Head movements during horizontal sound localization
Yuki Toyoda1, Daisuke Morikawa2, and Tatsuya Hirahara3
1-3

Toyama Prefectural University

5180 Kurokawa, Imizu, Toyama 939-0398 JAPANABSTRACT
We measured subject’s head movements during horizontal sound localization experiments to determine
how little the people move their heads in a head-still condition, and how they move their heads in ahead-movement condition. The head movements of eight subjects in a head-still condition were measured
while localizing six kinds of band-pass noises, and the head movements of five subjects in ahead-movement condition were measured while localizing 500-Hz low-pass noise, 12-kHz high-pass noise,
and white noises. In the experiments, all stimuli were presented from one of loudspeakers placed 30º apartin a circle of 1 m radius. In the head-still condition, we confirmed that all subjects kept their heads fairly
still. The maximum head movement was 1° in roll angle, 3° in pitch angle, and 1.5° in yaw angle. These
results suggest that the subjects followed the instruction of not moving their heads while the stimuli werebeing presented in the head-still condition. In the head-movement condition, all subjects turned their heads
toward the presented sounds, yet they did not necessarily turn their heads to face the sound. The maximum
head movement angle in yaw angle among the subjects was about ±60°. The head movement patterns,
however, were different among subjects, suggesting that each subject seemed to have his/her own strategy.
Keywords: Head Movement, Sound Localization, Head-still Condition, Head-movement Condition

1. INTRODUCTION
We move our heads when we listen to the sound because we implicitly know sound localization
becomes more accurate by moving our head. The impact of head movement on sound localization has
been known since Wallach [1-11]. However, it is not clear how we move our heads when we localize
sound.
Thurlow et al. reported that there are many horizontal head rotations when we locate sounddirections [11]. Blauert reported that people move their heads so that the difference limen of soundlocalization may decrease [10]. This means that they face the sound source because the minimum
audible angle is smallest [12]. Iwaya et al. reported that subjects tended to move their heads more
dynamically when the sound was presented away from front (0°), trying to capture it directly in frontof them [5]. Recently, Nojima et al. reported that subjects moved their heads toward the sound image
on the lateral sides and they can localize the sound even when they do not face the sound image [13].
Toshima et al. noted that subjects do not necessarily turn their heads to capture the sound in front ofthem [6].
To clarify how we move our heads when we localize sound, we measured subject’s head
movements during horizontal sound localization experiments with real sound sources in the head-still
and head-movement conditions. We first show data on how little the subjects moved their head in
accordance with the experimenter's instructions of not to move their heads during the experiment.
Next, we present data on how much the subjects moved their heads to localize sound when head
movement was allowed. Finally, we discuss the head movement strategy on sound localization.

1

t054013@st.pu-toyama.ac.jp
t074001@st.pu-toyama.ac.jp
3hirahara@pu-toyama.ac.jp
21

2. METHODS
Three-dimensional positions and Euler angles of a subject’s head were measured using a motion
sensor (Flock of Birds, Ascension Technology) placed on top of the head. The accuracy of the motion
sensor was static positional accuracy 1.8 mm RMS and static angular accuracy 0.5° RMS. Figure 1
depicts a block diagram of the experimental system. Two PCs were used; one for controlling thesound localization experiment and the other for simultaneously recording the stimuli sound and head
movement data to confirm their synchronization. Sampling frequency of the sound was 48 kHz andthat of the Flock of Birds was 128 Hz.
The stimulus duration and inter-stimulus interval were both 3 s. Stimuli were presented from one ofloudspeakers placed 30° apart in a circle of 1 m radius. Each session consisted of 60 trials, and thestimuli were presented in random order from the 12 loudspeaker. One experiment consisted of four
sessions. Subjects sat on a chair placed at the center of the speaker array without using any head
fixing device. Their head movements were measured during horizontal sound localization
experiments in the head-still and head-movement conditions.
In the head-still condition, six kinds of band-pass noises (BPNs) were used as stimuli for soundlocalization. Their pass-bands were 2-12, 2-8, 2-4, 4-12, 4-8, and 8-12 kHz. Broad-band BPNs canbe localized easily while narrow-band BPNs are difficult to localize in the head-still condition [14].
The subjects were instructed to close their eyes and keep their heads still while each stimulus waspresented. When each stimulus was stopped, they opened their eyes and marked the perceived
sound-image location at one of the 12 horizontal directions on an answer sheet. Eight males in their
20s participated in this experiment.
In the head-movement condition, white noise (WN), 500-Hz low-pass noise (LPN), and 12-kHz
high-pass noise (HPN) were used as stimuli for sound localization. WN can be localized easily while
LPN and HPN are difficult to localize because many front-back errors occur. The subjects were
instructed to close their eyes and were encouraged to rotate their heads freely while each stimulus waspresented. Five males in their 20s participated in this experiment.

DAC1

Edirol UA-101

1m
PC1

30°
PC2

DAC2

Fs = 48 kHz
Nb = 24 bit

Figure 1 – Block diagram of experimental system (left) and photograph of experiment room(right). PC1 was used to for stimuli presentation for sound localization experiment. PC2
was used to simultaneously record sound stimuli and subject's head movement data.

3. RESULTS
3.1 Head-still condition
Figure 2 shows an example of the typical head movement of the subject in the head-still condition.
The gray lines represent the stimulus waveform. In Figure 2(a), the blue, green and red lines
respectively represent the x, y, and z directions of the head. In Figure 2(b), the blue, green and redlines respectively represent the roll, pitch, and yaw angles of the head. The subject kept his heads
fairly still while each stimulus was presented. When each stimulus presentation ended, he moved his

2head downward to mark the perceived location on the answer sheet. Thus, the x direction increased,
the y direction decreased, and pitch angle decreased. Then, he returned his head back to the listening
position for the next stimulus, but these returned positions were not always the same. The head
positions while listening to stimuli differed by up to 8.2 cm in the x direction and 20º in pitch angle.
However, the variation in the yaw angle, which should largely impact horizontal sound localization,
was less than 2.6° for each subject.

40

y

x

(a)

40

z

Angle [degree]

Position [cm]

20
0
-20
-40

0

10

20

20
0
-20
-40
0

30

yaw

pitch

roll

(b)

10

Time [s]2030Time [s]Figure 2 – Example of typical head trajectories of subject in head-still condition;(a) three-dimensional positions data and (b) Euler angles data.
Figures 3(a) and (b) show the mean maximum head movement positions and angles during thestimulus presentation for each subject. References for measuring the maximum head movements
were the positions and angles at the beginning of each stimulus. The blue, green, and red lines
respectively represent the x, y, and z directions of the head (Fig. 3(a)), and the blue, green, and redlines respectively represent roll, pitch, and yaw angle of the head (Fig. 3(b)). The mean maximum
head movement of all subjects was less than 1.3 cm in each direction and less than 3° in each Euler
angle. The maximum pitch of head movement was largest among Euler angles because the subject’s
heads moved downward when they marked their answers on the answer sheet. These data suggest thatall subjects kept their heads fairly still when each stimulus was presented in the head-still condition.

20

40
x

(a)

y

z

(b)

Angle [degree]

Position [cm]

10
0
-10
-20

A

B

C

D E
Subject

F

G

yaw

pitch

20
0
-20
-40

H

roll

A

B

C

D E
SubjectF

G

H

Figure 3 – Maximum head movement for each subject during stimulus presentation in head-still condition;(a) maximum head position movement and (b) maximum head angular movement.

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3.2 Head-movement condition
Figure 4 shows the typical head-yaw angle trajectory of the subject head for LPN. The gray line
represents the stimulus waveform and the red line represents the head-yaw angle trajectory. The blueand green circles respectively represent the presented and perceived azimuthal angle of the stimulus.
All stimuli were localized correctly.
In most cases, the subject turned his head towards the presented stimulus direction, then hesometimes swung his head. For the first stimuli, presented at the front-right (60°), he turned his headright and swung it to right. For the third stimuli, presented at the right (90°), he turned his head rightonce. For the 5th stimulus, presented at the rear left (-120°), he turned his head left and swung it toboth sides. For the 7th stimuli, presented at the front left (-30°), he turned his head left and swung it
to left. For the 8th stimulus, presented at the front (0°), he turned his head right once. Usually, themaximum yaw angles at which he turned his head were smaller than the stimulus 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 when a subject turned and swung his head once or
twice to either the right or left. The both-side head swing pattern is when a subject swung his head
both to the right and left. Stimuli numbers 1, 3, 4, 6, 7, 9, and 10 resulted in a single-side head swing
pattern and stimuli numbers 2, 5, and 8 resulted in a both-side head swing pattern.

180
Azimuthal / yaw angle [degree]presented angle of the stimulus

120

perceived angle
head rotation angle trajectory60

stimuli waveform

0
-60
-120
-180

0

6

12

18

24

30

36

42

48

54

60

Time [s]Figure 4 – Example of typical head-yaw angle trajectory of subject for LPN. Head movement can beclassified into two patterns; single-side head swing and both-side head swing patterns.
Figure 5 shows the mean, maximum, and minimum angles of the head-yaw angle in single-side
head swing (upper panel) and both-side head swing (lower panel) patterns for each angle of thepresented stimuli. N represents the number of single-side or both-side head swings. The blue, red, andgreen circles respectively represent the mean, maximum, and minimum yaw angles. There are somepoints where data is not drawn for the single-side head swing, indicating the subject always swung hishead to both sides. In the single-side head swing pattern, the maximum or minimum yaw angle was
not necessarily 0°, because the subjects did not return his head to the initial position punctually wheneach stimulus presentation began.

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WN

HPN

LPN
90
60

Head-yaw angle [degree]

30

mean

0

max
Single-side
head swing

min

-30
-60

N=101

N=170

-90

N=40

90
60
30

Both-side
head swing

0
-30
-60

N=70

-90
-120 -60

0

60

N=200

N=139

120 180 -120 -60

060

120 180 -120 -60

0

60

120

180

Presented angle of stimulus [degree]

Figure 5 – Mean, maximum, and minimum head-yaw angle of subject for LPN, HPN, and WN.
Upper panels show those of single-side head swing pattern and lower panels show those of
both-side head swing pattern.
When stimuli were presented from front (0°) or behind (180°), the subjects always swung theirheads to both sides. When stimuli were presented from the lateral sides, the subjects turned their heads
toward the stimuli. However, the maximum head-yaw angle was about ±60°. The subjects did not turntheir heads to face the stimuli when they were presented over ±60°.
Among LPN, HPN, and WN, the mean head-yaw angle for each stimulus was smallest for WN.
WN was almost perfectly localized by all subjects even in the head-still condition because it
contained rich acoustical cues for localization. The dynamic acoustical cues provided by the head
movement are probably not necessary. Small head movements should be enough to confirm thesound direction for WN. By contrast, the correct localization performances of LPN and HPN wereless than 75% in the head-still condition. The performances improved more than 17% when head
movement was allowed because front-back confusion disappears due to head rotation. Obviously, thedynamic cues from head movement play important roles in localizing LPN and HPN, which only
contain either inter-aural time difference (ITD) or inter-aural level difference (ILD). Thus, a larger
amount of head movement is necessary for localizing LPN and HPN than that for WN.
All five subjects showed similar head-movement strategies. They did not turn their heads tocapture the sound stimuli in front of them, the maximum head-yaw angles were about ±60°, and theamount of head rotation was small for WN compared to that for LPN and HPN. Detailed head
movement strategies, however, differed by subject. For example, one subject always swung his heada certain amount despite the stimulus presentation azimuthal angle in the single-side head swing
pattern. Another subject preferred to use the single-side head swing pattern than the both-side headswing pattern.

4. DISCUSSION
The results of the head-still condition showed that the subjects kept their heads still during thestimuli presentation without using any head fixing devices and that they returned their heads to theright position no later than the start of the next stimulus. The subjects followed the experimenter's
instructions very well. These results confirm the validity of the experimental procedures for thesound localization tests we have been conducting. It must be, however, very difficult to keep thehead still for longer the sessions.
The results of the head-movement condition showed that the subjects did not turn their heads tocapture the stimulus in front of them. The yaw angles at which they turn their heads were always
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smaller than the stimulus azimuthal angles. Even though the maximum yaw angle at which humans
can turn their heads is about 70° [9], the subjects did NOT turn their heads to this extent. Presumably,
they can obtain enough 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 decrease
the difference limen of the sound localization. They all swung their heads while localizing the sound.
The variation in ITD and ILD, i.e. ΔITD and ΔILD or time derivative of ITD and ILD, must be usedas dynamic cues to localize sound. In fact, the sound image of the stimuli swings around the head
when rotating the head, resulting in a dramatic reduction in the ambiguity of sound image location.

5. CONCLUSIONS
We measured subject’s head movements during horizontal sound localization experiments in thehead-still and head-movement conditions.
In the head-still condition, subject’s maximum head position movements and maximum head
angular movements were small when each stimulus was presented. Therefore, all subjects kept theirhead fairly still when they were instructed to do so.
In the head-movement condition, subjects turned their heads toward the stimuli when each stimulus
was presented. The head movement patterns were either single-side head swing or both-side headswing. The maximum head-yaw angle, however, was less than ±60° despite the stimulus direction.
The subjects did not turn their heads to capture the stimulus in front of them over ±60°. The amountof head rotation was small for WN compared to that for LPN and HPN, suggesting that the lessacoustical cues are involved in a stimulus, the more head movements are necessary to localize it.
Detailed head movement strategies, however, differed by subject.

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

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