A Flexible Polyvinylidene Fluoride Film-Loudspeaker

中國機械工程學刊第三十六卷第一期第 59~66 頁(民國一百零四年) Journal of the Chinese Society of Mechanical Engineers, Vol.36, No.1, pp 59~66 (2015)

A Flexible Polyvinylidene Fluoride Film-Loudspeaker Yi-Ta Wang*, Yuh-Chung Hu** and Kuan-Ru Chen*** Keywords ： loudspeaker, vibration.

piezoelectric,

electromagnet acting on a permanent magnet. But it has the limitation of dimension for its inherent driving structure and hence still can’t overcome the requirement of volume reduction. Newell (2007) Electrostatic loudspeakers (ESL), with lower distortion while lack of bass response, are driven by an electrostatic field acting on a thin flat diaphragm. Though ESL can satisfy the volume-reduction requirement, it requires high voltage (several hundred volts) to generate sufficient field strength for large audio signal. Walker (1955) Piezoelectric material can overcome the high-voltage problem of electrostatic loudspeaker because it directly converts electrical energy into mechanical deformation. Piezoelectric loudspeakers, sometime used as tweeters in computer speakers and portable radios, are driven by a piece of piezoelectric material acting a mechanical diaphragm. The electrostatic construction is in effect a capacitor, and current is only needed to charge the capacitance created by the diaphragm and the stator plates. The structure of piezoelectric loudspeaker is thinner and simpler than that of moving-coil loudspeaker, and therefore is better to meet the requirement of miniaturization. If the frequency response of piezoelectric loudspeaker can be improved, it could bring advantages of miniaturization, flexibility, high potential and instant response into the main trend of loudspeaker industry. Ohga (1983) In compare with moving coil loudspeaker, piezoelectric loudspeaker has some benefits below: 1. Without moving coil, it has better transient response and lower damping loss; 2. Without large permanent magnet, it does not impacted by the outside magnetic field and is more compact; 3. It has high impedance, 1 kΩ to 20 Ω as the frequency is 1 kHz to 40 kHz respectively; 4. Its harmonic distortion is very low, about 1%, which means that it is good in replay; 5. The great transient response, which reduces ringing to the minima, makes it a more clearly frequency response; 6. With the characteristic of high-power output, it can develop large transform when input a weak current, namely sensitive to signal. The word piezoelectricity means electricity resulting from pressure. Piezoelectricity was

PVDF,

ABSTRACT Polyvinylidene Fluoride (PVDF), a piezoelectric polymer, is suitable for flexible devices because of its flexibility and lightweight. In this article, the authors adopt commercial PVDF film to make a flexible PVDF film-loudspeaker. The proposed flexible PVDF film-loudspeaker is a sandwiched membrane structure which contains a PVDF film sandwiched in between two silver electrode-layers. An audio signal is applied to the PVDF film, which responds by the mechanical deflection in proportion to the voltage applied across the PVDF film, thus converting electrical energy into mechanical vibration. Therefore, the coupled electromechanical characteristic of the composite membrane dominates the performance of the loudspeaker. This work is to find the resonant frequencies and vibration mode shapes of the proposed flexible PVDF film-loudspeaker as well as its frequency response.

INTRODUCTION Nowadays, loudspeakers play more and more important roles in the entertainment products of our life. Peoples pursue for ideal loudspeakers of smaller, thinner, and low power consumption. There are kinds of loudspeakers, such as coil loudspeakers, electrostatic loudspeakers, piezoelectric loudspeakers, etc. Borwick (1994) Coil loudspeakers, the most common in hi-fi stereo because of their high-quality reproduction of sound, are driven by a voice coil Paper Received December, 2013. Revised Jan, 2014, Accepted February, 2014, Author for Correspondence: Yi-Ta Wang. *

Assistant professor, Department of Mechanical and Electro-Mechanical Engineering, National ILan University, ILan, 26041 Taiwan.

** Professor, Department of Mechanical and Electro-Mechanical Engineering, National ILan University, ILan, 26041 Taiwan *** Graduate student, Department of Mechanical and Electro-Mechanical Engineering, National ILan University, ILan, 26041 Taiwan

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g, it has a very high mechanical toughness and strength that can withstand very high pressures but not easy to damage. In a word, PVDF is softer and has respectable piezoelectric coefficient, hence this work chooses PVDF as the actuator of loudspeaker.

discovered in 1880 by French physicists Jacques and Pierre Curie. This ceramic is administered to various transducers for this great property of electro-mechanical transformation. Piezoelectric ceramic has fruitful piezoelectric property. The most common material is Lead Zirconate Titanate (PZT). But it is too hard to get a better response in lower frequency. In order to get a better frequency response in lower frequency, advantages of piezoelectric polymer film loudspeaker are invented. PZT possesses better high-frequency response but narrower frequency domain. PVDF, a piezoelectric polymer, is suitable for flexible devices because of its flexibility and lightweight. Fukada (2000) Kim and Han (1999) proposed a parylene-diaphragm piezoelectric loudspeaker and found that polymer diaphragm had a better sound pressure. However, that parylene-diaphragm has to be actuated by ZnO and the response in low frequency required significantly improvement. Lee (2003) used PVDF to produce a flexible piezoelectric speaker which has the sound pressure of 80 dB as the frequency response upper to 400 Hz. Sugimoto et al. (2009) presented a flexible and transparent loudspeaker driven by PVDF. Piezoelectric materials are classified into the following categories: 1. Single crystal: quartz, tourmaline, Rochelle salt (potassium sodium tartrate), tantalite, niobate …etc.; 2. Thin film: ZnO, lead lanthanum zirconate titanate (PLZT), lead zirconate titanate (PZT)…etc.; 3. Polymer: polyvinylidene fluoride (PVDF), nylon, VF2/VF3…etc.; 4. Ceramic: PZT, BaTiO3 …etc.; 5. Composite: PZT composite with Si, SiO2, rubber…etc.; Commercial piezoelectric materials are ceramics material such as BaTiO3 and PZT. In a nutshell, ceramics piezoelectric materials are good at small in size, faster in response, low power consumption. But there are some limits in application, it is hard that can stand huge normal stress but could be distracted by uneven stress. Table 1 shows a comparison of different piezoelectric materials in descending order of stiffness. Heywang (2008) Quartz is the most rigid piezoelectric material displays only small values for d33 and g33. It has many applications because of its excellent mechanical properties. PZT shows larger piezoelectric coefficient d33 and comparative small voltage coefficients g that it is always be made of ultrasound transducers and ceramic capacitors. Nowadays, PZT piezoelectric speaker is the most popular piezoelectric loudspeaker because of that it’s hard in structure that produces large sound pressure output but not be good at low frequency. Ho (2008) Moreover, it can’t be made of a flexible loudspeaker. Polymer PVDF is very flexible and deformable which is much softer than the ceramic materials. Because of the moderate coefficients values of d and

Table 1. Comparison of common piezoelectric materials. Material E (GPa) d33 [pC/N] g33 [Vm/N] Quartz 72 2 (d11) 0.05 (g11) PZT 50 171 0.01 PVDF 2 20 0.2 Kawai (1969) found that PVDF has strong piezoelectric property and great ferroelectric property, which had brought an opportunity for expanding the application of polymeric material towards active utilization of its properties from hitherto use as a passive insulation material. Phones and microphones which made by PVDF are small distortion, good sound quality, and stable. PVDF is a semi-crystalline polymer made from many CH2-CF2 monomers which have solid and homogenous structure. There are four types of PVDF structure, namely α, β, γ, and δ types, which are defined by their crystallization temperature and time. Huang (2010) and Hsu (2012) PVDF gets piezoelectricity when pressing voltage in one direction that product transforms molecular chain CF2 parallel to each other to produce dipo1e. The piezoelectric diaphragm adopted in this work is implemented from commercial 110-µm thick PVDF film covered by 6-µm thick metal layer on both sides that was manufactured by Measurement Specialties Inc. The most common literatures are to develop the diaphragm materials to avoid the higher-frequency modes because the higher-frequency modes are regarded as the noise which cause the coloration of sound transmission. The conventional opinions always aim at suppressing the higher-frequency modes of vibration because the only vibrating object is the diaphragm itself and therefore the material of the diaphragm became the pivotal design parameter. The range of vibration of the diaphragm is related in its size and supply voltage. This work uses a commercial PVDF piezoelectric film to find the different mode shapes. This work designs a flexible PVDF film-loudspeaker whose core structure is a commercial polymer piezoelectric material PVDF film that is manufactured by Measurement Inc. The proposed flexible PVDF film-loudspeaker is 3cm long, 3cm wide, and 122um thick. The PVDF film is covered by 6-μm silver electrode layers on both sides. The thicknesses of the electrode layers are very thin compared to the PVDF film and hence their influence on the vibration of the film can be ignored. A sine wave signal with 5 V peak to peak is applied on the electrode layers. Since the vibrating behaviors are

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Y.T. Wang et al.: A Flexible Polyvinylidene Fluoride Film-Loudspeaker.

PVDF, 110 μm 3 cm 3 cm

Ag, 6 μm Fig. 1

Signal wires

The architecture of the proposed flexible PVDF film-loudspeaker. whole measuring system. The function generator can generate many kinds of signals with ±10 V from 2 Hz to 6 MHz, like sine, square, and triangle waves…etc. In this work, a sine-wave signal of 5 V, peak to peak, is applied on the PVDF film loudspeaker. The Laser Doppler Vibrometer senses the frequency shift of the back scattered light from a moving surface. The vibration of the PVDF film loudspeaker is measured by a Laser Doppler Vibrometer. The Laser Doppler Vibrometer can detect the vibration of the PVDF film loudspeaker via measuring the frequency shift between the incident ray on the loudspeaker and the reflected ray by the loudspeaker. It works based on the optical interference which requires two coherent light rays, namely the incident and reflected rays. Fig. 4 illustrates the structure of the LDV. Changing the optical path length per unit of time manifests itself as the Doppler frequency shift of the measurement ray. The resulting intensity is not just

highly related to sound pressure level, then this work is to determine the vibration behaviors of the PVDF film by using Laser Doppler Vibrometer via finding the mode shape and frequency response of this flexible PVDF film-loudspeaker. The results could become an important reference before testing the sound pressure of the loudspeaker in the future.

ARCHITECTURE OF THE FLEXIBLE PVDF-LOUDSPEAKER The proposed flexible PVDF film-loudspeaker, with the dimension of 3 cm by 3cm, is a sandwiched membrane structure which contains a PVDF film of 110-μm thick sandwiched in between two silver electrode-layers, each of 6-μm thick. The total thickness is 122 μm. Fig. 1 shows the architecture of the proposed flexible PVDF film-loudspeaker. An audio signal is applied to the PVDF film and by which a mechanical vibration responded in proportion to the applied voltage, thus convert electrical energy into mechanical energy. Therefore, the coupled electromechanical characteristic of the compound membrane dominates the performance of the loudspeaker.

Film loudspeaker

EXPERIMENT SETUP To measure the dynamic response of the PVDF film-loudspeaker to driving voltage, the authors make a test frame by acrylic plates. The PVDF film-loudspeaker is clamped in the test frame by four screws, as shown in Fig. 2. This work is to find the resonant frequencies and vibration mode shapes of the proposed flexible PVDF film-loudspeaker as well as its frequency response. The authors choose the non-contact laser Doppler vibrometer to measure the vibration of the flexible PVDF film-loudspeaker since it is flexible and lightweight. Fig. 3 shows the

1

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5

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3 cm Fig. 2 The proposed flexible PVDF loudspeaker clamped by the test frame. -61-

film

J. CSME Vol.36, No.1 (2015)

Film loudspeaker

1 2 3 5 4 6 7 8 9

Function generator

Laser Doppler vibrometer

Fig. 3

The architecture of the proposed flexible PVDF film-loudspeaker.

Fig. 4

Schematics of Laser Doppler Vibrometer.

the sum of the two rays but is according to the interference term,

investigation (rreflected is function of time) generates a dark and bright (fringe) pattern typical of interferometry on the detector. One complete dark bright cycle on the detector corresponded to an object displacement of exactly half of the wavelength of the light used. Changing the optical path length per unit of time manifests itself as the Doppler frequency shift of the measurement ray. This means that the modulation frequency of the interferometer pattern determined is directly proportional to the velocity of the object.

I total  I incident  I reflected



 2 I incidentI reflected cos 2 rreflected  rincident / 



where the interference term relates to the path length difference between the two rays. As the path length of the reference ray is constant over time (rincident is constant), a movement of the object under

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Doppler-effect; sensing the frequency shift of back scattered light from a moving surface We choice nine equi-spaced points (Fig. 2) in the PVDF film and used Laser Doppler Vibrometer to get the displacment of each point and plot the mode shape figure. Fig. 5 shows the frequency responses of the nine measured points. The points located near the four screws, namely the points 1, 3, 7 and 9, show smaller displacements than the other points. This is due to that the film is clamped by the four screws. Point 5 has the largest displacement because which located at the center of the proposed PVDF film-loudspeaker. By scanning the frequencies, one can find three resonant frequencies, those are 40, 290, and 530 Hz respectively. The higher resonant frequencies do not appear because the driving voltage (5 V peak to peak) of the function generator is too low to activate the higher vibration modes. Fig. 6 shows the frequency response average of the PVDF film loud speaker and that the first three resonant frequencies are 40, 290, and 530 Hz respectively. Fig. 7(a)-(c) show the measured vibration shapes of the first three modes.

The function generator, FG-506 by MOTECH Inc., supply sine-wave AC voltage of 5 V peak-to-peak with different frequencies to the proposed flexible PVDF film-loudspeaker. An oscilloscope is used to monitor the wave form of the signal outputted by the function generator. The Laser Doppler vibrometer is adopted to measure the vibration of the proposed PVDF film-loudspeaker. Nine equal-spaced points on the membrane of the proposed loudspeaker are measured. The measured data are recorded by the laptop (NB). After detect the vibration data, Laser Doppler Vibrometer transforms analog data to digital data to the Vibsoftare in NB. In order to make sure the vibration comes from loudspeaker but the other. LDV was putted on air cushion and the device is placed on tripod which on the ground.

RESULTS AND DISCUSSION The resonant frequencies and vibration mode shapes of the proposed flexible PVDF film-loudspeaker as well as its frequency response are measured. The experiments is based on the

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Frequency response Frequency Response Point 9of pt. 9 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0

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The frequency response of each measured point of the proposed flexible PVDF film-loudspeaker.

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Frequency response pt. 7 Frequency Response Point of 7 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 00 500 1000 1500 2000 2500 3000 0 0.5 1.0 1.5 2.0 2.5 3.0 Frequency[Hz] Frequency (kHz)

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Fig. 7(a)

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Frequency[Hz] The frequency response average of the PVDF film loudspeaker.

The measured vibration shapes of the membrane with respect to the resonant frequencies of 40 Hz.

conceptualization for designing a simple loudspeaker construction that utilizes a sheet of commercial piezoelectric polymer. Because the vibration behaviors are highly related to the sound pressure level, this work determines the vibration of the loudspeaker diaphragm by the use of laser Doppler vibrometer. The vibration mode shape and resonant frequency is found. To measure the dynamic response

CONCLUSION Here we design a flexible PVDF film-loudspeaker. The proposed flexible PVDF film-loudspeaker is a sandwiched membrane structure which contains a PVDF film sandwiched in between two silver electrode-layers. This article reports on a

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of the PVDF film loudspeaker to driving voltage, the authors make a test frame by acrylic plates. The PVDF film loudspeaker is clamped in the test frame by four screws. This testing ignored the impedance of

loudspeaker. A sine-wave AC voltage of 5 V, peak to peak, is applied on the PVDF film loudspeaker by a function generator.

Fig. 7(b)


Fig. 7(c)


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flexible and transparent loudspeaker,” Applied Acoustic, Vol.70, pp. 1021-1028(2009). Walker, P.J., Wide Range Electrostatic Loudspeakers, Wireless World magazine, pp. 208-384(1955).

ACKNOWLEDGMENT This work is financially supported by the National Science Council of Taiwan under the grant no. NSC 100-2628-E-197-001-MY3.

REFERENCES Borwick, J., Loudspeaker and Headphone Handbook, 2nd edition, Read Educational and Professional Publishing Ltd., Great Britain, pp. 28-105(1994). Fukada, E., “History and Recent Progress in Piezoelectric Polymers,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 47, No. 6, pp.1277-1290(2000). Heywang, W., Lubitz, K., and Wersing, W., Piezoelectricity-Evolution and Future of a Technology, Springer Series in Materials Science, pp.158-177(2008). Ho, J.H., Master D. Thesis, Optimizing Acoustic Frequency Response of Piezoelectric Loudspeakers Based on Structural Design, Graduate Department of Engineering Science and Ocean Engineering, National Taiwan University, Taipei, Taiwan (2008). Hsu, S.W., Master D. Thesis, Investigation of Piezoelectric Flexible Thin Speakers, Master's Program of Electro-Acoustics, Feng Chia University (2012). Huang, S.H., Master D. Thesis, Flexible and Directional Loudspeaker Array by Using Piezoelectric Transducers, Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Tainan, Taiwan (2010). Kawai, H, “The piezoelectricity of poly (vinylidene fluoride),” Japan. J. Apply Physic, No.8, pp.975-976(1969). Kim, E.S., and Han, C.H., “Parylene-diaphragm piezoelectric acoustic transducers,” IEEE Micro Electro Mechanical System, pp.148-152(1999). Lee, C.K., “Flexible and transparent organic film speaker by using highly conducting PEDOT PSS as electrode,” Synthetic Metals, Vol. 139, pp.457-461(2003). Measurement .Inc., Piezoelectric Polymer Speakers Application Note 1242138. Newell, J.P., Newell, Philip Richard, and Holland, Keith R., Loudspeakers: For music recording and reproduction, Elsevier/Focal Press (2007). Ohga, J., “A Flat Piezoelectric Polymer Film Loudspeaker as a Multi-resonance System,” The Acoustical Society of Japan (E), Vol. 4, No. 3, pp. 113-120(1983). Sugimoto, T., Ono, K., Ando, A., Kurozumi, K., Hara, A., Morita, Y., Miura, A., “PVDF-driven

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可撓式聚氟化二乙烯薄膜喇叭王宜達

胡毓忠

陳冠如

國立宜蘭大學機械與機電工程學系

摘要聚氟化二乙烯是一種質輕且易彎曲的壓電高分子材料，適合應用於製作可撓性元件。本文以商用的聚氟化二乙烯薄膜製作可彎曲的薄膜喇叭；該薄膜喇叭的構造由上下二層銀電極層中間包夾一層聚氟化二乙烯薄膜所組成。該薄膜喇叭的操作原理乃是將音訊號轉成驅動電壓，透過銀電極層來驅動聚氟化二乙烯薄膜產生振動進而發出聲音。該聚氟化二乙烯薄膜的振動頻率與振幅則正比於驅動電壓，簡言之，該薄膜喇叭的發聲原理便是透過壓電薄膜的機電轉換機制將電能轉換為機械能。因此，該薄膜喇叭的效能取決於其結構的機電耦合特性。本文旨在探討該可彎曲的聚氟化二乙烯薄膜喇叭的共振頻率與振動模態以及其頻域響應。