PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Spatial sound segregation in the monaural listening condition
Daisuke MORIKAWA1; Daiki KOJIMA1; Tatsuya HIRAHARA1
1

Toyama Prefectural University, JapanABSTRACT
A sound image revolves around the head with head rotation in the monaural listening condition. The soundimage moves by a large angle around the listener’s head when the sound source moves from the open-ear side
to the blocked-ear side, and vice versa, based on the listener’s head rotation. Sounds presented from a certain
direction actually jump from rear to front or from the center line to the left/right side of a listener’s head andvice versa, and a listener perceives segregated sound images at a certain head rotation angle. We measured
the sound image trajectory of a fixed sound source with head rotation while listening under the monaural
listening condition. Results show that sound image segregation tends to occur when the sound sourcecrosses the boundary between the open-ear and blocked-ear sides as a result of head rotation. Two soundimage locations calculated from the direct and diffracted waves along the head from the sound source cancause spatial sound image segregation under the monaural listening condition.
Keywords: Monaural, Sound image trajectory, Sound segregation

1. INTRODUCTION
The interaural time difference (ITD), interaural level difference (ILD), and spectral cue (SC) are
well-known important cues for sound localization (1). However, ITD and ILD cannot be cues in themonaural listening condition because they are binaural cues. SCs are cues that can be used to localize
sound in the monaural listening condition. Unilateral deaf persons (2) or normal-hearing persons with
one ear blocked by an earplug (3) can actually localize sound sources on the horizontal plane of theavailable-ear side, suggesting that the SCs of the ear can be cues to localize sound when ITD and ILD
are unavailable.
The temporal variations of the ITD, ILD, and SCs produced by head rotation are also well-known
significant contributors to the improvement of binaural sound localization accuracy (4, 5, 6). Thus, thetemporal variation of SCs produced by head rotation is also expected to improve the accuracy inmonaural sound localization. However, monaural sound image localization experiments under thehead rotation condition have shown that the sound image flies around the head with head rotation in
the monaural listening condition. The sound image moves by a large angle around the head when thesound source moves from the open-ear side to the blocked-ear side, and vice versa, with head rotation(7, 8). All listeners perceive the sound image flying around the head with head rotation when thestimulus sound is presented from a certain direction. The sound image jumps from rear to front or fromthe center line to left/right side of a listener’s head, and vice versa. Furthermore, some listeners
perceive two segregated sound images at a certain head rotation angle.
This study describes this spatial sound segregation phenomenon in the monaural listening conditionand discusses its cause.

2. METHODS
2.1 Experimental system
Figure 1 shows the experimental system. The sound reproduction system used herein comprised aWindows-based PC, two digital-to-analog converters (DACs; RME, Fireface UFX), 12 power
amplifiers (ONKYO, CR-N755), and 12 loudspeakers (Vifa, MG10SD-09-08). The sampling
frequency of the DACs was 48 kHz. The loudspeakers were placed around a chair centered on ahorizontal circle of 1 m radius at 30° intervals. The loudspeakers were mounted at a height of 1.1 m.
1dmorikawa@pu-toyama.ac.jp

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The answering system comprised a tablet-type device (apple, iPad), a stylus (Just Mobile,JTM-PD-000019), and a Mac-based PC. The tablet-type device has an answer interface function that
sends answered coordinate values to the Mac-based PC via wireless UDP/IP communication. The
communication protocol used was Open Sound Control (OSC). The answering interface wasdeveloped using an Mrmr OSC controller (9). The Mac-based PC received the coordinate values.
Figure 2 shows the graphical user interface (GUI) of the answering interface on the tablet. Thelistener drew the sound image trajectory and the presence or absence of sound image segregation
using the GUI. A discontinuous trajectory of the sound image was drawn when sound image
segregation occurred.
A listener could push the “next” or “repeat” buttons after the sound image trajectory of thestimulus was drawn. When the listener pressed the “next” button, a trigger signal was transmitted
from the Mac-based PC to the Windows-based PC via a serial port, and the next stimulus waspresented. When the listener pressed the “repeat” button, another trigger signal was transmitted, andthe same stimulus was presented again. The listener was allowed to listen to the stimulus as manytimes as he/she wanted until a satisfying trajectory was drawn.
2.2 Stimulus
Broadband noise (i.e., Gaussian-distributed random noise) was used as the stimulus in theexperiment. The duration of each stimulus was 5 s, and a 30-ms linear taper window was applied at its
beginning and end. The A-weighted sound pressure level of the stimulus L A was set to 40 dB at thehead center position. This low-sound-pressure-level stimulus was used to ensure the monaural
listening condition. All normal-hearing listeners wearing earplugs (Moldex, Pure-Fit 6800s) in bothears could not hear the stimulus at this level. The earplug attenuated sound by 30 to 50 dB from 100 Hz
to 20 kHz; however, some listeners wearing earplugs on both ears could still hear the stimuluspresented at LA = 60 dB or 50 dB, possibly because of incomplete/ill-fitting of the earplugs in theear-canals.
The experiment was performed in an experimental room with the walls and the ceiling covered
with sound-absorbing materials. The A-weighted background noise level of the room was 23 dB.
Figure 3 shows the stimulus spectrum, the background noise spectrum of the room, the hearing
thresholds (ISO 226-2003), and the hearing threshold with an earplug, considering the vertical axis
as the sound pressure level, ref. 20 μPa.
monitorMac PC
Wi-Fi router

Tablet iPaddevice apple

DAC

Win PC

3800

USB1100(height)

30r
DAC

Fire face UFX
RME
CR-N755
ONKYO3300

Figure 1 – Experimental system.

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Vifa
MG10SD-09-08

Figure 2 – Answering interface.

80

Sound Pressure Level [dB]60

Hearing Threshold with Ear plug

4020ISO226-2003

stimulus, LA: 40dB

0−20100

Noise floor of the room
1k
Frequency [Hz]

10k

20k

Figure 3 – Stimulus spectrum, background noise spectrum of the room, hearing thresholds (ISO 226-2003),
and hearing threshold with an earplug.

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2.3 Procedure
First, a listener put ear plugs in both ears and sat on the chair at the center of the circular speakerarray. Second, the experimenter confirmed that the listener cannot hear the stimuli presented fromthe sides of either the left or the right ear (90° or 270°). The experimenter then removed one ear plugfrom the left or right ear, making that side the normal-hearing side. This procedure was performed toguarantee the monaural listening condition.
The listener under the monaural listening condition was instructed to rotate his/her head byapproximately 60° to the left or right from the front when the stimulus was presented. Pitch and rollhead rotations were not allowed. In order to ensure that the listener’s head rotation was confined to ayaw angle within an acceptable range, the Euler angle of the listener’s head was monitored using a
motion sensor (Ascension, Flock of Birds or Logical Product, LP -WSD004-0A) mounted on the topof the listener’s head. The listener’s task was to draw a sound image trajectory on the tablet-typedevice, after listening to the stimulus while turning his/her head. The listener could listen to thestimulus as many times as needed to obtain a reliable sound image trajectory. Thus, the depictedsound image trajectory data were not linked to the head rotation angle data of the listener being
monitored. In addition, the listeners were instructed to place the tablet-type device on their laps
while the stimulus was being presented, to avoid producing a reflected wave.
Thirteen normal-hearing listeners participated in the experiments. All listeners participated inboth right-ear blocked and left-ear blocked condition. They had experience in binaural soundlocalization experiments with static as well as dynamic head movement conditions.

3. RESULTS
Eleven listeners reported that sound image segregation occurred at some stimulus locations,whereas the remaining two listeners reported that no sound image segregation occurred at any of thestimulus locations. Figure 4 shows examples of the sound image trajectories of a typical listener whoperceived the sound image segregation.
When a listener with a blocked right ear turned his head counterclockwise, he perceived that thesound image discontinuously moved around the head from the front to the rear with the stimulus
presented from the frontal (θs = 0°) loudspeaker. He perceived two segregated sound images atapproximately −30° and −120° when he turned his head leftward (Fig. 4(a)).
When a listener with a blocked right ear turned his head clockwise, he perceived a sound image
that jumped from rear to front with the stimulus presented from the rear (θs = 180°) loudspeaker. He
perceived two segregated sound images at approximately −15° and −165° when he turned his headrightward (Fig. 4(c)).
Furthermore, when a listener with a blocked left ear turned his head clockwise, he perceived asound image that jumped from rear to front with the stimulus presented from the right-front (θs =30°) loudspeaker. He perceived two segregated sound images at approximately 60° and 140° when
he turned his head rightward (Fig. 4(b)).
When a listener with a blocked left ear turned his head counterclockwise, he perceived a soundimage that jumped from rear to front with the stimulus presented from the left-front (θs = −60°)
loudspeaker. He perceived two segregated sound images at approximately 75° and 25° when heturned his head leftward (Fig. 4(d)).
In this manner, 11 listeners perceived two segregated sound images when they rotated their heads
to the angle where the sound image had just jumped. When sound segregation occurred, the soundimage jumped from rear to front or from the center to the left/right side, and vice versa.
Figure 5 summarizes the number of responses in which the listeners perceived sound image
segregation in each stimulus direction for each of the two head rotation directions and two earocclusion conditions. The abscissa of each panel represents the azimuth of the stimulus presented.
The azimuthal angle was 0° at the front of a listener and positive, clockwise.
When the listeners rotated their heads toward the open-ear side, most listeners perceived soundimage segregation with the stimulus presented at 0° ± 30° (Figs. 4(a) and (b)). In contrast, when thelisteners rotated their heads toward to the blocked-ear side, most listeners perceived sound image
segregation with the stimulus presented at 60° to 120° with the blocked right ear (Fig. 4(c)) and 270°
and 300° with blocked left ear (Fig. 4(d)).

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(a)

(b)

Perceived
two sound images

(d)

(c)

Figure 4 – Examples of the sound image trajectory. (Red dot: sound image position at start, Green dot:
perceived two sound image, Orange dot: Sound image position at the end)

Number of responses [listener]

10

Rotated toward
opened side

Rotated toward
blocked side

Blocked left ear

Blocked right ear
(a)

(b)

(c)

(d)

5010

50060060 120 180 240 300
120 180 240 300
Sound source azimuth [degree]

Figure 5 – Number of responses in which the listeners perceived sound image segregation. (a) Rotated to theopen-ear side with a blocked right ear; (b) rotated to the open-ear side with a blocked left ear; (c) rotated tothe blocked-ear side with a blocked right ear; and (d) rotated to the blocked-ear side with a blocked left ear.

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Open-ear side

Blocked-ear side

Number of responses [times]

Figure 6 – Open- and blocked-ear sides
Rotated to open ear side(a)Rotated to blocked ear side(b)151050-60

060060 120
120 180 240 -60
Sound source azimuth [degree]

180240Figure 7 – Sum of the number of sound image segregation occurrences when the direction of the blocked-ear
side was positive. (a) Rotated to the open-ear side and (b) rotated to the blocked-ear side.

4. DISCUSSION
The results showed that sound image segregation mostly occurred when the sound source
direction crossed the boundary between the open- and blocked-ear sides (Figure 6) as a result ofhead rotation.
Assuming left-right symmetry in the shape of the head and pinnae, the stimulus direction
observed from the open-ear would have been the same under the following two conditions: turning
the head clockwise with an open left ear, and turning the head counterclockwise with an open rightear. Thus, each of the two pairs of Figures 5 (a) & (b) and (c) & (d), can be combined into one figure
respectively by using the abscissa of the stimulus azimuth between −60° and 240°. Figures 7(a) and(b) show the sum of the number of sound image segregation occurrences for each stimulus azimuth
when the listeners turned their heads to the open and blocked-ear sides.
As shown in Fig. 7, sound image segregation tended to occur with stimulus presented at 0° and±30° when the head was turned toward the open-ear side and at ±60° and ±90° when the head wasturned toward the blocked-ear side. The stimulus azimuths that provided a large number of image
segregation occurrences differed by 60° or more between the two head-turning directions.
The sound presented at the front (0°, ±30°) transitioned from the open-ear side to the blocked-ear
side when the head was rotated to the open-ear by approximately 30° or less. The sound presented atthe blocked-ear side (60°, 90°) transitioned from the blocked-ear side to the open-ear side when thehead was rotated to the blocked-ear side.
These facts suggest that front–back sound segregation occurs when sound is presented at theboundary between the open- and blocked-ear sides as a result of head rotation. Under the monaural
listening condition, the sound image of a sound source in the open- and blocked-ear sides islocalized in the open-ear side. Therefore, the perceived sound image jumps when a sound source
location passes across the boundary between the open- and blocked-ear sides according to the headrotation, and two sound images are simultaneously perceived when the relative sound source location

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with respect to the head is on the boundary. This sound segregation cannot be considered as based on
the cone of confusion, on which ITD and ILD adopt the same value, because binaural cues areunavailable under the monaural listening condition. This sound segregation can be considered to bebased on the application of direct waves to the open ear and diffracted waves along the head to theopen ear from the sound source. One sound image location is that computed from the sound source’sdirect wave convolved with the head-related transfer function (HRTF) of its incident angle to theopen-ear. Another sound image location is that computed from the diffracted waves along the head
convolved with the HRTFs of their incident angles to the open-ear.
Additionally, sound image segregation occurs more frequently with a stimulus presented between
120° and 180° when the head is rotated toward the blocked-ear side, rather than a stimulus presented
between 60° and 120° when the head is rotated toward the open-ear side. The difference between thetwo head rotation directions appears to depend on whether the open-ear is in the front of or above theshoulder. The reflection from the shoulder might affect the perception of the sound image
segregation.
The range of the open- or blocked-ear side must be vague because it depends on the soundpressure level of the stimulus and the hearing threshold at each frequency. The range and theboundaries should vary among listeners because the size and shapes of the head and the pinnae
depend on the individual. Measurements of the HRTF and the absolute threshold of hearing of eachlistener are necessary to clarify the range of each listener. These remain as future work.
The head rotation angle at which the sound image segregation occurre d was not measured hereinbecause drawing the sound image trajectory on a tablet in real time was difficult while listening to
the stimuli when the head is rotating. However, the measurement of head rotation angle along withthe sound image trajectory is necessary to verify the relative angles of the head direction and thesound source location at which the sound image segregates. This will also be investigated in futurework.

5. CONCLUSIONS
Herein, we measured the sound image trajectory of a fixed sound source with head rotation while
listening under the monaural listening condition. The results showed that sound image segregation
tended to occur when the sound source was located on the boundary between the open- andblocked-ear sides according to the head rotation. Two sound image locations calculated from thedirect and diffracted waves along the head from the sound source can cause spatial sound image
segregation under the monaural listening condition.

ACKNOWLEDGEMENTS
Part of this work was supported by KAKENHI (17K00244).

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