided
or both-sided head swings. The black, red, and blue circles are the MEAN j (T ) , MAX j (T ) , and MIN j (T ) ,respectively. There are some points where data is not drawn for the single-side head swing, indicating the listeneralways swung his head to both sides. In the single-side head swing pattern, MAX j (T ) or MIN j (T ) was notnecessarily 0° because the listener did not return his head to the initial position punctually when each stimulus
presentation began. MAX j (T ) was about 60° and MIN j (T ) was about –60°.
MAX j (T )presented stimulus angle

Head rotation (yaw) angle [㼻]

120

120

perceived angle
Head-rotation angle trajectory

60

60

stimuli waveform

0

0

–60

–60

–120

–120
–180
60

–180
0

6

12

18

24

30 36
Time [s]4248

Presented / perceived azimuthal angle [㼻]

180

180

54

FIGURE 2. Example of typical head-yaw angle trajectory of a listener for LPN. Head movement can be classified into twopatterns: single-side head swing and both-side head swing.

HPN 12 kHz

LPN 500 Hz
60

White noise

MAX
MEAN

Head-yaw angle [㼻]0Single-side
head swing

MIN

–60
N = 40N = 101

N = 170

60
Both-side
head swing

0–60

N = 139

N = 70–120 –60

0

60

120

180 –120 –60

0

60

120

180 –120 –60

N = 200

0

60

120

180

Presented stimulus azimuthal angle [㼻]FIGURE 3. MEAN j (T ) , MAX j (T ) , and MIN j (T ) of the listener for LPN, HPN, and WN. Upper panels show those of thesingle-side head swing pattern, and lower panels show those of the both-side head swing pattern.

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Morikawa et al.

AMAX j (T ) and AMIN j (T ) are the average values of the absolute value of MAXi , j (T ) , and MINi , j (T ) for
each of listener, respectively. The means of AMAX j (T ) and AMIN j (T ) for all the listeners for 0°, ±30° and ±60°of
the presented stimuli are shown in Figure 4(a) and are given by Eqs. (4)–(5). n represents the number of listeners
who moved their heads toward the sound image on the lateral sides.
1 n(4)
AMAX j (T )(T 0°, 30°, 60°)
¦ MAXi, j (T )
ni1
1 n
(5)
(T 0°, -30°, -60°)¦ MINi, j (T )ni1
The listeners did not turn their heads to face the stimuli when the stimuli were presented over ±60°.
MMEAN(T ) (Figure 4b) is the average value of MEANi , j (T ) and is given by Eq. (6), where n is the number ofstimulus for each angle and s represents the number of listeners.
1 s §1 n·
MMEAN(T )(6)¦ ¨ ¦ MEANi , j (T ) ¸
s j 1© n i 1¹
MMEAN(T ) for each stimulus was the smallest for WN than for LPN and HPN. The detailed head movements
differed among the listeners. For example, one listener always swung his head a certain amount even though thestimulus azimuthal angle was that of the single-side head swing pattern. Another listener preferred to use the singleside head swing pattern than the both-side head swing pattern. However, all six listeners showed similar headmovement strategies. They did not turn their heads to capture the sound stimuli in front of them, MAX j (T ) and
MIN j (T ) were about ±60°, and the amount of head rotation was small for WN compared to that for LPN and HPN.
AMIN j (T )90

20 (b)LPN HPN WN

60

30

Head-yaw angle [㼻]Head-yaw angle [㼻]

(a)

10
WN

0
–10

HPN

–20
0

0

s30

s60

Presented stimulus azimuthal angle [㼻]

–120

–60

LPN
0

60

120

180

Presented stimulus azimuthal angle [㼻]FIGURE 4. (a) Mean of AMAX j (T ) and AMIN j (T ) of all listeners for LPN, HPN, and WN at 0°, ±30° and ±60°.
(b) MMEAN(T ) for LPN, HPN, and WN for all azimuthal angles.

Median-plane sound localization
Figure 5 shows a typical head-yaw angle trajectory of the listener’s head for LPN. The gray line represents thestimulus waveform, and the red line represents the head-yaw angle trajectory. The blue and green circles represent
the presented and perceived elevation angles of the stimulus, respectively. All the stimuli were localized correctly
except for stimuli number 2 and 7.
In all cases, the listener swung his head right and left centering on 0°, irrespective of the stimulus elevation angle.
Thus, in median sound localization, there was almost no single-side head swing pattern but mostly the both-side
head swing pattern.
Figure 6 shows the MEAN j (T ) , MAX j (T ) , and MIN j (T ) in the both-side head swing patterns for each angleof the presented stimuli. N represents the number of the both-side head swings. The black, red, and blue circles are
the MEAN j (T ) , MAX j (T ) , and MIN j (T ) , respectively.
The detailed head movement differed among the listeners. For example, among LPN, HPN, and WN, MAX j (T )for each stimulus was the smallest, and MIN j (T ) for each stimulus was the largest for WN for this listener. Other
listeners showed the same head movement regardless of the stimulus. However, all four listeners showed similar

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head-movement strategies. They mostly swung their heads right and left centering on 0°, irrespective of the stimulus
elevation angle and stimulus.

presented stimulus angle

Head rotation (yaw) angle [㼻]120perceived angle

120
stimuli waveform

60

60

0

0

–60
head-rotation angle trajectory

–120
–180
0

6

12

18

24

30 36
Time [s]

42

48

54

Presented / perceived elevation angle [㼻]

180

180

60

FIGURE 5. Example of typical head-yaw angle trajectory of a listener for LPN.

HPN 12k Hz

Head-yaw angle [㼻]LPN 500 Hz

White noise

MAX

60

MEAN

0

Both-side
head swing

MIN

–60

N = 140

0

60

120

180 0

N = 140

60

120

180 0

N = 140

60

120

180

Presented stimulus elevation angle [㼻]FIGURE 6. MEAN j (T ) , MAX j (T ) , and MIN j (T ) of the listener for LPN, HPN, and WN for the both-side head swing pattern.

DISUCUSSION
In the horizontal-plane sound-localization experiments, WN was almost perfectly localized by all the listeners
because it contains rich acoustic cues for localization. This was observed even in experiment under the head-still
condition [14]. The dynamic acoustic cues provided by large head movement are probably not necessary in the WN
case where small head movements are sufficient to confirm the sound direction. In contrast, the correct localization
performances of LPN and HPN were less than 75% under the head-still condition [14]. The performances improved
more than 20% when head movement was allowed because the front–back confusion disappeared due to the headrotation. It is clear that the dynamic cues from head movement play an important role in localizing LPN and HPN,
which contain either an interaural time difference (ITD) or an interaural level difference (ILD). Thus, a larger
amount of head movement is necessary for localizing LPN and HPN than for localizing WN.
In median-plane sound-localization, the performance of WN was about 70% under the head-still condition [14],because it contained only spectral cues (SC). This is similar to the performance of LPN and HPN under the headstill condition of horizontal sound localization. In addition, the performances of LPN and HPN were less than 40%[14]. The performances of WN, LPN, and HPN improved more than 20% when head movement was allowed. For
median-plane sound localization, a large amount of head movement is necessary for WN as well as for LPN and
HPN, unlike the case of horizontal-plane sound localization.
Our experiments demonstrated that the listeners did not turn their heads to capture the stimulus in front of them.
The yaw angles at which they turned their heads were always smaller than the stimulus azimuthal angles. Even
though the maximum yaw angle at which humans can turn their heads is about 70° [10], the listeners did not turn

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their heads to this extent, indicating that they can obtain sufficient information for sound localization with smaller
head yaw angles. Namely, they did not face the sound stimuli, suggesting that they do not move their heads in order
to use the smallest minimum audible angle direction. They all swung their heads while localizing the sound. The
variation in ITD, ILD, and SC, i.e., ΔITD, ΔILD, and ΔSC or the time derivative of ITD, ILD, and SC must be usedas dynamic cues to localize sound. In fact, the sound image of the stimuli swings around the head when the head is
rotated, resulting in a dramatic reduction in the ambiguity of the sound image location.

CONCLUSIONS
We carried out experiments to measure listeners’ head movements during horizontal- and median-plane soundlocalization experiments under head-rotation conditions. In the horizontal-plane sound localization, listeners turned
their heads toward the stimuli when each stimulus was presented. The head movement patterns were either singleside head swing or both-side head swing. The maximum head-yaw angle, however, was less than ±60° even though
the stimulus direction. The listeners did not turn their heads to capture the stimulus in front of them when thestimulus was presented beyond ±60°. The amount of head rotation was small for WN compared to that for LPN and
HPN, suggesting that if the acoustic cues included in the stimulus are less, the head movements necessary to localize
the sound are more. In the median-plane sound localization, the listeners swung their heads right and left, centering
on 0°, irrespective of the stimulus elevation angle and stimulus. The head movement pattern was mostly both-side
head swing.

ACKNOWLEDGMENTS
Part of this work was supported by KAKENHI (22300061).

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