Analysis of head rotation trajectories during a sound localization task
Yukio IWAYA (1) , Brian FG KATZ (2)(1) Tohoku Gakuin University, Japan/Sorbonne Université, France, iwaya@ieee.org(2) Sorbonne Université, CNRS, Institut Jean le Rond d’Alembert, Lutheries - Acoustique - Musique, Paris, France,

brian.katz@sorbonne-universite.fr

Abstract
Dynamic changes of the Head-Related Transfer Function renderings as a function of head movement have beenshown to be an important cue in sound localization. To investigate the cognitive process of dynamic soundlocalization, quantification of the characteristics of head movements is needed. In this study, trajectories of headrotation in a sound localization task were measured and analyzed. Listeners were asked to orient themselves
towards the direction of active sound source via localization, being one of five loudspeakers located at 30◦
intervals in the horizontal plane. A 1 s pink noise burst stimulus was emitted from different speakers in random
order. The range of expected head rotations (EHR) for a given stimulus were, therefore, from 30◦ to 120◦ .
Head orientation was measured with a motion capture system (yaw, pitch, and roll). Analysis examined angularvelocity, overshoot, and reaction time (RT). Results show that angular velocity increased as EHR increased. Norelationship between overshoot and EHR was observed. RT was almost constant (≈260 ms) regardless of EHR.
This may suggest that dynamic sound localization studies could be difficult for a short stimulus with duration
less than 260 ms.
Keywords: Dynamic sound localization, Head rotation, Reaction time1INTRODUCTION

Sound localization of human beings can be accomplished via the Head-Related Transfer Functions (HRTFs)
empirically [1] in our daily lives. When we can move our head, the accuracy of sound localization improves [2,3, 4]. This tendency can be observed in virtual conditions with dynamic binaural renderings [5]. In consequence,
it can be concluded that dynamic cues in sound localization are important in addition to HRTFs. Several
previous studies have investigated the perceptional process of dynamic sound localization using the notion of‘active listening’ [6, 7]. However, the details of these dynamic cues has not been sufficiently detailed. In thisstudy, therefore, the cognitive process of dynamic sound localization was quantitatively investigated with regards
to head rotation specifically. In previous studies [5], front-back confusions in sound localization were analyzed.
We observed that front-back confusion rates decreased when the duration of test stimulus was long. This result
indicates that head rotation in dynamic sound localization tasks needs sufficient time to be beneficial. In otherwords, we must be aware of the existence of a sound and roughly localize the corresponding direction of thesound before we rotate our head. In this paper, the time trajectories of head rotation in a sound localization
task were measured and analyzed.

2METHOD

Six university students (in their 20’s with normal hearing) participated in the study. Five loudspeakers (C50A,Beringer) were arranged on the horizontal plane at a radius of 2 m at −60◦ intervals to 60◦ , where 0◦ indicatedthe direction in-front of a listener. A listener sat on a chair at the center of the circle. The height of listener’s
ear entrance and center of loudspeakers were 1.2 m. A 1 s pink noise at 70 dBA was emitted randomly throughone of the five loudspeakers, via a USB audio interface (UR824, Steinberg) controlled by a PC1 (MicrosoftWindows 10, Core i7-4790, 16GB Memory). Five markers were placed on a headband attached to the listenerenabling monitoring of and head movement, captured via a motion capture system (V120 Trio, OptiTrack) on7247

another PC2 (Microsoft Windows 10, Core i7-4790, 16GB Memory). The electric signal of the stimulus from aloudspeaker was fed back to a USB audio interface (UR824, Steinberg) attached to PC2, while the stimulus andhead movement data were simultaneously recorded (Motive, OptiTrack). The update rate of the motion capturesystem was 1/120 s.
The listener was asked to localize sound direction by facing (nose pointing protocol) to the direction of thesource. In each trial, three stimuli were emitted continuously at 1 s intervals. For each listener, 35 trials wereperformed, with a total of 105 localizations.

3RESULTS AND ANALYSIS

3.1 Abstract of head movement
An exmaple of a typical head movement data log is shown in Fig. 1. Head rotation (yaw, pitch, and roll)and the recorded sound envelope are plotted in Fig. 1. We observed small overshoot in roll and yaw angle.
Furthermore, start time is delayed from sound emitted time. These tendencies were observed for all participants.
Therefore, we analyzed, overshoot, reaction time (RT), and angular velocity of roll.
3.2 Reaction TimeWe defined reaction time as followings: First, we calculated the amount of angle change of roll angle between
five frames, as shown in Fig. 2 (one frame is 1/120 s). Small changes were eliminated as data noise using
threshold processing. Subsequently, a tangent line was drawn at the time of 2◦ , when we decided the participant
started movement. Finally, the moment of the 0◦ crossing of the tangent was calculated as the starting time forhead movement. The difference between the calculated start time and the sound envelope rise time was definedas the reaction time.
The five loudspeakers were arranged at 30◦ intervals. The range of expected head rotations for a given stimulus
was, therefore, from −120◦ to 120◦ . Table 1 shows the average RT for expected amount of head rotation.
Positive and negative angular signs indicate right and left rotations, respectively. From Table 1, it can beobserved that RT is almost constant, ≈0.26 s.
Table 2 shows the results of RT averaged across trials for each participant. Subject B was the fastest (0.18 s)with Subject C the slowest (0.30 s). As a result, it can be assumed that there may be individual difference
among individuals.
Table 1. Average RT to amount of head rotation.
Expected amount of head rotation (◦ )30–3060–6090–90
120-120
Average

RT (s)
0.31
0.27
0.27
0.27
0.24
0.24
0.22
0.23
0.26

3.3 Maximum Angular velocity.
We calculated the time to the maximum value of each movement in Fig. 2 and the slope at the time was used
to determine the maximum angular velocity.

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Figure 1. Example of head movement (yaw, pitch, roll, and sound envelope) data log. Target angles were 30◦ ,60◦ , and −60◦ .
Table 2. Average RT of each participant.
Participant
ABC
DEFAverage

RT (s)
0.29
0.18
0.30
0.290.24
0.22
0.26

From Table 3, the maximum angular velocities were larger than that of expected amount of head rotation in 1 s.
It was observed that as the total amount of head movement increased, the maximum angular velocity tended toincrease.
Table 3. Maximum angular velocity to expected head rotation
Expected amount of head rotation (◦ )
30-30
60-60
90-90
120-120

max. angular velocity of rotation (◦ ).
139.8
143.6
200.1
189.6
251.8
239.5
300.6
288.1

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Figure 2. Example of amount of change of roll angle in 5 frames.
3.4 OvershootParticipants did not always face the emitted speaker correctly. To quantify this, the difference between the angleof the peak in one localization and the angle at the start time of the next localization was calculated as theovershoot.
Average overshoot for each expected amount of head rotation is shown in Table 4. No systematic change inovershoot was observed as a function of the target position.
Table 4. Maximum angular velocity to expected head rotation.
Expected amount of head rotation (◦ )
30-30
60-60
90-90
120-1204average overshoot (◦ ).
2.4
0.4
2.4
1.8
2.6
2.4
2.6
1.7

SUMMARY

In this paper, the time trajectories of head rotation in a sound localization task were measured and analyzed.
Reaction time, overshoot, and maximum angular velocity of the head rotation were investigated. From theresults, reaction time was more than 0.2 s, irrespective of the expected amount of rotation. Results show thatangular velocity increased as the angular distance to the target increased. No relationship was observed between
overshoot and rotation. Reaction time was almost constant (about 260 ms) regardless of expected rotation. This
may suggest that dynamic sound localization studies could be difficult for a short stimulus. However, further
investigations are needed because the duration 260 ms in this paper might be a task dependent result.

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ACKNOWLEDGEMENTS
Authors would like to thank Mr. Shoji and Mr. Otake for their contribution in conduction experiments. This
study was partially supported by JSPS Kakenhi (19K12286).

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