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

Left-right sound localization outside loudspeaker positions in stereo
reproduction with parametric loudspeakers
Shigeaki Aoki, Kouki Ito, Kazuhiro Shimizu, Suehiro Shimauchi
Kanazawa Institute of Technology

ABSTRACTThe parametric loudspeaker is known as a super directional loudspeaker by using nonlinear interaction
between ultrasounds. We present some listening test results in stereo reproduction with the parametricloudspeakers. At first, ILDs were used as binaural information. It was confirmed that the sound images
reproduced by the parametric loudspeakers were perceptually localized more outside than those by theordinary loudspeakers. Especially when the ILDs (Interaural Level Differences) were minus or plus infinity,
the sound images by the parametric loudspeakers were localized outside the actual position of theloudspeakers, while the images by the ordinary loudspeakers were localized rather slightly inside. It was also
confirmed that the sound images by the parametric loudspeakers can be controlled to be localized more
explicitly outside by using HRTFs (Head-Related Transfer Functions) instead of ILDs as the binauralinformation.
Keywords: Parametric loudspeaker, Sound localization, Super-directivity

1. INTRODUCTION
The parametric loudspeaker is known as a super directional loudspeaker based on nonlinear
interaction between ultrasounds. The parametric loudspeaker is one of the prominent applications ofnonlinear acoustics [1,2]. The main applications of the parametric loudspeaker are monaural
reproduction. It is expected that a stereo reproduction using the parametric loudspeakers will lead toenjoy music and make a conversation without other persons' annoyance and trouble.
Recently, it was confirmed that parametric loudspeakers were available to stereo reproduction [3].
It was suggested that binaural information i.e. ILD (Interaural Level Difference) and ITD (InterauralTime Delay) had stronger effect on sound localization than in using ordinary loudspeakers. That is,sound images are localized in the more outside direction [4]. The influence of listener’s head in stereo
reproduction using parametric loudspeakers is investigated [5]. Interesting characteristics of verticalsound image control with parametric loudspeakers are found [6]. Moreover, an audio system with goodreality using the parametric loudspeakers has been investigated.
In this report, the direction of sound localization between with the parametric loudspeakers andwith the ordinary loudspeakers, which are set at angle of -30 and +30 degrees, are compared. The ILD
is focused on as binaural information. The test method [4] is modified to make clear detailed
characteristics. Moreover, in order to pursue the possibility of sound localization explicit outside theright and left loudspeakers, the listening tests are conducted not only with pure ILD but also withHRTFs (Head-Related Transfer Functions). The obtained listening test results are analyzed anddiscussed.

2. LISTENING TEST I (WITH PURE ILD)
2.1 Test condition
The placement of the listener and the parametric loudspeakers is illustrated in Fig.1. The listening
tests are conducted in an anechoic room. A young female and four young males with normal hearing
ability attend. The relationship between the right and left loudspeakers and the listening pos ition arevertices of regular triangle with 1.8m length side. Both parametric loudspeakers are placed 0.3 m apartfrom the wall. The acoustical axis of the right or left parametric loudspeaker is set to the right or left
ear of a listener, respectively.

2696

The estimate paper is put on the wall for pointing the direction of sound localization. Meshes of a
net are described on the paper. The origin points of x and y are the center and the level of both ears. Theranges of x and y are from -3.8 to +3.8 m and from -50 to +60 cm, respectively. The parametricloudspeaker was shaped as an equilateral hexagon. The inner and outer diameters were 99 mm and 112
mm, respectively.

Figure 1 – Loudspeakers and listening positions. The relationship between the right and left loudspeakers andthe listening position are vertices of regular triangle with 1.8m length side.
2.2 Test signal
The test signal is pink noise. The ultrasonic wave is modulated by the preprocessed test signal. SSB(Single-sideband) modulation with the lower sideband is used. The level differences of radiatedsignals between the right and left loudspeakers are f , -9, -6, -3, 0, +3, +6, +9 and f dB. The
duration of test signal is 5 seconds.
2.3 Test procedure
The listening tests are conducted in the following steps.
1. Before every listening test, a listener listens to some test signals in order to experience the soundlocalization preliminarily.
2. The listener listens to a set consisted of 45 trials with randomly edited test signals, five times foreach of type reproductions.
3. After the listener listens to a test signal, she or he points a position of sound localization on an
estimate paper by a laser pointer. Horizontal and vertical angles of sound localization are calculated by
the pointed position.
2.4 Test results
The obtained sound localization in the horizontal direction is shown in Fig.2. The horizontal axis is
the level difference between the right and left loudspeakers. The vertical axis is the angle of soundlocalization in the horizontal direction. Symbols are the mean value and vertical lines indicate thedegree of dispersion of data by connecting positive and negative standard deviations are connected. In
the both cases of parametric loudspeaker and ordinary loudspeaker, the standard deviations were sosmall that sound localization was controlled by the ILDs. The directions of sound localization with theparametric loudspeakers were more outside than those with the ordinary loudspeakers. For example,loudspeakers were set at angle of -30 and +30 degrees and ILDs were f and f . In using theparametric loudspeakers, the directions of sound localization were outside the loudspeaker. On theother hand, in using the ordinary loudspeakers, those were not outside the loudspeaker but inside. The
parametric loudspeaker has such a sharp directivity that a cross talk level and diffraction are smaller
than the ordinary loudspeaker has. It is estimated that the substantial ILD wi th the parametricloudspeakers is larger than those with the ordinary loudspeakers.

2697

The obtained sound localization in the vertical direction is shown in Fig. 3. The horizontal axis is
the level difference between the right and left loudspeakers. The vert ical axis is the angle of soundlocalization in the vertical direction. There was small difference between the parametric loudspeaker
and the ordinary loudspeaker. It was the same tendency that the directions of sound localization wereslightly up in the center direction. The degree was different. The extent of the parametric loudspeaker
was twice as much as that of the ordinary loudspeaker.

Figure 2 – Sound localization in the horizontal direction. The horizontal axis is the level difference between
the right and left loudspeakers. The vertical axis is the angle of sound localization in the horizontal direction.

Figure 3 – Sound localization in the vertical direction. The horizontal axis is the level difference between theright and left loudspeakers. The vertical axis is the angle of sound localization in the horizontal direction.

3. LISTENING TEST II (WITH HRTF)
3.1 Test signal
The test condition is the same as in the previous listening test. It is anticipated that the obtained
characteristics of the parametric loudspeaker in the previous test probably enable sound localization
more outside of loudspeakers. The cross talk of the parametric loudspeaker is so small that thelistening situation is probably realized to be similar to the listening by headphone s. Therefore,binaural signals are created by convolving pink noise with HRTFs. The HRTFs are provided by MIT[7]. Therefore, in this listening test, individualities of listeners' HRTFs are not considered. The
selected directions of the HRTF’s sound localization are -90, 0 and +90 degrees. The ultrasonic wave2698

is modulated by preprocessing the modulated signal. The SSB (Single -sideband) modulation with thelower sideband is used. The duration of test signal is 5 seconds, which is the same as in the previoustest.
3.2 Test procedure
The listening tests are conducted in the following steps.
1. Before every listening test, a listener listens to some test signals in order to experience the soundlocalization preliminarily.
2. The listener listens to a set consisted of 15 trials with randomly edited test signals, five times foreach of type reproductions.
3. After the listener listens to a test signal, she or he points a position of sound localization on an
estimate paper by a laser pointer. Horizontal and vertical angles of sound localization are calculated by
the pointed position.
3.3 Test results
As for reference, angles of sound localization and the standard deviation in the horizontal
direction in Fig.2 are picked up in Table 1. The used binaural information was ILDs. The
loudspeakers were set at angle of -30 and +30 degrees. The angles the ILDs are f and f areaveraged. It is easy to compare between the results of the ordinary and parametric loudspeakers. In
using the parametric loudspeakers, the directions of sound localization were outside the loudspeaker.
On the other hand, in using the ordinary loudspeakers, those were not outside the loudspeaker but
inside. The standard deviations are small enough to distinguish these re sults between bothloudspeakers.
Table 1 – Direction of sound localization in using ILD (∞) as binaural information
Binauralinformation
ILD (҄ dB)

Direction of soundStandard Deviation,localization, deg

deg

Ordinary

27.8

3.5

Parametric

35.5

4.6

Loudspeaker

Angles of sound localization in the horizontal direction when the HRTFs were used as binaural
information are shown in Table 2. The averaged angles and standard deviations when the directionsof HRTFs are -90 and +90 degrees are noted. It is easy to compare between the results of theordinary and parametric loudspeakers. The standard deviations are small enough to distinguish theseresults between both loudspeakers. It was confirmed that by using HRTFs of -90 and +90 degrees, thesound images were able to control to localize outside the parametric loudspeakers which were placedat angles of -30 and 30 degrees.
Table 2 – Direction of sound localization in using HRTF (90 deg) as binaural information
Binauralinformation
HRTF (90 deg)

Direction of soundStandard Deviation,

localization, deg

deg

Ordinary

26.8

4.1

Parametric

48.3

6.0

Loudspeaker

4. CONCLUSIONSThe listening tests in stereo reproduction with the parametric loudspeakers and the ordinary
loudspeakers were conducted. These loudspeakers were set at angle of -30 and +30 degrees. The2699

obtained test results were analyzed and discussed.
It was confirmed that when ILDs were used as binaural information, the directions of soundlocalization with the parametric loudspeakers were more outside t han those with the ordinary
loudspeakers. The obtained results were interesting especially when ILDs were f and f . In
using the parametric loudspeakers, the directions of sound localization were outside of the loudspeaker.
On the other hand, in using the ordinary loudspeakers, those were not outside the loudspeaker butinside. Moreover it was confirmed that by using HRTFs of -90 and +90 degrees, the sound images wereable to control to localize more outside the parametric loudspeakers.

ACKNOWLEDGEMENTS
This work was partly supported by a Grant-in-Aid for Scientific Research (No. 19K12053)

REFERENCES
1. Westervelt P. J. Parametric Acoustic Array J. Acoust. Soc. Am. 1963;35(4):535-537,.
2. Yoneyama M., Fujimoto J., Kawano Y. Ssabe S. The audio spotlight: An application of nonlinear
interaction of sound waves to a new type of loudspeaker design J. Acoust. Soc. Am.
1983;73(5);1532-1536,.
3. Aoki S., Toba M. Tsujita N. Sound localization of stereo reproduction with parametric loudspeakers
Applied Acoustics 2012;73(12) 1289-1295.
4. Tsujita N., Wakabayashi T., Aoki S. Stereo Sound Localization using Parametric Loudspeakers Proc. ofAutumn Meeting of the ASJ 2012.p.969-970.
5. Tsujita N., Shimizu K. Aoki S. Influence of listener’s head in stereo reproduction using parametricloudspeakers Proc. IEEE International Ultrasonics Symposium 2016.
6. Aoki S., Shimizu K. Itou K., Study of vertical sound image control with parametric loudspeakers Applied
Acoustics 2017;116(1) 164–169.
7. MIT HRTF Measurements of a KEMAR Dummy-Head Microphone:
http://sound.media.mit.edu/resources/KEMAR.html

2700