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Japanese Journal of Applied Physics 54, 087201 (2015)
http://dx.doi.org/10.7567/JJAP.54.087201

Airborne ultrasonic transducer using polymer-based elastomer
with high output-to-weight ratio
Jiang Wu*, Yosuke Mizuno, Marie Tabaru, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
E-mail: wujiang@sonic.pi.titech.ac.jp
Received February 6, 2015; accepted May 19, 2015; published online July 2, 2015
With the properties of low density, low elastic modulus, and low mechanical loss, poly(phenylene sulﬁde) (PPS) is a suitable material as theelastomer in an airborne ultrasonic transducer for generating large vibration velocity. In this study, we design and fabricate a transducer composedof a PPS-based longitudinal vibrator and a PPS-based disk of 0.3 mm thickness to obtain high-intensity ultrasound. The rated sound pressure at a
distance of 300 mm reached 38.9 Pa (125 dB, 0 dB re. 0.02 mPa) when the frequency and voltage were 58.90 kHz and 20 V. The weight of thistransducer is 6.3 g. The ratio of the sound pressure to the weight of the prototype transducer is 1.8 times larger than that of the commercial
transducer. The experimental results indicate that PPS is a good substitute for metal as the elastomer for manufacturing airborne ultrasonictransducers with a high output-to-weight ratio. © 2015 The Japan Society of Applied Physics

1.

Introduction

Airborne ultrasonic transducers have been widely used
in industry and daily life, for instance, in rat-repelling,
deforming,1–3) deliquoring,4) and distance detection.5–10)
Since its ﬁrst invention, many optimizations have been
carried out mainly on the structure of the transducers.2–4,11–19)
Gallego-Juárez et al. attached a large plate on top of aLangevin vibrator and introduced a stepped structure in theplate to adjust the phase diﬀerence in the bending vibration.2–4) This transducer supplies a very intense soundpressure spot. However, the volume of the transducer is toolarge to change the radiation direction. On the other hand, forsensing and ranging applications, a small airborne ultrasonictransducer composed of an aluminum ﬁlm cone and abending vibrator has been widely used for a long time.
Recently, 285 elements of such commercial transducers havebeen used to form a two-dimensional array. The controllable
removal of liquid and the levitation of tiny polystyrene balls
in a wide range have been demonstrated.20,21) For wider
applications, a lightweight airborne ultrasonic transducer thatcan produce high intensity ultrasound is highly required. The
adaption of some materials with low densities, low elastic
moduli, and low mechanical loss is applicable for achieving
high-intensity pressure and lightweight for the airborne ultrasonic transducer. With the recent development in material
industry, a variety of new polymers have been invented andsome of these industrial polymers were used as a matchinglayer and backing in MHz transducers for medical applications.14) In recent reports, one of the functional polymers,
namely, poly(phenylene sulﬁde) (PPS), has been used as thevibrator in an ultrasonic motor.22–24) The most important
property of the PPS-based vibrator for ultrasonic motor
was the high vibration amplitude; and thus, it is expectedthat PPS is also prospective to be applied to an airborne
ultrasonic transducer. In this study, using PPS as the elastomer, we design and fabricate a sandwich-shaped longitudinal transducer with a vibrating-disk ﬁlm for ultrasoundapplications in air.
2.

Materials and structure

The proposed airborne ultrasonic transducer is mainlycomposed of a bar-shaped longitudinal vibrator and a thin

Fig. 1. (Color online) Structure of the airborne ultrasonic transducer.

Table I. Mechanical parameters of the tested PPS.
Density (103 kg=m3)

1.35

Elastic modulus (GPa)

3.45

Poisson’s ratio

0.36

ﬁlm, as shown in Fig. 1. Both the longitudinal vibrator andthe thin ﬁlm are made of PPS. As a new functional polymer,
PPS has the properties of low elastic modulus, low density,and high mechanical quality factor,22,25–30) and some of themechanical parameters of the PPS used in this study are
shown in Table I. Five pieces of ring-shaped piezoelectric
ceramics (Fuji Ceramics C213) are clamped in the middle of
the PPS-based bar using a stainless steel bolt. The thin-ﬁlm
radiator is clamped on top of the longitudinal vibrator by astainless steel nut. In this study, a nut with a ﬂat surface isused to allow the ﬁlm to come into full contact with the topof the longitudinal vibrator with a uniformly distributedpressure. The height and diameter of the nut are 4 and 6 mm,
respectively. In conventional Langevin vibrators, both ends
are under free condition and act as wave antinodes. However,in the longitudinal vibrator proposed in this paper, thedisplacements at both ends of the nuts are extremely small
because the stiﬀness of the stainless steel bolt is much higher
than that of the PPS-based elastomer. Thus, in the numericalsimulation stated later, we used a simpliﬁed model, in which
the ends of the nuts are ﬁxed as an approximation.

087201-1© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 087201 (2015)

J. Wu et al.

(a)

(a)

(b)

Fig. 3. (Color online) Simulation results of modal analysis: the resonancefrequencies of (a) the longitudinal vibrator and (b) the thin ﬁlm underdiﬀerent dimensions.

(b)Fig. 2. (Color online) Meshing models of (a) the PPS-based thin ﬁlm and(b) the simpliﬁed meshing model of the longitudinal vibrator in modal
analysis.

To generate high vibration velocity, the resonancefrequencies of the longitudinal vibrator and the ﬁlm should
be identical. To ﬁnd the appropriate vibration modes anddimensions, we calculated the resonance frequencies of thelongitudinal vibrator and the thin ﬁlm separately using theﬁnite element method (FEM) software ANSYS (ver. 14.5).
Figures 2(a) and 2(b) show the meshing models of the PPSbased thin ﬁlm and the longitudinal vibrator, respectively. In
Fig. 2(a), the nodes located within the 6-mm-diameter circle
on the surface were set to be ﬁxed in all degrees of freedom(DOFs), because this part on the thin ﬁlm was clamped by a
6-mm-diameter nut. The resonance frequency of the PPSbased ﬁlm is determined by the thickness and diameter. The
resonance frequencies of diﬀerent vibration modes were
calculated and plotted as functions of the diameter from10 to 15 mm with the thickness ﬁxed at 0.3 mm. For thelongitudinal vibrator depicted in Fig. 2(b), the resonancefrequency is mainly determined by the length of the vibrator.
Thus, we recorded the resonance frequencies of the longitudinal vibration modes when the length l was varied from 20
to 50 mm. The outer and inner diameters of the PPS-based
bars were set to be 10 and 3.2 mm, respectively. Five piecesof piezoelectric ceramics with the outer and inner diametersof 10 and 4 mm were located between the PPS-based bars,
and the total height of these piezoelectric ceramics was set tobe 3 mm. In the calculation model, we used the simpliﬁed
model with the ﬁxed–ﬁxed condition as explained before,
and the outer surfaces of the nuts were restricted in all DOFs.
Figure 3 shows the simulation results of the resonancefrequencies for the second longitudinal mode excited in thelongitudinal vibrator and some bending modes in the thinﬁlm. For the longitudinal vibrator, the second longitudinalmode (L2) shows a higher eﬃciency than the other modes
because the piezoelectric ceramics were set in the middle ofthe vibrator body. The simulation results of the resonancefrequencies in the longitudinal vibrator as a function oflength are shown in Fig. 3(a). To verify the eﬀectiveness ofthe simpliﬁed model, we tested a complete model including
the stainless steel bolt. The resonance frequencies calculatedusing the simpliﬁed model and complete model at 25 mm
length were 53.173 and 54.271 kHz, respectively, whichshowed that the error is less than 2%. To reduce the calculation time, the simpliﬁed model is used for exploring theresonance frequencies in this study. For the thin ﬁlm, among
a number of vibration modes that can be generated, theappropriate mode should be selected carefully. In this paper,
we paid attention to the sound pressure in free space generated by the diﬀerent vibration modes. The resonance frequencies of the bending vibration modes are listed in Fig. 3(b). It
is obvious that they can be categorized into three groups
according to the deformation shapes. The sound pressure
distribution generated in front of the vibrator is determinedby the vibration distributions (modes) on the thin ﬁlm. Whenthe ﬁlm is located at the xy-plane deﬁned in Fig. 2, the soundpressure distributions pðx0 ; y0 ; z0 Þ for the typical modes ofB10, B08, and B13 are calculated using the formula:ZZvðx; yÞ
pðx0 ; y0 ; z0 Þ ¼
2 Sðx;yÞ dðx; yÞ exp½j  k  dðx; yÞ dx dy
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
dðx; yÞ ¼ ðx0  xÞ2 þ ðy0  yÞ2 þ z20 ;ð1Þ

where ρ is the density of air, k is the wave number, anddðx; yÞ is the distance between the observation position
ðx0 ; y0 ; z0 Þ and the element of the vibrating ﬁlm located on
ðx; y; 0Þ. The center of the ﬁlm is set at the origin of thecoordinate system. vðx; yÞ is the vibration velocity distribution on the vibrating ﬁlm, which is calculated using theharmonic analysis in ANSYS. Figure 4(a) shows the meshingmodel in harmonic analysis. Diﬀerent from the models inmodal analysis, the thin ﬁlm was clamped on the xy-plane.
The length of the longitudinal vibrator is set to be 24 mm andthe diameter of the ﬁlm is approximately 12 mm. A driving
voltage of 20 V was applied on each piece of piezoelectric

087201-2© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 087201 (2015)

J. Wu et al.

(a)

(b)

(c)

(d)

Fig. 4. (Color online) Simulation results of harmonic analysis: based on the model shown in (a), the vibration displacement is calculated and thedeformation contour is shown in (b). The vibration displacements of the longitudinal transducer and the ﬁlm are shown in (c) and (d), respectively.

ceramic as an excitation. Figure 4(b) shows the deformation
contour at the frequency of 55.4 kHz. The vibration displacements of the longitudinal transducer and the thin ﬁlm areshown in Figs. 4(c) and 4(d), respectively. The L2 mode andB10 mode were generated successfully on the longitudinal
vibrator and thin ﬁlm. The interface between the two parts
acted as a node of the transducer as predicted. Substituting
the vibration displacements of the ﬁlm into Eq. (1), we
obtained the sound pressure distributions in the xz-plane for
the B10, B08, and B13 modes, as shown in Fig. 5. In the B10
mode, the main beams appear at 0 and ±45° and the pressureof the central beam at a distance of 0.3 m in the z-axis reaches
approximately 30 Pa. By contrast, with the B08 and B13
modes, sound beams do not appear in the z-direction andthe sound pressures are extremely low. These are mainly
caused by the low vibration velocities and the interference
between the waves generated from the in-phase and out-ofphase vibrations along the circumference of the vibrating
ﬁlm in B08 and B13 modes. Based on the above analysis, wefabricate a prototype transducer with the L2 and B10 modes
shown in Fig. 6(a) and measure the sound pressure in freespace under diﬀerent driving voltages. The dimensions of the
prototype transducer are the same as those of the simulation
model shown in Fig. 4(a).
3.

(a)

(b)

Experimental results

The experimental setup shown in Fig. 6(b) was built toevaluate the prototype transducer. The transducer was ﬁxed
at the center of a rotating stage, and the angle θ was changed
by rotating the stage. A 1=8-in. microphone (ACO 7118)
was used as a calibrated sound sensor. A sound absorption
sponge was arranged to decrease the eﬀects of reﬂection on
the measured results. The transducer was ﬁxed by inserting
one end of the bolt into the foamed styrol. As the displacement of the bolt is small and the foamed styrol is soft,the vibration velocities are insensitive to the insert length of
the bolt. This ﬁxing method simpliﬁes the assembly of thetransducer.

(c)Fig. 5. (Color online) Simulation results: sound pressure distributions on
xz-plane under (a) L2-B10 mode, (b) L2-B08 mode, and (c) L2-B13 mode.

087201-3© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 087201 (2015)

J. Wu et al.

(a)

(b)

Fig. 6. (Color online) (a) Transducer prototype and (b) experimental setup.

(a)

(b)

Fig. 7. Sound pressure as a function of driving frequency: (a) 20–80 kHz
and (b) 58.6–59.4 kHz.

First, the working frequency for the prototype was determined. We measured the sound pressure, where the driving
frequency was swept from 20 to 90 kHz at intervals of 1 kHz,
while keeping the zero-to-peak voltage at 20 V. The distance
L and the angle θ were 300 mm and 0, respectively. The
experimental results in Fig. 7(a) showed that a sharp peak
appeared at the frequency of approximately 59 kHz. Detailed
changes around 59 kHz were observed with a ﬁne intervalof 0.05 kHz. Figure 7(b) shows that the maximum soundpressure is up to 38.9 Pa (125.8 dB) at 58.90 kHz. The soundpressures are larger than 19.5 Pa for the frequency band from58.70 to 59.25 kHz, which means that the −6 dB bandwidthis 0.55 kHz. Figure 8 shows the vibration velocity distribution on the ﬁlm at the driving frequency of 58.90 kHz, which
was measured using a laser Doppler velocimeter (Polytec,
NLV1232). It is clear that the vibration mode B10 was
successfully generated.
Second, the dependences of the sound pressure on therotating angle θ and the distance L were measured for adriving frequency of 58.90 kHz and a voltage of 20 V.
Figure 9 shows the directivity pattern at a distance of 300 mm.
For the central beam, the sound pressure is larger than 19.5 Pa
for the angle from −10 to 10°. Comparing with the simulation
results shown in Fig. 5, other than the main beams, two side
lobes appear at the angles of ±65°, which might be caused by
the reﬂections by the bolt. Figure 10 shows the sound pressure
as a function of the distance L. The pressure exceeds 100 Pa(134.0 dB) if the distance was less than 100 mm.

Fig. 8. (Color online) Vibration velocity distribution on PPS-based ﬁlm
measured with a laser Doppler velocimeter.

Fig. 9. (Color online) Directivity pattern measured at a distance of
300 mm.

Third, we investigated the dependence of the soundpressure on the driving voltage as shown in Fig. 11, where
the frequency was approximately 59 kHz, and L and θ were
kept at 300 mm and 0. The driving frequency was adjusted
every time the driving voltage was changed to obtain a higher
sound pressure. The sound pressure approached 71.1 Pa(131.0 dB) under the zero-to-peak voltage of 40 V. Whenthe driving voltage is lower than 40 V, the sound pressure
linearly increases and the optimal frequency decreases, as thedriving voltage increases.
Comparing with conventional transducers, the airborne
ultrasonic transducer proposed in this paper has the featuresof higher sound pressure at rated voltage and higher outputto-weight ratio. The rated sound pressure of the typical

087201-4© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 087201 (2015)

J. Wu et al.

we designed and fabricated an airborne ultrasonic transducer
using PPS as the elastomer, and measured the sound pressure
distribution in free space. The experimental results showedthat the sound pressure reached 38.9 Pa under the same
intensity of electric ﬁeld on the piezoelectric ceramics, whichwas 3.5 times as large as that of the commercial transducer.
The ratio of the sound pressure to the weight of thistransducer is 1.8 times larger than that of the commercial
transducer. PPS, as a functional polymer with the propertiesof low density, low elastic modulus, and low mechanicalloss, is expected to be applied in powerful and lightweightairborne ultrasonic transducers. In the future, we will
integrate the polymer-based transducers to an array to obtain
more powerful and controllable ultrasounds.

Fig. 10. Sound pressure as a function of the distance L between thetransducer and the microphone.

Acknowledgment

The authors thank Daicel Corporation for providing some ofthe polymer samples tested in the experiments.

Fig. 11. (Color online) Sound pressure and resonance frequency asfunctions of driving voltage.

commercial ultrasonic transducer is 11.2 Pa (115 dB) whenthe maximum intensity of electric ﬁeld in the piezoelectric
ceramic is 47 V=mm, assuming that the thickness of thepiezoelectric ceramic is 0.3 mm and the zero-to-peak voltage
is 14.1 V. The sound pressure of the prototype transducer
under the zero-to-peak voltage of 20 V, which was equivalent
to the maximum intensity of the electric ﬁeld of 40 V=mm,
reached 38.9 Pa (125.8 dB). This value is 3.5 times higher
than that of the commercial transducer. The weight of the
prototype transducer is 6.3 g, 60% of which is that of thebolt and nuts made of stainless steel. The weight of thecommercial transducer is approximately 3.2 g. Thus, theratio of the sound pressure (output) to the weight of the
prototype transducer is about 1.8 times larger than that of
the conventional transducer. Furthermore, if the material withhigh elastic modulus and low density was used as thebolts, the weight of this transducer would be reduced. The
experimental results indicate that PPS is a good substitutefor metal as the elastomer in airborne ultrasonic transducers.
The multiple-pole radiation pattern is another feature of thistransducer. The angle coverage provided by the transducer
with the multiple-pole radiation pattern is wider than thatgenerated by the conventional one-main-pole type. In some
applications, for instance, rat-repelling instruments andautomobile radars, the numbers of transducers can be reduced
if this new transducer is applied.
4.

Conclusions

To achieve higher sound pressure with minimum weight,

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