Audio Engineering Society
Convention Paper 9283
Presented at the 138th Convention
2015 May 7–10 Warsaw, Poland
This paper was peer-reviewed as a complete manuscript for presentation at this Convention. This paper is available in the AES
E-Library, http://www.aes.org/e-lib All rights reserved. Reproduction of this paper, or any portion thereof, is not permitted
without direct permission from the Journal of the Audio Engineering Society.
Loudspeaker systems by linear motion type
piezoelectric ultrasonic actuators
Daichi Nagaoka1 , Juro Ohga2 , Hirokazu Negishi3 , Ikuo Oohira4 , Kazuaki Maeda5 , and Kunio Oishi1
1
Tokyo University of Technology, Hachioji-shi, Tokyo, 192-0914, Japan
2
Shibaura Institute of Technology/ MIX corporation, Kamakura-shi, Kanagawa, 247-0071, Japan
3
MIX corporation, Yokosuka-shi, Kanagawa, 238-0026, Japan
4
Self-employed, Yokohama-shi, Kanagawa, 227-0026, Japan
5
TOA Corporation, Takarazuka-shi, Hyogo, 665-0033, Japan
Correspondence should be addressed to Daichi Nagaoka ([email protected])
ABSTRACT
The research group of authors has been developing completely new loudspeaker constructions which are
driven by piezoelectric ultrasonic motors. This paper proposes two sorts of applications of piezoelectric
linear actuators to both direct radiator and horn loudspeakers.
A direct-radiator loudspeaker with a cone radiator driver by piezoelectric actuators shows smooth frequency
characteristics in low frequency region because its radiating motion includes no significant resonance in the
working frequency region. Therefore, it is useful for radiation of the lowest frequency part of audio signal.
A horn loudspeaker by the same actuators works in rather moderate frequency region which is higher than
the cut-off frequency of horns of ordinary size.
1. INTRODUCTION
The conventional electrodynamic loudspeakers were
developed about 60 years ago and their fundamental
construction is not changed.
These sorts of loudspeakers, however, include some
limitations in its performances, due to that the
electro-dynamic transducers for the conventional
loudspeakers do not have satisfactorily high driving
impedance because its driving force is induced via
an air gap [1].
Nagaoka et al.
Loudspeaker using ultrasonic actuators
To avoid this defect, use of an electromechanical
transducer with large mechanical impedance for a
driver is necessary to drive a sound-radiating diaphragm by a strong, as well as highly controlled
force. A direct-radiator loudspeaker with this driver
will work without any significant resonance. It may
be suitable to very low frequency signal radiation.
A horn loudspeaker with this driver will be able to
use rather simple throat construction with a smaller
diaphragm. The sound equalizer in front of the diaphragm may not be necessary.
(a) Photograph
The authors are studying the completely new loudspeaker driven by piezoelectric ultrasonic motors or
actuators [2–4].The ultrasonic motors (USM), and
also the ultrasonic actuators, are characterized by
very high driving mechanical impedance because its
mover contacts its stator tightly. The authors already presented the direct-radiator loudspeaker using rotational ultrasonic motors [4]. They proposed
so-called ”QMDS” construction using four ultrasonic
motors as a new direct-radiator loudspeaker suitable
for radiation at a low frequency range.
However, use of rotational motors requires a mechanism which induces a reciprocal motive force from
a continuous revolution of the motors. This condition results somewhat complicated construction. In
this paper, the authors propose use of ultrasonic linear actuators. A few compact linear actuators are
already developed to be commercially available [5].
2. PIEZOELECTRIC ULTRASONIC LINEAR
ACTUATOR
The linear actuator that the authors use is Type
NU30, which was developed by Dr. M. Takano et
al. in 2006 and delivered by NIKKO Co. It includes
a multilayer piezoelectric actuator [6].
(b) Piezoelectric ceramic rods
Fig. 1: Structure and photograph of NU30.
and bending vibration, which are independent each
other.
Fig. 1 (a) shows photograph of NU30 actuator. A
small elliptical motion of the finger tip of it actuates
reciprocal motion of a driving plate connected to a
diaphragm.
The cooperation of two vibrating modes will be explained by Fig. 2. A small elliptical motion of the
finger tip is induced by a composite of longitudinal
vibration (L1) and bending vibration (B2) of the
piezoelectric ceramic rod having four divided electrodes. Its resonant frequencies of longitudinal extension vibration and transverse bending vibration
of the ceramic are designed to be close frequency
proximity. In the case of NU30, the impedance of
L1 and B2 is 55 Ω and 20 Ω in a driving frequency
respectively.
Fig. 1 (b) describes structure of two multilayer
piezoelectric ceramic rods included in the actuator. The electrodes of them are divided so as to
induce two vibration modes: longitudinal vibration
Fig. 3 shows relationship between applied voltage
and velocity of the mover. Input-output linearity is
excellent even very low input voltage region [5]. Relationship between input voltage and vibrating ve-
2.1. Construction of NU30
Fig. 1 shows transducer construction of the piezoelectric ultrasonic actuator NU30 [7].
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Nagaoka et al.
(a) L1 mode
Loudspeaker using ultrasonic actuators
(b) B2 mode
Fig. 2: Two vibration modes.
Fig. 4: Driving circuit block diagram.
tioin of the finger tip at the same frequency as the
audio signal.
Fig. 4 shows a block diagram of the driving circuit
developed by the authors.
Two power amplifiers drive B2 and L1 electrodes.
Signal for B2 is modulated by using a multiplexer.
The signal for L1 is shifted by 90 degree to the signal
for B2 by an all-pass filter.
Fig. 3: Velocity characteristics.
locity of chip top of the actuator is almost linear.
2.2. Driving circuit
The multilayer piezoelectric actuator used here requires two sorts of signal waves.
One is a high-frequency sinusoidal ac signal, 55 kHz
of frequency, for example. The other is an audio signal to be radiated. The high-frequency signal shall
be arranged as two signals with phase difference of
90 degree, i.e., “sin” and “cos” signals to generate
elliptical motion of the finger tip.
One of the high-frequency signals is modulated by
the audio signal as DSB (Double Side Band) wave.
The high-frequency wave and the DSB wave are
added to L1 and B2 respectively, rotational direction of finger tip is dependent on sign of the audio
signal. When the sinusoidal voltage of the audio signal is positive, the rotational direction is clockwise,
and vice verse. A driving target responds to the mo-
3. EXPERIMENTAL MODEL OF DIRECTRADIATOR LOUDSPEAKER
As the first application of the linear actuators,
the authors examined a direct-radiator loudspeaker
model with a cone radiator to compare the former
model by using rotational motion of the ultrasonic
motors [4]. The former model was suitable for low
frequency radiation, lower than 200 Hz, for example.
However, the new experimental loudspeaker system
driven by the linear motion type ultrasonic actuators
were rather suitable for radiation of mid frequency
range signal than. low frequency radiation. The proposed loudspeaker is denoted as USMSP (UltraSonic
Motor loudSPeaker).
3.1. Construction of direct-radiator loudspeaker
model
Loudspeaker construction of this model is simpler
than the former models because the cone radiator is
driven by linear motion of a driving plate induced
by the actuators directly. Figs. 5 and 6 show the
construction and a photograph.
Two NU30 actuators are mounted vertically. Reciprocal motion of the driving plate induces vibration
of the cone radiator. Fine adjustment of alignment
between actuators and driving plate is done by a
micrometer-head.
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Loudspeaker using ultrasonic actuators
Fig. 7: Output sound pressure level and distortion
level.
Fig. 5: Linear motion type USMSP construction
(overview).
conventional USMSP [8].
4. HORN LOUDSPEAKER MODEL
The loudspeaker driver by the linear motion type
piezoelectric ultrasonic driver seems suitable for a
horn loudspeaker for public address systems etc.
because its driving mechanical impedance is high.
It means large driving force to the horn driver diaphragm.
The authors applied this sort of driver to a horn
loudspeaker model. The proposed horn loudspeaker
is denoted as USMCD (UltraSonic Motor Compression Driver).
Fig. 6: Photograph of USMSP.
Fig. 7 shows an example of frequency response of
output sound pressure and second and third distortion levels of this model in front of the cone radiator. Radiation characteristics of this model for signal frequency of less than 800 Hz seems good. Frequency response is smooth and distortion is less than
−20 dB. The result described above shows that this
USMSP is sutable for a higher frequency than the
4.1. The conventional horn loudspeakers
The throat of a horn is designed to be smaller than
wavelength of the highest frequency sound to be radiated. The conventional horn loudspeakers use a
driver diaphragm larger than size of the throat to
get a satisfactorily large volume velocity. Therefore,
the diaphragm is coupled via an acoustical transformer so-called as an equalizer.
By using the piezoelectric ultrasonic actuators, the
authors examined to drive by a diaphragm of as
small as the horne throat size, coupled directly without any equalizer.
4.2. Construction of USMCD
Figs. 8 and 9 show our experimental horn loudspeaker model driven by a mechanism similar to that
shown in Fig. 5.
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Fig. 8: Construction of the experimental driver.
Fig. 10: Output sound pressure level and distortion
level.
Fig. 9: Photograph of horn loudspeaker model.
The authors constructed an experimental horn loudspeaker model using a reflex horn of 500 mm in diameter for frequency range of more than 200 Hz,
commercially available by TOA Co. [9].
The horn driver construction is a cylinder-piston system whose piston is connected to a driving plate
vibrated by piezoelectric actuators. The clearance
between cylinder and piston is 20 micrometer which
is sufficiently narrow because cut off frequency by
leak of the clearance is lower than the lower limiting
frequency of the horn. The plate driven by the actuators is also supported at the opposite end by using
a loudspeaker damper to vibrate linearly and maintain the initial position at no signal. A laboratory
jack was used for height adjustment of driver.
4.3. Input-output characteristic of USMCD
Fig. 10 shows an example of frequency response of
output sound pressure and second and third distortion levels of this model at the open end of the horn.
This model can radiate sound from cut-off frequency
of the horn to 1000 Hz. The conventional horn loudspeaker TH-750 can produce a sound level of 110 dB
in the frequency range of 200 to 6000 Hz with the
corresponding driver unit TU-750 [10]. Still availability of the experimental loudspeaker explained
above is poor, the future improvement of construction will bring a practical performance. The authors
are trying to a new design of the cylinder for converting the mechanical motion into an acoustic signal.
5. CONCLUSION
In this paper, the authors propose use of commercially available ultrasonic linear actuators for two
sorts of loudspeaker constructions. One is a directradiator loudspeaker with a cone-radiator. The
other is a horn loudspeaker. Both models are suitable for radiation of higher frequency sound than former models by using rotational revolution of piezoelectric ultrasonic motors which the authors proposed already.
6. ACKNOWLEDGMENT
The authors wish to thank Prof. K. Nakamura of
Tokyo Institute of Technology and Dr. M. Takano
of Industrial Research Institute of Ishikawa for their
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valuable supports and discussions.
7. REFERENCES
[1] John M. Eargle, “Loudspeaker Handbook,”
Chapman & Hall (USA, 1997), pp. 22-26.
[2] J. Ohga, “Sound system with wideband piezoelectric rectangular loudspeakers using a tuck
shaped PVDF bimorph,” Proc. Acoustics 2008,
pp. 4573-4578 (Paris, France, June 29-July 4,
2008).
[10] TOA Co., “TU-750 Driver unit,” http://www.
toa.co.jp/products/prosound/speakers/
speakers_phone_separate/tu-750.htm
[in Japanese].
[11] H. F. Olson and F. Massa, “Applied Acoustics,” second edition, P. Blankiston’s Son & Co.
(USA, 1939), p. 214.
[3] J. Ohga, “Variety of electroacoustical devices
by piezoelectric materials - Is the loudspeaker
without magnet nor coil practical? -,” Fundamentals Review, IEICE, 1, 4, pp. 46-61 (2008)
[in Japanese].
[4] J. Ohga, R. Suzuki, K. Ishikawa, H. Negishi,
I. Oohira, K. Maeda, and H. Kubota, “Loudspeaker for low frequency signal driven by four
piezoelectric ultrasonic motors,” AES 132nd
Convention, no. 0040 (Apr. 2012).
[5] M. Takano, M. Takimoto, and K. Nakamura,
“Electrode design of multilayered piezoelectric
transducers for longitudinal-bending ultrasonic
actuators,” Acoust. Sci. & Tech. vol. 32, no. 3,
(2011), pp.100-108.
[6] M. Takano, A. Nakashima, M. Taka, and T
Ishii,“Development of Linear Ultrasonic Actuator with Elastic Hinge (Evahuation of Elastic
Hinge Structure using Finite Element Analysis),” The Japan Society of Mechanical Engineers, vol. 72, no. 721 (2006), pp. 209–214 [in
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[7] New queyras series editorial board, “Application of piezoelectric ceramics,” Gakken Co.
(Japan, 1989) pp. 101–102.
[8] Juro Ohga, Hiroya Saito et al., “Loudspeaker for low frequency signal driven by four
piezoelectric ultrasonic motors,” International
Congress on Acoustics 2013 Montreal, 1pEAb5
(2013).
[9] TOA
Co.,
“TH-750
Reflex
horn,”
http://www.toa.co.jp/products/prosound/
speakers/speakers_phone_separate/
[in Japanese].
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