Applied Acoustics 73 (2012) 1282–1288
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Applied Acoustics
journal homepage: www.elsevier.com/locate/apacoust
Three-dimensional acoustic sound field reproduction based on hybrid
combination of multiple parametric loudspeakers and electrodynamic subwoofer
Yutaro Sugibayashi a, Sota Kurimoto a, Daisuke Ikefuji a, Masanori Morise b,⇑, Takanobu Nishiura b
a
b
Graduate School of Science and Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan
College of Information Science and Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan
a r t i c l e
i n f o
Article history:
Available online 12 April 2012
Keywords:
Three-dimensional acoustic sound field
reproduction
Parametric loudspeaker
Electrodynamic subwoofer
Sound image localization
Sound quality improvement
a b s t r a c t
Auditory Mixed Reality (MR) systems that reproduce Three-Dimensional (3-D) acoustic sound fields have
recently become a research focus because the combination of visual and auditory MR systems can achieve
a greater sense of presence than conventional visual MR systems. General auditory MR systems usually
use a headphone-based system with a Head-Related Transfer Function (HRTF), which is a major system
for reproducing 3-D acoustic sound fields. However, the localization accuracy of sound images with a
HRTF depends on the individual. On the other hand, we have already proposed a system for reproducing
a 3-D acoustic sound field with parametric loudspeakers instead of headphones. The 3-D acoustic sound
field reproduced by this system has achieved a highly accurate localization of sound images. However,
one problem is that it is difficult to reproduce lower frequency sounds using parametric loudspeakers,
which causes a poorer sound quality. We tried to accomplish a greater sense of presence for 3-D acoustic
sound fields based on a hybrid combination of an electrodynamic subwoofer and the parametric loudspeakers by improving the sound quality. Sound images were formed at the target location using the
parametric loudspeakers, and a lower frequency sound was compensated for by using the electrodynamic
subwoofer. Subjective evaluation experiments were conducted to verify the effectiveness of the proposed
system. We confirmed the improved sound quality while maintaining a higher accuracy of sound image
localization by using the proposed system. We also confirmed the optimum parameters of the proposed
system to achieve a greater sense of presence.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Mixed Reality (MR) systems that seamlessly merge real and virtual spaces have recently become a research focus for use as an applied Virtual Reality (VR) technology that presents a visual sense of
presence to users [1]. Conventional MR systems have presented
only Three-Dimensional (3-D) visual Computer Graphics (CG) objects to users. Auditory MR systems which reproduce 3-D acoustic
sound fields have also recently been a focus because the combination of visual and auditory MR can present a greater sense of presence to users than that when using conventional visual MR
systems [2].
Auditory MR systems require the reproduction of a 3-D acoustic
sound field as virtual sound by forming sound images at target
locations and to seamlessly merge real and virtual sounds. A
two-by-two audio-visual MR system that compatibly manages
both the visual and auditory MR systems has been proposed [3].
⇑ Corresponding author. Tel./fax: +81 77 561 5075.
E-mail addresses: [email protected] (Y. Sugibayashi), cm005068
@ed.ritsumei.ac.jp (S. Kurimoto), [email protected] (D. Ikefuji), mor
[email protected] (M. Morise), [email protected] (T. Nishiura).
0003-682X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.apacoust.2012.03.009
This system has merged real and virtual sounds without shutting
out the real sound by using open-air headphones and used a
headphone-based system with a Head-Related Transfer Function
(HRTF) [4] to reproduce 3-D acoustic sound images surrounding
the human head, which is a major 3-D acoustic sound reproduction
system.
Sound images can be accurately formed at target locations by
using a headphone-based system with a HRTF. However, a
headphone-based system with a HRTF requires the measurement
of the personal HRTF of each user because the shapes of human
heads or ears differ based on the person. It has been proposed to
select or generate an optimum HRTF from the previously measured
HRTF database [5] because of the many computational costs of
measuring HRTF [6]. However, users often confuse the front-back
localization of sound images [7]. Auditory MR systems should
overcome these problems.
We have already proposed a system for reproducing a 3-D
acoustic sound field by using parametric loudspeakers [8] instead
of headphones to overcome these problems [9]. As a result, the
proposed system achieved a higher level of accuracy for the sound
image localization, which did not depend on the individual users.
However, one problem with this system is that it is difficult to
Y. Sugibayashi et al. / Applied Acoustics 73 (2012) 1282–1288
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The 3-D acoustic sound fields have higher localization accuracy
of sound image without depending on the individual.
The tightness the user feels on their head because of the headphones is reduced.
We have already proposed a system of reproducing the 3-D
acoustic sound field by using parametric loudspeakers instead of
headphones to fulfill these requirements [9].
We discuss the underlying principle and characteristics behind
the parametric loudspeakers used in the proposed system in next
section. We then discuss the sound image localization by using
parametric loudspeakers as the underlying principle of the proposed system.
Fig. 1. Directional patterns of parametric and electrodynamic loudspeakers.
reproduce lower frequency sounds by using the parametric loudspeakers, which causes a poorer sound quality.
We attempted to achieve a greater sense of presence for 3-D
acoustic sound fields based on a hybrid combination of an electrodynamic subwoofer and parametric loudspeakers to improve the
sound quality for this paper.
2. Problems with headphone-based system with HRTF
and requirements for auditory MR
Auditory MR systems require the formation of sound images at
the target locations and to seamlessly merge real and virtual
sounds. A two-by-two audio-visual MR system [3] has reproduced
a virtual 3-D acoustic sound field by using a headphone-based system with a HRTF and merged real and virtual sounds by using
open-air headphones. However, the headphone-based system with
a HRTF has two problems mentioned below.
It is difficult to measure the personal HRTF of each user due to
the computational costs although a personal HRTF is required to
accurately present sound images. The localization accuracy of
sound images depends on the individual, provided that the user
uses a HRTF of another person. In particular, users often confuse
the front-back localization of sound images.
The users may feel an increased amount of the tightness on
their head because they have to be equipped with headphones
in addition to Head Mounted Displays (HMD).
The MR system should not only overcome these system problems in order to reproduce a 3-D acoustic sound field, but also fulfill the requirements of an auditory MR system. Therefore, the MR
system should also fulfill the following two requirements.
3. 3-D acoustic sound field reproduction with parametric
loudspeakers
3.1. Principle behind parametric loudspeaker
Parametric loudspeakers with sharper directivity can emit audible sounds to a particular area in contrast with conventional loudspeakers that emit widely spreading acoustic sound. The particular
area where a listener can hear the audible sound is defined as an
audio spot in this paper.
Parametric loudspeakers use ultrasounds as the carrier sounds,
which have sharper directivity characteristics. Fig. 1 outlines the
directional patterns of parametric and electrodynamic loudspeakers. Fig. 2 outlines the principle behind parametric loudspeakers.
The amplitude of an ultrasound is modulated with an audible
sound. The modulated ultrasound consists of the frequencies of
the carrier sound and the adjacent sidebands. Parametric loudspeakers emit an intense modulated ultrasound. A difference tone
or combination tone is then generated because of the nonlinear
interaction in the air. The difference tone between the carrier
sound and each sideband is equal to the original audible sound.
In other words, the emitted ultrasound is demodulated into the
original audible sound because of a nonlinear interaction in the
air. The modulated ultrasound vAM(t) with an audible sound is calculated as
v AM ðtÞ ¼ V cm ð1 þ mV S ðtÞÞV C ðtÞ;
ð1Þ
V sm
m¼
;
V cm
ð2Þ
where, Vcm represents the maximum amplitude of the carrier sound,
m represents the amplitude modulation factor, and Vsm represents
the maximum amplitude of the audible sound. Here, VS(t) represents the audible sound and VC(t) represents the carrier sound.
Fig. 2. Underlying principle of parametric loudspeaker.
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Fig. 4. Concept behind proposed system.
3.3. Overview of proposed system
Fig. 3. Concept behind reflective audio spot.
Based on this method, parametric loudspeakers with the sharper
directivity characteristics can be created.
3.2. Sound image localization with parametric loudspeakers
The audible sound emitted from parametric loudspeakers is reflected from the walls while maintaining a sharper directivity [10].
This particular area affected by the reflection, where a listener can
hear audible sounds, is defined as a reflective audio spot in this paper. Fig. 3 outlines the concept behind a reflective audio spot in
which the listener can perceive an acoustic sound image from
the location of a wall, and not from that of the a loudspeaker
[11]. This is because only reflected sound not direct sound is transmitted to the listener because of sharper directivity. A steering
sound image can thereby be achieved by steering the emission angle of the parametric loudspeaker. Therefore, we have proposed a
system to reproduce a 3-D acoustic sound field using these characteristics of reflective audio spots with parametric loudspeakers.
Fig. 4 outlines the concept behind the proposed system. The
proposed system uses a unit with multiple parametric loudspeakers mounted on it. The emitted sounds from the unit are reflected
from the walls, ceiling, or floor, similarly to the principle of a light
planetarium. Sound reflections with parametric loudspeakers can
form sound images at various target locations. Listeners using
the proposed system can experience 3-D acoustic sound fields
without having to wear headphones.
3.4. Configuration for proposed system
The proposed system consists of a unit that has ten parametric
loudspeakers mounted on it (as shown in Fig. 5a) and the reflectors
(as shown in Fig. 5b). The direction of emissions for all the parametric loudspeakers can be adjusted. The sounds emitted from
the unit are reflected from the reflector in addition to the walls,
ceiling, or floor. We used acrylic boards (500 500 5 mm) as
reflectors and constructed a 3-D acoustic sound field with them.
The directions of the reflectors were adjusted so that the emitted
sound arrived at the listening location.
Fig. 5. Configuration for proposed system.
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Table 1
Experimental conditions.
Parametric loudspeaker
Electrodynamic subwoofer
Microphone
Loudspeaker amplifier
Microphone amplifier
A/D, D/A converter
Sampling
Background noise level
Reverberation time
MITSUBISHI, MSP-50E
YAMAHA, YST-SW225
HOSIDEN, KUC-1333
YAMAHA, P2500S
Thinknet, MA2016
Roland, UA-101
96.0 kHz, 16 bit
36.7 dBA
670 ms
3.5. Problem with proposed system
The 3-D acoustic sound fields produced by the proposed system
have achieved a highly accurate localization of sound images.
However, the main problem is that it is difficult to reproduce lower
frequency sound by using parametric loudspeakers, which causes
poorer sound quality. We tried to accomplish a greater sense of
presence with 3-D acoustic sound fields based on a hybrid combination of an electrodynamic subwoofer and parametric loudspeakers by compensating for the low frequency sound.
4. 3-D acoustic sound field reproduction based on hybrid
combination of multiple parametric loudspeakers and
electrodynamic subwoofer
Fig. 6. Frequency characteristics of each loudspeaker and proposed system.
frequency characteristics of the parametric loudspeakers, the electrodynamic subwoofer, and the proposed system to find what effect the electrodynamic subwoofer has on the sound images from
the parametric loudspeakers.
4.2. Experiment to measure frequency characteristics
We proposed a 3-D acoustic sound field reproduction that is
based on hybrid combination of multiple parametric loudspeakers
that reproduce higher frequency sound and an electrodynamic
subwoofer that reproduces a lower frequency sound. Sound images
were formed at the target location with the parametric loudspeakers, and lower frequency sound was compensated for by the electrodynamic subwoofer. Here, we attempted to meet the following
three requirements to achieve a greater sense of presence.
Sound quality of 3-D acoustic sound field is improved.
The higher localization accuracy of the sound image at the target locations, which is formed by the parametric loudspeakers,
is maintained even though the parametric loudspeakers and
electrodynamic subwoofer are combined.
The sound image is not localized at the electrodynamic subwoofer location because the sound images should be localized at
only the target locations.
The experiments to measure the frequency characteristics of
the loudspeakers were conducted to confirm the frequency characteristics of the parametric loudspeakers, the electrodynamic subwoofer, and the proposed system. Table 1 summarizes the
experimental conditions. Fig. 6 plots the experimental results.
We confirmed that the electrodynamic subwoofer used in these
experiments could emit sound up to 1 kHz as a result of its frequency characteristics. It could compensate for the low frequency
characteristics of the parametric loudspeakers by using the pro-
In other words, the electrodynamic subwoofer should not affect
the sound images of the parametric loudspeakers while improving
the sound quality for achieving a greater sense of presence. Therefore, we needed to investigate the influence of the electrodynamic
subwoofer on the sound images of the parametric loudspeakers.
4.1. Influence of electrodynamic subwoofer on sound images
of parametric loudspeakers
Studies have found that the localization accuracy decreases
where people localize a lower frequency sound [12] because lower
frequency sound spreads more widely [13]. Therefore, people do
not generally localize sound images on an electrodynamic subwoofer that emits sound below the 200-Hz frequency band. However,
because it is also difficult to reproduce lower frequency sound
above 200-Hz with parametric loudspeakers, we also need an electrodynamic subwoofer to emit sound above this band to completely compensate for the low frequency band. Therefore, the
electrodynamic subwoofer may affect the sound images from the
parametric loudspeakers. Consequently, we should measure the
Fig. 7. Experimental environments.
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5. Evaluation experiments
We conducted two subjective experiments. The experiments to
evaluate the localization accuracy of the sound image were conducted to find what influence the electrodynamic subwoofer had
on the localization accuracy of the sound image at the target locations, which was formed by the parametric loudspeakers, when
using the proposed system.
The subjective experiments using the Mean Opinion Score
(MOS) were conducted to determine the optimum cut-off frequency for the LPF of the electrodynamic subwoofer and the optimum LHR to improve the sound image localization for the humans.
The subjects evaluated the sound quality and sound image localization using the MOS.
Fig. 8. Results from experiments to evaluate localization accuracy of sound image.
posed system (as shown in Fig. 6). However, the electrodynamic
subwoofer may have affected the sound images from the parametric loudspeakers when it emitted sound whose frequency was up
to 1 kHz.
The higher localization accuracy of the sound image at the target locations, which was formed by the parametric loudspeakers,
may therefore decrease when the proposed system was used.
Therefore, for the evaluation experiments, we first evaluated the
localization accuracy of the sound image at the target locations,
which was formed by the parametric loudspeakers, when the proposed system was used.
A sound image may also be localized at the electrodynamic subwoofer location although the sound images should be localized at
only the target locations. To overcome this problem, we should
optimize the parametric loudspeakers and electrodynamic subwoofer parameters, which may affect the sound image localization,
and we should achieve the sound image that can be localized at
only the target location, not at the electrodynamic subwoofer.
5.1. Experiments to evaluate localization accuracy of sound image
and results
Fig. 7 outlines the experimental environments. Seven sound
images were randomly presented to the subjects by using the seven parametric loudspeakers while using the proposed system as
shown in Fig. 7. The subjects were asked to select one of the seven
directions, in which they localized the sound image. The stimulus
was white noise and its duration was 10.0 s. Five subjects (males
aged 22–24) with normal hearing took part in the evaluation.
Fig. 8 plots the results obtained from the experiments to evaluate the localization accuracy of the sound image, which indicates a
95.7% accuracy. These results correspond to the results obtained
without the use of an electrodynamic subwoofer [9]. Furthermore,
the subjects did not confuse the front-back localization of the
sound images, while front-back confusion was a main problem of
the headphone-based system. These suggest that the higher localization accuracy of the sound image at the target locations, which
was formed by the parametric loudspeakers, was maintained when
using the proposed system.
5.2. Subjective evaluations on sound quality and sound image
localization and results
4.3. Optimization for cut-off frequency of low pass filter
and sound pressure levels
We attempted to optimize the two parameters of the proposed
system, which may affect the sound image localization, and attempted to achieve the sound image localized at only the target
location, not at the electrodynamic subwoofer. We optimized the
cut-off frequency of the Low Pass Filter (LPF) with which the sound
emitted by the electrodynamic subwoofer was processed. We also
optimized the Sound Pressure Level (SPL) of the electrodynamic
subwoofer to that of parametric loudspeaker ratio through the
evaluation experiments. The SPL of the electrodynamic subwoofer
to that of the parametric loudspeaker ratio was defined as a Low to
a High frequency Ratio (LHR) in this paper. The observed output
signal Y(x) and LHR were calculated as
YðxÞ ¼ aPðxÞ þ bSðxÞLðxÞ;
ð3Þ
LHR ¼ 20 logðb=aÞ;
ð4Þ
where P(x) represents the output signal of the parametric loudspeaker, S(x) represents the output signal of the electrodynamic
subwoofer, and L(x) represents the signal of the LPF. x represents
the angular frequency, and a and b represent the amplification coefficients of each loudspeaker.
Subjective evaluations were conducted to determine the optimum cut-off frequency for the LPF for the electrodynamic subwoofer and optimum LHR to achieve the sound image localized at only
the target location, not at the electrodynamic subwoofer.
Subjective evaluations were conducted on the sound quality
and sound image localization using the MOS. The subjects evaluated the sound quality and sound image localization, according
to the standards listed in Table 2. All the subjects listened to a reference sound (Score 1 in Table 2) before the experiments to evaluate the sound quality. The parametric loudspeaker presented the
reference sound that was a stimulus for score 1. The stimuli were
randomly presented to the subjects. The stimuli consisted of a
voice (male), and music (orchestra and cello). The durations of
the stimuli were 10.0 s. Five subjects (males aged 22–24) with normal hearing took part in the evaluation. The cut-off frequencies for
the LPF were 200, 400, 600, 800, 1000, and 1 Hz. The slope was
Table 2
Score and opinion.
Score
Sound
quality
Sound image localization
5
Excellent
4
Good
3
2
Fair
Poor
1
Worst
Sound image is at only target direction (not at subwoofer
direction)
Sound image is at almost only target direction (almost
not at subwoofer direction)
Fair
Sound images are at almost two directions (also slightly
at subwoofer direction)
Sound images are at two directions (also at subwoofer
direction)
Y. Sugibayashi et al. / Applied Acoustics 73 (2012) 1282–1288
Fig. 9. Results from subjective evaluation on sound quality for cut-off frequency of
LPF.
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Fig. 11. Results from subjective evaluation of sound quality for LHR.
Fig. 12. Results from subjective evaluation on sound image localization for LHR.
Fig. 10. Results from subjective evaluation on sound image localization for cut-off
frequency of LPF.
30 dB/oct. for the lower frequency. The LHRs were 20, 15, 10,
and 5 dB.
Fig. 9 plots the results obtained by subjectively evaluating the
sound quality for the cut-off frequency of the LPF. The error bars
in the barcharts represent the standard deviations. We confirmed
an improvement in the sound quality accomplished by using a hybrid combination because score 1 represents the sound quality
equivalent for the parametric loudspeakers. The sound quality
especially improves with the increasing cut-off frequency of the
LPF. These results suggest that the lower frequency sound emitting
from the parametric loudspeakers could be compensated for by the
electrodynamic subwoofer.
On the other hand, Fig. 10 plots the results obtained from subjectively evaluating the sound image localization for the cut-off
frequency of the LPF. We confirmed that the MOS for the sound image localization was extremely higher when using cut-off frequencies from 400 to 800 Hz, while the MOS for the sound image
localization differed depending on the sound source. The MOS for
the sound image localization was higher for the voice (male) and
music (cello). Although we could not confirm the tendency for
improvement by varying the cut-off frequencies with the other
sound sources, the sound quality and the sound image localization
effectively improved on average with the cut-off frequencies for
the electrodynamic subwoofer from 400 to 800 Hz as can be seen
from Fig. 10.
Fig. 11 plots the results obtained by the subjective evaluation of
the sound quality for the LHR. We confirmed that the sound quality
especially improved with the increasing LHR from 20 to 10 dB.
Moreover, the sound quality improved even under the conditions
where the LHR was 20 and 15 dB because score 1 represents
the sound quality equivalent to that of the parametric loudspeakers. To achieve a greater sense of presence, we should also take
the sound image localization into consideration.
On the other hand, Fig. 12 plots the results obtained from subjectively evaluating the sound image localization for the LHR. We
could confirm that the MOS for the sound image localizations
was extremely higher when decreasing the LHR, while the MOS
for the sound image localization differed depending on the sound
source. The MOS for the voice (male) was 4.9 and 4.7, and that
for music (cello) was 4.7 and 4.6 when the LHR was 20 and
15 dB. The minimum MOS at 20 and 15 dB was 3.1 and 2.9
for music (orchestra). Score 3 represents fair as can be seen in Table 2. Therefore, the MOS for the sound image localization was
higher, provided that the LHR was 20 and 15 dB.
6. Discussion
Results from experiments to evaluate the localization accuracy
of the sound image suggested that the highly accurate localization
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of the sound images at the target locations, which were formed by
the parametric loudspeakers, was maintained when using the proposed system. Therefore, the proposed system was effective at
accurately locating the sound image. Furthermore, both a higher
sound quality and an improved sound image localization were
effectively achieved using the cut-off frequencies from 400 to
800 Hz for the electrodynamic subwoofer and LHRs of 20 and
15 dB. Therefore, the cut-off frequencies from 400 to 800 Hz
and LHRs of 20 and 15 dB were optimum for achieving a greater
sense of presence.
On the other hand, we confirmed that the MOS for the sound
image localization differed depending on the sound source. Therefore, we need to investigate the frequency characteristics of the
sound source with high and low MOSs for sound image localization
to achieve a greater sense of presence in the future.
7. Conclusion
We tried to achieve a greater sense of presence for 3-D acoustic
sound fields based on a hybrid combination of an electrodynamic
subwoofer and parametric loudspeakers. We found from the
experimental results that a higher localization accuracy of the
sound images at the target locations, which were formed by using
parametric loudspeakers, was maintained even though the parametric loudspeakers and an electrodynamic subwoofer were combined. Furthermore, we attempted to optimize the parameters of
the parametric loudspeakers and electrodynamic subwoofer,
which may affect the sound image localization, and attempted to
improve the sound image localization for humans. We confirmed
from the experimental results that the optimum cut-off frequencies for the electrodynamic subwoofer ranged from 400 to
800 Hz, and LHRs of 20 and 15 dB were optimum for achieving
both an improved sound quality and sound image localization.
We intend to investigate the frequency characteristics of a
sound source with high and low MOSs for the sound image localization, and will attempt to achieve a greater sense of presence for
3-D acoustic sound fields using a hybrid combination of an electrodynamic subwoofer and parametric loudspeakers in future studies
by further improving the sound image localization.
Acknowledgement
This work was partly supported by a Grand-in-Aid for Scientific
Research funded by the Ministry of Education, Culture, Sports, and
Science (MEXT) of Japan.
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