directivity control of loudspeaker system in low frequency

7-07 Directivity Control Of Loudspeaker System In Low Frequency Range
DIRECTIVITY CONTROL OF LOUDSPEAKER SYSTEM
IN LOW FREQUENCY RANGE
Vecky Canisius POEKOEL*, Yoshifumi CHISAKI* and Tsuyoshi USAGAWA*
* Human and Environment Informatics, Graduate School of Science and Technology,
Kumamoto University, JAPAN
2-39-1 Kurokami, Chuo-Ku, Kumamoto City, 860-8555 Japan
email : vecky@hicc.cs.kumamoto-u.ac.jp , chisaki@cs.kumamoto-u.ac.jp, tuie@cs.kumamoto-u.ac.jp
ABSTRACT
This paper proposes a directivity control method of
loudspeaker system in low frequency range to
reproduce sound in hemispere, i.e. uni-directional
reproduction. Uni-directional sound signage is
attractive application, and is able to guide people
during
evacuations
in
smoke-filled
fire
emergencies. Usually, a single loudspeaker has
omni-directional characteristics in low frequency.
A multiple-loudspeaker system using active noise
control (ANC) technique is used to develop unidirectional characteristics in low frequency range.
This research investigates a two-loudspeaker system
that is configured by a primary loudspeaker faced to
0 degree and a secondary loudspeaker faced to
+130 degree or +180 degree as examples.
Filtered-x Least Mean Square (FxLMS) adaptation
is carried out to configure the digital filter of the
system. Experimental results show that the system
can control the directivity on target direction at low
frequencies. More than 10 dB gain reduction
at 200 Hz is obtained for the range of +90 degree
around the target direction.
Keywords : multiple-loudspeaker system, unidirectional characteristics, low frequency range,
FxLMS.
1
INTRODUCTION
Generally, a single loudspeaker has omnidirectional characteristics in low frequency ranges.
Therefore, a single loudspeaker reproduces sound to
all directions in a low frequency range. The unidirectional characteristics of sound reproduction is
needed to propagate the sound in one direction as
spotlight sound towards the listener, while reducing
undesired gain at backside especially in low
frequency range, in order to improve the quality of
sound or speech at location of the listener. Because
the backside gain will be undesired echoes at
listener's location due to the reflections.
A single loudspeaker is used as the sound
reproduction system of emergency system for
evacuation either inside building or tunnel to guide
the direction for evacuation. For such cases, it is
necessary to have the uni-directional sound
propagation characteristics.
The use of bells on the emergency system can
provide information about state of emergencies, but
they are not able to give the direction of evacuation.
Several studies have been done to develop the
emergency evacuation system for maximizing
human safety in an emergency situations[1-3].
The combination of a conventional bell sound
with certain tones have been developed to improve
the quality of emergency information through
sound, to be easily perceived as an emergency and
dangerous situation[1].
However, in emergencies such as smoke-filled
fires, it is necessary to provide sound guidance for
emergency evacuation that can lead people to
emergency exits. Yokoyama, et al., has developed
a system of directional sound guidance to give
directions using evacuation voice in event of a fire
emergency in a smoke-filled tunnel. The system
uses array loudspeakers by implementation timedelay technique with appropriate distance between
loudspeakers[2][3].
Kakuhari, et al., has developed point sound
source using multiple loudspeakers to reproduce
unidirectional sound propagation. This system is
very effective in shaping uni-directional
propagation characteristics of sound source on the
speech frequency range of 500 Hz-4000 Hz[4][5],
but it is not designed for low frequency range.
A multiple loudspeakers system that is focused
to produce a uni-directional propagation behavior,
is also proposed. Two loudspeakers are configured
back to back, and an adaptive filter using
FxLMS algorithm, is used to develop the uni-
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The Proceedings of The 7th ICTS, Bali, May 15th-16th, 2013 (ISSN: 9772338185001)
directional propagation of sound at low frequencies.
The proposed system is effective to reduce the
backside gain around +130 degree to +230 degree at
a frequency range of 100 Hz - 600 Hz.
one (S) at +180 degree (180) to reduce undesired
gain at around +180 degree direction.
This research investigates the proposed multiple
loudspeakers system. The system will be used to
control the directional of sound propagation of
reproduction system, that is focused to uni
directional that is around 0 degree, and at the same
time to reduce undesired gain around +90 degree to
+180 degree directions.
2
MULTIPLE LOUDSPEAKERS
SYSTEM
The proposed control system consists of two
loudspeakers, an adaptive Finite Impulse Response
(FIR) filter using FxLMS algorithm, and a delay
filter to adjust time-alignment between the desired
sound and the controlled sound at control point.
A block diagram of the proposed control system is
shown in Fig.1. There is a microphone which will
be used to adapt the adaptive FIR filter coefficients
in adaptation stage, and also to monitor the
performance in control stage. Figure 1 also shows
two acoustic paths, i.e. a primary acoustic path,
P'(z), and a secondary acoustic path, S1 ' z .
2.1
Loudspeakers Configurations
Based on the purpose of this study, the primary
loudspeaker is directed toward 0 degree for
propagating the desired sound, while the secondary
loudspeaker is directed toward +130 degree for
reducing gain at this direction. This configuration
is called P0-S130. Figure 2 shows two kinds of
configurations that will be used to control the
directivity of sound propagation. As shown in
Fig.2, there are two sides of the acoustical field
which are left and right sides of vertical axis. For
uni-directional control purpose, right side field
represents direction of the desired sound
propagation, while left side field is the controllable
zone.
Figure 2(a) shows the P0-S130 configuration,
where the primary loudspeaker (P) is faced to 0
degree (0) direction and the secondary loudspeaker
(S) is faced to +130 degree (130) direction to
control the undesired gain at around +130 degree.
Other configuration that is shown in Fig.2(b) is
P0-S180 configuration, where the primary
loudspeaker (P) faced at 0 degree (0) and secondary
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Figure 1. Block diagram of multiple two loudspeakers system
for uni-directional control of sound propagation
2.2
FIR Filter Setting
FIR filter, H(z), that is shown in Fig.1, will be
used to produce a controlling sound. This
controlling sound will be introduced into the
acoustic field through the secondary loudspeaker to
the microphone side as the controlled side.
Although it is possible to determine the
characteristics of FIR filter, H(z), an adaptive
method is more convenient and easy to manage the
system. In this study, FxLMS which is popular in
active noise control (ANC) technique is used.
A block diagram of the adaptation process
is shown in Fig. 3, there are the primary path
P z P ' zS2  z,
the
secondary
path
'

S1  zS1 z S2 , a model of secondary path Sz , a
system delay L(z), and an adaptive filter H(z).
A system delay is installed to compensate the delay
(a) P0-S130 Configuration
(b) P0-S180 Configuration
Figure 2. Two loudspeakers configurations
7-07 Directivity Control Of Loudspeaker System In Low Frequency Range

, is
for secondary path. A secondary path, Sz
installed to implement the filtered input signal,
i.e. filtered-X. Estimation of the secondary path

Sz
, and primary path P(z)P'(z)S2(z) are obtained
by impulse response measurement method using
time-stretched pulse (TSP) [6].
3.1
FIR Filter Setting
Figure 3 shows block diagram used in
adaptation stage. Devices and setting parameters
used for both configurations are shown in Table 1,
and their snapshots are shown in Fig.4.
Table 1. Devices and Parameters Setting
The coefficients of adaptive digital filter (ADF),
H(z), are adapted using FxLMS algorithm, and
white noise is used as input. After the ADF
converged, the output of ADF, Y(z) = H(z)X(z),
minimized the power of E(z) which is expressed as
Eq.(1). This condition means that the primary and
secondary sound are in anti-phase at control point.
.
E z =S2 z  {P' z  Pz L zS'1  zS1  z H z} X z  (1)
  z, will be
The obtained ADF coefficients, H
an estimate of ideal coefficients shown in Eq.(2).
.
Parameters / Equipments
Setting/Specification
Measurement Environment Anechoic chamber
Sampling Frequency
8 kHz
Tap Length of adaptive
FIR filter
150 taps
Tap Length of System
Delay
120 taps for P0-S130
1 tap for P0-S180
Loudspeakers
BOSE-101VM x 2
Microphone
Sony-EMC44B
'
H  z = −
3
P  z P  z L z
S'1 z S1 z 
(2)
RESULTS OF EXPERIMENT
The proposed two-loudspeaker system is tested
in anechoic room by using two kinds of
configurations, i.e. P0-S130 and P0-S180. The
configuration P0-S130 is used to highlight desired
sound directional toward 0 degree and to observe
the gain controlling at around +130 degree.
P0-S180 is used to highlight at 0 degree and to
observe the gain controlling at +180 degree.
Distances between primary and secondary
loudspeakers are 650 mm, and 550 mm for P0-S130
and P0-S180 respectively.
Figure 3. Block diagram of single FxLMS filter adaptation
3.2
Obtained Directivity
As mentioned earlier, the propagation
characteristics of a single loudspeaker at low
frequencies
are
almost
omni-directional.
Measurement results of a single loudspeaker
characteristics using different frequencies is shown
in Figure 5, that show almost omni-directional
pattern at 100 Hz and 500 Hz and uni-directivity
pattern at high frequencies, such as 1000 Hz and
above.
The directivity characteristics of the proposed
multiple loudspeakers control system are shown in
Fig.6.
Experimental results using P0-S130
configuration are shown in the first column of
Fig.6, and using P0-S180 configuration are shown
in the second column. Each figure shows the
propagation characteristics with and without control
at frequencies of 100 Hz, 200 Hz, and 500 Hz. The
black color solid lines are directional characteristics
under control condition, and the red color dashed
lines are obtained by single loudspeaker.
According to experimental results of P0-S130
configuration, there are more than 8 dB gain
reduction around +100 deg. to +170 deg. at 100 Hz,
more than 10 dB around +110 deg. to +190 deg. at
200 Hz, and more than 9 dB around +120 deg. to
170 deg. at 500 Hz. Furthermore, on P0-S180
configuration, there are more than 15 dB gains
reduction around +120 deg. to 240 deg. at 100 Hz,
more than 9 dB around +130 deg. to 220 deg.
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The Proceedings of The 7th ICTS, Bali, May 15th-16th, 2013 (ISSN: 9772338185001)
P0-S130 and P0-S180, to make uni-directional
characteristics to reduce the gain at +130 deg. and
+180 deg., respectively.
(a) P0-S130 Configuration
(b) P0-S180 Configuration
Figure 4. Snapshot of loudspeakers configurations
Experimental results show that P0-S130
configuration system is able to attenuate 10 dB for
70 deg. range at 200 Hz and P0-S180 one is able to
attenuate more than 9 dB for 90 deg. range at
200 Hz. In order to implement actual evacuation
guide systems, it is necessary to evaluate the
system's performance in reverberant environments
such as hallways and tunnels.
at 200 Hz, and more than 6 dB around +150 deg.
to +220 deg. at 500 Hz.
REFERENCE
Directivity characteristics that are obtained from
those systems show that the P0-S180 configuration
reduces gain focus at area around +140 deg. to
+220 deg., and P0-S130 configuration focus at area
around +90 deg. to 180 deg. There are further
reductions by P0-S180 configuration; more than
4 dB at 100 Hz, and more than 6 dB at 200 Hz and
500 Hz at the target of acoustic field of +100 deg
to +170 deg.
[1] L.C. Broer, S.J. Wijngaar, Directional sound
evaluation from smoke-filled tunnels, Proc.
First International Symposium Safe and
Reliable tunnels innovative European
achievements, 2004, 33-41
Figure 5. Directivity pattern of single loudspeaker
4
CONCLUSION
This paper investigates the proposed system by
using two kinds of loudspeaker configurations,
260
[2] S. Yokoyama, S. Sakamoto, S. Tazawa,
Subjective experiment on speech-rate of
emergency evacuation announcement in a
tunnel, Proceedings Euronoise, 1994.
[3]
S. Yokoyama, S. Sakamoto, H. Tachibana, S.
Tazawa, Study on the Application of Timedelay Technique to Public Address System in
a Tunnel, Proc. Inter-noise, 2005.
[4]
I. Kakuhari, H. Hashimoto, K. Terai,
Development of a loudspeaker system with a
unidirectional radiation pattern in a speech
frequency range, Proc. 106th Convention of
Audio Engineering Society, 4867, 1999.
[5]
I. Kakuhari, H. Hashimoto, K. Terai,
Development of a loudspeaker system with a
unidirectional radiation pattern in a speech
frequency range, Journal of AcousticSociety of
Japan, 21, 2002, 369-372.
[6]
Y. Suzuki, F. Asano, H.Y. Kim, T. Sone, An
optimum computer-generated pulse signal
suitable for the measurement of very long
impulse responses, J. Acoustic Society of
America, 2, 1995, 1119-1123.
7-07 Directivity Control Of Loudspeaker System In Low Frequency Range
(a) P0-S130 configuration, at 100 Hz
(b) P0-S180 configuration, at 100 Hz
(c) P0-S130 configuration, at 200 Hz
(d) P0-S180 configuration, at 200 Hz
(e) P0-S130 configuration, at 500 Hz
(f) P0-S180 configuration, at 500 Hz
Figure 6. Comparison of propagation directivity patterns with/without control between P0-S130 and P0-S180 configurations at 100 Hz, 200
Hz, and 500 Hz. Red color dashed lines show the directivity of single loudspeaker, i.e. one without control and black color lines show one
with control
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