Proceedings of 20th International Congress on Acoustics, ICA 201023-27 August 2010, Sydney, Australia

Signal bandwidth necessary forhorizontal sound localization
Daisuke MORIKAWA, Tatsuya HIRAHARA
Faculty of Engineering, Toyama Prefectural University, Toyama, Japan

PACS: 43.66.Qp

ABSTRACTThis paper clarifies how much signal bandwidth is necessary for horizontal sound localization. Horizontal sound localization experiments were conducted with sixteen listeners using white noise, and fourteen listeners using high-passnoise whose cut-off frequency (Fc) was 2, 4, 8, 12, or 16 kHz, or low-pass noise whose Fc was 0.5, 1, 2, 4, or 8 kHz.
Four listeners participated in an experiment on band-pass noise whose FcL-FcH was 2-12, 4-12, 2-8, 2-4, 4-8, or 8-12
kHz. It was very difficult to localize sound for high-pass noise whose Fc was high. Sound localization performancewas 64% for 12-kHz high-pass noise while it was 27% for 16-kHz high-pass noise. In contrast, sound localization
was possible even for 500-Hz low-pass noise. Sound localization performance was 81% for 1-kHz low-pass noise and
67% for that at 500-Hz. It was also difficult to localize sound for narrowband band-pass noise. Sound localization
performance was 56% for 2-4-kHz band-pass noise and 67% for that at 8-12-kHz. These results suggest that the interaural time difference, mainly calculated from low-frequency components, the interaural level difference, mainly calculated from high-frequency components, and spectral cues, mainly calculated from middle-frequency (5-10 kHz)components each play roles in sound localization for band-limited noise. We clarified that signal bandwidth from 2
kHz to 12 kHz is necessary to perform good horizontal-sound localization.

Three-dimensional (3-D) sound can be reproduced eitherwith binaural, transaural, wave-field synthesis, or multichannel surround-sound technologies. The perceptual characteristics of these 3-D sounds have been extensively investigated. It is well known that the interaural time difference(ITD), interaural level difference (ILD), and spectral cues,
which are involved in HRTF or HRIR, greatly contribute to
3-D sound localization [1]. However, there have been few
reports on how broad the bandwidth needs to be to reproduce
sound as 3-D sound. With regard to the real sound source,
Morimoto and Saito reported that a signal bandwidth from
4.8 to 9.6 kHz was the minimum necessary for sound localization in the median plane [2]. Nakabayashi reported thathorizontal localization is incomplete with one octave bandwidth noise [3]. Blauert reported on a directional band, viz.,certain frequency band signals are subject to becoming localized in a certain direction [1]. Kondou et al. re-examined thisdirectional band phenomenon [4]. Morimoto et al. investigated the role of low-frequency components in localizationon the median plane. They found that higher frequency components are dominant in median plane, whereas lowerfrequency components do not contribute significantly to localization [5]. Nojima et al. recently demonstrated thatfront/back judgment is difficult for 500-Hz low-pass noise
when head movement is not allowed [6]. With regard to thevirtual sound source, Arrabito and Mendelson reported that
14-kHz high-pass noise contributes little to vertical- or horizontal-sound localization [7]. This article clarifies the signal
bandwidth necessary for horizontal-sound localization using
band-limited noise from real sound sources.
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EXPERIMENTAL SYSTEM
SystemFigure 1 shows the experimental system. The system consisted of a Windows-based PC, two 8-channel digital-toanalog converters (DACs) (Roland, UA-101), 12 power amplifiers (BOSE, 1705II), and 12 loudspeakers (Vifa,
MG10SD-09-08). The sampling frequency of the DACs was
48 kHz. The 12 loudspeakers were placed around a chair in ahorizontal circumference at a 1-m radius at 30-degree intervals. The height of the loudspeakers was 1.1 m. The soundlocalization experiment was carried out in an experimental
room, whose walls and ceiling were covered with soundabsorbing materials. The A-weighted noise-floor level of theroom was 53 dB.

RolandEDIROL
UA-101
USB

BOSE
MODEL
1705Ⅱ

Vifa
MG10
SD-09-08

1100(height)

DA

PC

3800

INTRODUCTION

30°
DA

00
10

3300

Figure 1. Experimental system and setup1

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Proceedings of 20th International Congress on Acoustics, ICA 2010

Stimuli

16-kHz high-pass noise. Sound localization was impossible
for subjects who could not hear the stimulus.

Broadband noise, i.e., Gaussian distributed random noise,high-pass (Fc = 2, 4, 8, 12, 16 kHz), low-pass (Fc = 0.5, 1, 2,
4, 8 kHz) and band-pass (FcL-FcH = 2-12, 4-12, 2-8, 2-4, 48, 8-12 kHz) filtered noise were used as the stimuli. The
high-pass, low-pass and band-pass filters were 512 tap FIR
filters with –60-dB stop-band attenuation. The FIR filters
were designed with the window method [8].
The stimulus duration and inter-stimulus intervals were 3 sec.
A 30-ms linear taper window was applied at the beginningand end of the stimuli. As the frequency response of the loudspeakers was fairly flat (±8.7 dB) between 0.1 kHz to 20 kHz,no inverse filter was used to correct the speaker response in
the experiment. The sound-pressure level of the broadbandnoise stimuli was 80 dB at the head-center position. The
sound-pressure level of the high-pass, low-pass and bandpass filtered noise stimuli decreased depending on the filter
bandwidth.

Low-pass Noise
Figures 3 (a) and (b) plot the pooled sound-localization results for fourteen subjects for 1-kHz and 500-Hz low-passnoise. Sound localization was also poorer for low-pass noises
than for broadband noise. As seen in Fig. 3, many stimuli
were localized with front-back confusion. Sound images werelocalized away from the head for all low-pass noise.
Figure 4 plots the mean sound localization performance andstandard deviations for the fourteen subjects against the filter
cut-off frequency for high-pass and low-pass noise. Sound
localization performance was over 85% with 2-, 4-, and 8kHz high-pass noise. The performance dropped rapidly for
12- and 16-kHz high-pass noise. In contrast, the sound localization performance was over 85% for 8-, 4-, and 2-kHz lowpass noise, and it dropped gradually when the cut-off frequency of the low-pass filter was lowered.

Procedure

Subjects
Sixteen subjects participated in the sound-localization experiments. Table 1 lists the details on the number of subject
for all the stimulus experiments. Five subjects could not hear
the stimulus of 16-kHz high-pass noise due to severe hearing
loss above 16 kHz. Of these, three subjects who were in their
20s, 40s and 50s each participated in the sound-localization
experiments on 16-kHz high-pass noise. Subjects who could
hear the stimulus were placed in group E, and subjects who
could not hear the stimulus were placed in group D.

Table 1. Details on subjects for each stimulus
20s 30s 40s 50s Total
Noise＼Age group
Broadband
10
1
2
3
16
High and low-pass
8
1
2
3
14
16-kHz high-pass (E)
7
1
1
0
9
16-kHz high-pass (D)
1
0
1
1
3
Band-pass
4
0
0
0
4
360
300

High-pass Noise
Fig. 2(b) plots the pooled sound localization results for fourteen subjects for 12-kHz high-pass noise. The sound images
were localized away from the head for all high-pass noise.
Fig. 2(c) plots the results for nine subjects who could hear the
stimulus for 16-kHz high-pass noise. Localization was poorer
for high-pass noise than for broadband noise. As seen in Fig.
2(c), sound localization was vague for stimuli that only contained very-high-spectral components. Fig. 2(d) plots theresults for three subjects who could not hear the stimulus for2(c) HPN16kHz(E)(d) HPN16kHz(D)

120
60
0
360
300
240
180
120
60
0
0

120

240

360 0

120

240

360

Target Azimuth [ degrees ]Figure 2. Horizontal-sound localization results for broadband
and high-pass noise
Perceived Azimuth [ degrees ]Figure 2(a) plots the pooled sound-localization results for the
sixteen subjects for broadband noise. The sound localization
was almost perfect for broadband noise. Sound localization
performance, i.e., the correct rate for localizing sound stimuli,
was 94%. The sound images were, of course, localized awayfrom the head. There were no differences in sound localization performances by the different age groups.

(b) HPN12kHz180

RESULTS
Broadband Noise(a) Broadband
noise

240

Perceived Azimuth [ degrees ]The experimental procedure was as follows. Subjects sat on achair placed in the center of the speaker array. The subjects
listened to stimuli reproduced from one of the loudspeakers.
They were asked to horizontally localize the sound-image
positions of the real sound sources and to mark the locatedposition on an answer sheet. The subjects were instructed toclose their eyes and keep their heads still when a stimulus
was reproduced. Each session consisted of 60 trials, and thestimuli were presented in random order from the 12 loudspeakers. One experiment consisted of four sessions, resulting in 20 trials from each of the 12 directions. The experiments were conducted separately for each low-pass, highpass and broadband noise stimulus.

360(a) LPN1kHz(b) LPN500Hz

300
240
180
120
60
0
0

120

240

360

0

120

240

360

Target Azimuth [ degrees ]Figure 3. Horizontal-sound localization results for low-passnoise

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Proceedings of 20th International Congress on Acoustics, ICA 2010
Perceived Azimuth [ degrees ]

80
60
40

(b) BPN2-4kHz

360 (a) BPN2-12kHz
300
240
180
120
60
0
0

120

240

360 0

120

240

360

Target Azimuth [ degrees ]

LPN
HPN

00 0.51

2
4
8
Cut off frequency [kHz]Figure 5. Horizontal-sound localization results for band-passnoise

12 16 24

Figure 4. Mean sound localization performance and standard
deviations for band-pass, high-pass, and low-pass noise
Band-pass Noise
Figures 5 (a) and (b) have the pooled sound-localization results for four subjects for 2-12-kHz and 2-4 kHz band-passnoise. The sound localization was almost perfect for 2-12kHz band-pass noise. The sound localization performancewas 95%. Localization was poorer for narrowband band-passnoise than for 2-12-kHz band-pass noise. As seen in Fig. 5(b),many stimuli were localized with front-back confusion.
Figure 6 plots the mean sound localization performance andstandard deviations for the four subjects against the filter cutoff frequency of band-pass noise. The sound localization
performance was over 85% with the 2-12 and 4-12-kHz
band-pass noise, and the performance dropped gradually as
the pass-bandwidth of the filter narrowed. Sound images
were localized away from the head. However, the same stimuli presented at the front were localized very close to thehead except for 2-12- and 4-12-kHz band-pass noise. The 24-kHz band-pass noise was particularly localized that way by
all subjects.
Figure 7 compares the mean sound localization performanceand standard deviations for the four subjects for band-passnoise and high-pass noise, and band-pass noise and low-passnoise. The sound localization performance of 8-12-kHz bandpass noise was poorer than 8-kHz high-pass noise. The soundlocalization performance for 2-4-kHz band-pass noise waspoorer than that for 4-kHz low-pass noise. The narrower the
bandwidth, the poorer the performance.

DISCUSSIONAs seen in Fig. 4, sound localization is difficult for stimuli
that only consist of high-frequency components over 12 kHz.
Sound localization performance largely decreases for 16-kHz
high-pass noise (27%), i.e., sound localization is very difficult for stimuli that only have very high-spectral components.
One reason for poor sound localization performance with 16kHz high-pass noise is that there are no low spectral components. The sound-image locations for this stimulus have to be
calculated only from the ILD of the high-spectral components.
ITD cannot be used as a cue because it is calculated fromlow-spectral components below 4 kHz at the medial superior
olive (MSO) [9]. The spectral notches around 5-10 kHz also
cannot be used as a cue for sound localization in the stimulus

100Sound localization performance [%]

20

80
60
40

BPN2-12kHz
BPN4-12kHz

20

BPN2-8kHz
BPN2-4kHz

02

BPN4-8kHz

BPN8-12kHz

4
8
Cut off frequency [kHz]12

Figure 6. Mean Sound localization performance and standard deviations for band-pass noise
100
Sound localization performance [%]Sound localization performance [%]

100

80
60
40
20
0

Sub.1
Sub.2
Sub.3
Sub.4
BPN8-12kHz HPN8kHz BPN2-4kHz LPN4kHz
StimuliFigure 7. Comparison of band-pass noise with high- andlow-pass noise
Another reason for the poor sound localization performancewith 16-kHz high-pass noise could be attributed to the low
sensation level (SL) of the stimuli. Hebrank and Wrightfound localization performance was level independent for
white-noise stimuli presented over 40 dB SL, but decreased
for stimuli below 40 dB SL [10]. Inoue reported that soundlocalization performance for white-noise stimuli was low at 0
to 20 dB SL [11]. The sound pressure level of the 16-kHz
high-pass noise used in our experiment was actually 72 dB
and its SL was 0 to 30 dB depending on subjects. The low SLof the stimuli, thus, is a possible cause for the low soundlocalization performance of the stimulus.
Some visually impaired people who cannot see are able topredict the locations of visual stimuli and this is known as
blind sight [12]. Our experiment revealed that subjects who

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could not hear 16-kHz high-pass noise could not localize
sound at all. The hearing loss of the three subjects at highfrequency is most likely the result of malfunctions at thesensor stage due to aging. Thus, it is not surprising that they
could not hear the stimuli and they also could not localize thestimuli.

(2) Low-frequency components below 2 kHz or highfrequency components over 12 kHz contributed little tohorizontal-sound localization.

As seen in Fig. 4, sound localization is difficult for stimuli
that only consist of low-frequency components below 1 kHz.
Sound localization performance decreases for 500-Hz lowpass noise (64%); however, it is still better than that for 16kHz high-pass noise. The major cue for sound localization of
500-Hz low-pass noise was the ITD calculated from the lowspectral components. The ILD of the low-spectral components can be used as another cue, yet ILD was mainly calculated from the high-spectral components at the lateral superior olive (LSO) [9]. The spectral notches around 5-10 kHz
also cannot be used as cues in the sound localization of thestimuli. The ITD and possibly some ILD from low-spectral
components give better estimates of the location of the soundimage than the ILD from high-spectral components alone
gives.
As shown in Fig. 6, sound localization is difficult for stimuli
that only consist of middle-frequency components between 24 kHz, 4-8 kHz, or 8-12 kHz. Sound localization performancelargely decreases for 2-4-kHz band-pass noise (56%), i.e.,sound localization is very difficult for stimuli that have narrow-spectral components. Fig. 6 also supports the finding thatthe signal bandwidth necessary to perform horizontal soundlocalization is from 2 to 12 kHz.
As seen in Fig. 7, Localization is poorer for 8-12-kHz bandpass noise than for 8-kHz high-pass noise. The reason for thepoor sound localization performance with 8-12-kHz bandpass noise is that there were no low-spectral components or
narrow-spectral components. The major cue for this stimulus
for sound localization is the ILD calculated from the highspectral components from 8 to 12 kHz. The spectral notches
around 5-10 kHz can be used as another cue. However, thisstimulus does not include spectral components above 8 kHz.
The ILD information from 8-12-kHz band-pass noise is insufficient compared with that from 8-kHz high-pass noise.
Spectral components over 12 kHz contributed to the calculations of ILD.
In addition, Fig. 7 shows that individual differences in soundlocalization performance for 8-12-kHz band-pass noise arelarge. The reason for this is shat the weight of ILD and spectral cues used to localize this stimulus may vary from person
to person.
As seen in Fig. 7, Localization is poorer for 2-4-kHz bandpass noise than for 4-kHz low-pass noise. The reason for thepoor sound localization performance with 2-4-kHz band-passnoise is that there are no high-spectral components. The
sound-image locations for this stimulus only have to be calculated from the ITD of the low spectral components from 2
to 4 kHz. However, this stimulus does not include spectral
components below 2 kHz. The ITD information from 2-4
kHz band-pass noise is insufficient compared with that from
4-kHz low-pass noise. Spectral components below 2 kHz
contributed to the calculations of ITD.

CONCLUSION
Horizontal sound localization experiments for broadband
noise, high-, low- and band-pass noise were conducted with
sixteen subjects. The obtained results are as follows:(3) Horizontal-sound localization was very difficult for
high-pass noise. Sound localization performance
dropped to 64% for 12-kHz high-pass noise and 27% for
16-kHz high-pass noise.
(4) Horizontal-sound localization was impossible for highpass noise with subjects who could not hear the stimuli
due to severe hearing loss above 16 kHz.
(5) Horizontal-sound localization was difficult for low-passnoise with a 500-Hz cut-off frequency. Sound localization performance dropped to 81% for 1-kHz low-passnoise and 67% for 500-Hz low-pass noise.
(6) Horizontal-Sound localization was difficult for narrow
band-pass noise. Sound localization performance
dropped to 56% for 2-4-kHz band-pass noise, 69% for
4-8-kHz band-pass noise, and 67% for 8-12-kHz bandpass noise.

ACKNOWLEDGEMENTPart of this work was supported by KAKENHI (22300061)

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[9] J.O. Pickles, An Introduction to the Physiology of Hearing (Second Edition), (Academic press, London, 1988),
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[10] J. Hebrank, D. Wright “The effect of stimulus intensity
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(1) It was necessary to have a bandwidth from 2 to 12 kHz
to perform horizontal-sound localization.
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