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Designing Two-Dimensional Film Speakers Using Piezoelectric Materials Keehong Um Department of Information Technology, Hansei University Gunpo-city, Kyunggi-do, Korea [email protected] Dong-Soo Lee R & D Center of Industrial Materials FILS CO., Ltd. Sungnam-city, Kyunggi-do, Korea [email protected] Abstract—A speaker is an acoustic device which produces sound through electromechanical operations. Most speakers are comprised of three-dimensional devices such as a voice coil, a permanent magnet, and an enclosure. An ac voltage applied to the voice coil generates displacement of the cone due to the magnetic interaction between the magnet and the voice coil. The displacement produces audio sound waves. Recently, many mobile electronic devices such as mobile phones have become smaller, lighter, and thinner for the sake of convenience and due to advanced technologies. A problem with this miniaturization, however, is that the volume of the produced output sounds has also decreased. In contrast to conventional three-dimensional speakers, we have designed a new type of two-dimensional flexible speaker based on the use of reverse piezoelectric effect on certain piezoelectric materials. This would be very useful in today’s increasingly thinner consumer electronics devices such as mobile phones. There are other applications such as clothes, wearable computers, raincoats, etc. Keywords-component; piezo material; film speaker; surface impedance; SPL I. INTRODUCTION A speaker is a system which produces acoustic waves through electromechanical operations. It transforms electronic signals into audible sound signals. At the end of the 19 th century, Preece and Braun et al. reported that when an ac current passed through a very thin metal foil, the thermophone produced sound. But the thermophone emitted extremely weak sound [1]. A thermophone is an electroacoustic transducer in which sound waves are produced by the expansion and contraction of the air adjacent to a strip of conducting material. Arnold and Crandall proposed the correct physical diagram for the thermophones, saying, “When an ac current is passed through a thin conductor, periodic heating takes place in the conductor following the variation in the current strength. This periodic heating sets up temperature waves which are propagated into the surrounding medium; the amplitude of the temperature falls off very rapidly as the distance from the conductor increases. On account of the rapid attenuation of these temperature waves, their net effect is to produce a periodic rise in temperature in a limited portion of the medium near the conductor, and thermal expansion and contraction of this layer of the medium determines the amplitude of the resulting sound waves [2].” Piezoelectricity is the ability of some materials to generate an electric field or electric potential in response to applied mechanical stress. The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state in crystalline materials with no inversion symmetry [3]. Piezoelectricity is useful in applications such as the production and detection of sound, generation of high voltages, and electronic frequency generation. The piezoelectric effect is a reversible process in that materials exhibiting the direct piezoelectric effect (the internal generation of electrical charge resulting from an applied mechanical force) also exhibit the reverse piezoelectric effect (the internal generation of a mechanical force resulting from an applied electrical field). We have designed a new type of two-dimensional flexible speaker by utilizing the reverse piezoelectric effect on certain piezoelectric materials. The speaker we have proposed can produce sounds as loud as a conventional voice-coil three-dimensional speaker. II. CONFIGURATION Lin Xiao et al. used CNT thin film to design speakers. They put CNT thin film directly on the two electrodes to form a simple loudspeaker [4]. Suzuki et al. reported a simple and easy growth method for an ultra-long, vertically aligned multi- walled CNT (MWNT) array. They studied the thermoacoustic properties of CNT speakers made from MWNT webs. They described the MWNT web speaker as the CNT speaker. They used two palladium electrodes at the end of the CNT film [5]. We have designed two-dimensional film speakers by adding two parallel electrodes on both sides of the coating material. The main parts are piezoelectric films, coating materials, and electrodes. Therefore, the two-dimensional film speaker we propose is comprised of the following three parts in five pieces: (1) a piece of piezoelectric material (called piezo film), (2) two pieces of coating material , and (3) and two electrodes. The three-dimensional side view of newly designed film speakers is shown disassembled in Fig. 1. It is clear that the structure is comprised of five pieces [6]. 2011 IEEE International Conference on Consumer Electronics - Berlin (ICCE-Berlin) 978-1-4577-0234-1/11/$26.00 ©2011 IEEE 120

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Page 1: [IEEE 2011 IEEE First International Conference on Consumer Electronics - Berlin (ICCE-Berlin) - Berlin, Germany (2011.09.6-2011.09.8)] 2011 IEEE International Conference on Consumer

Designing Two-Dimensional Film Speakers Using Piezoelectric Materials

Keehong Um

Department of Information Technology, Hansei University

Gunpo-city, Kyunggi-do, Korea [email protected]

Dong-Soo Lee R & D Center of Industrial Materials

FILS CO., Ltd. Sungnam-city, Kyunggi-do, Korea

[email protected]

Abstract—A speaker is an acoustic device which produces sound through electromechanical operations. Most speakers are comprised of three-dimensional devices such as a voice coil, a permanent magnet, and an enclosure. An ac voltage applied to the voice coil generates displacement of the cone due to the magnetic interaction between the magnet and the voice coil. The displacement produces audio sound waves. Recently, many mobile electronic devices such as mobile phones have become smaller, lighter, and thinner for the sake of convenience and due to advanced technologies. A problem with this miniaturization, however, is that the volume of the produced output sounds has also decreased. In contrast to conventional three-dimensional speakers, we have designed a new type of two-dimensional flexible speaker based on the use of reverse piezoelectric effect on certain piezoelectric materials. This would be very useful in today’s increasingly thinner consumer electronics devices such as mobile phones. There are other applications such as clothes, wearable computers, raincoats, etc.

Keywords-component; piezo material; film speaker; surface impedance; SPL

I. INTRODUCTION A speaker is a system which produces acoustic waves

through electromechanical operations. It transforms electronic signals into audible sound signals. At the end of the 19th century, Preece and Braun et al. reported that when an ac current passed through a very thin metal foil, the thermophone produced sound. But the thermophone emitted extremely weak sound [1]. A thermophone is an electroacoustic transducer in which sound waves are produced by the expansion and contraction of the air adjacent to a strip of conducting material. Arnold and Crandall proposed the correct physical diagram for the thermophones, saying, “When an ac current is passed through a thin conductor, periodic heating takes place in the conductor following the variation in the current strength. This periodic heating sets up temperature waves which are propagated into the surrounding medium; the amplitude of the temperature falls off very rapidly as the distance from the conductor increases. On account of the rapid attenuation of these temperature waves, their net effect is to produce a periodic rise in temperature in a limited portion of the medium near the conductor, and thermal expansion and contraction of this layer of the medium determines the amplitude of the

resulting sound waves [2].” Piezoelectricity is the ability of some materials to generate an electric field or electric potential in response to applied mechanical stress. The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state in crystalline materials with no inversion symmetry [3]. Piezoelectricity is useful in applications such as the production and detection of sound, generation of high voltages, and electronic frequency generation. The piezoelectric effect is a reversible process in that materials exhibiting the direct piezoelectric effect (the internal generation of electrical charge resulting from an applied mechanical force) also exhibit the reverse piezoelectric effect (the internal generation of a mechanical force resulting from an applied electrical field). We have designed a new type of two-dimensional flexible speaker by utilizing the reverse piezoelectric effect on certain piezoelectric materials. The speaker we have proposed can produce sounds as loud as a conventional voice-coil three-dimensional speaker.

II. CONFIGURATION

Lin Xiao et al. used CNT thin film to design speakers. They put CNT thin film directly on the two electrodes to form a simple loudspeaker [4]. Suzuki et al. reported a simple and easy growth method for an ultra-long, vertically aligned multi-walled CNT (MWNT) array. They studied the thermoacoustic properties of CNT speakers made from MWNT webs. They described the MWNT web speaker as the CNT speaker. They used two palladium electrodes at the end of the CNT film [5]. We have designed two-dimensional film speakers by adding two parallel electrodes on both sides of the coating material. The main parts are piezoelectric films, coating materials, and electrodes. Therefore, the two-dimensional film speaker we propose is comprised of the following three parts in five pieces: (1) a piece of piezoelectric material (called piezo film), (2) two pieces of coating material , and (3) and two electrodes. The three-dimensional side view of newly designed film speakers is shown disassembled in Fig. 1. It is clear that the structure is comprised of five pieces [6].

2011 IEEE International Conference on Consumer Electronics - Berlin (ICCE-Berlin)

978-1-4577-0234-1/11/$26.00 ©2011 IEEE 120

Page 2: [IEEE 2011 IEEE First International Conference on Consumer Electronics - Berlin (ICCE-Berlin) - Berlin, Germany (2011.09.6-2011.09.8)] 2011 IEEE International Conference on Consumer

Figure 1. Five pieces disassembled.

From Fig.1, we can have a side view of five pieces assembled as shown in Fig. 2. In Fig. 3, the bird’s eye view of five pieces assembled on a plane is shown.

Compared to conventional three-dimensional speakers, this structure is flexible and it can be tailored into many different curved shapes, so it can be changed or crumpled in many shapes and can be put onto a variety of curved surfaces. It could be used in wearable computers, umbrellas, raincoats, hats, mobile phones, and other applications. The crumpled version of Fig.3 is shown in Fig. 4. It still emits sound audible sound in all directions. It could be useful in today’s increasingly thinner consumer electronic devices (such as mobile phones) and have other applications (such as clothes, wearable computers, raincoats, etc.).

III. COMPONENTS OF FILM SPEAKERS The component located in the center is a piece of piezo

film, and two pieces of coating material are put on both sides of it. The previous prototype speakers we designed were mainly composed of piezoelectric material (called piezo film), and coating material. The piezo film is made from polyvinylidene fluoride (PVDF), which is a highly nonreactive and pure thermoplastic fluoropolymer [7, 8]. PVDF is a specialized plastic material in the fluoropolymer family.

A. Piezoelectric Materials The piezoelectric effect in solids describes the relation

between a mechanical stress and an applied electrical voltage. The effect was discovered in 1880 by the Jacques and Pierre Curie brothers. They found that when a mechanical stress was

Figure 2. Side view of the film speaker

Figure 3. The bird’s eye view of five pieces assembled on a plane.

applied on piezoelectric materials (such as tourmaline, quartz, topaz, tourmaline, and Rochelle salt), electrical charges appeared, and the voltage was proportional to the stress applied. The piezoelectric effect occurs only in nonconductive materials. Piezoelectric materials can be divided in two main groups i.e., crystals and ceramics. The most well-known piezoelectric material is quartz (SiO2). Piezoelectric materials exhibit the reverse piezoelectric effect. That means when a voltage is applied across the surface of a piezoelectric crystal, it responds by generating a mechanical force. A surrounding mechanical diaphragm converts the electrical energy into acoustic energy. Recently, piezoelectric PVDF has been studied in order to make acoustic transducers, either being used as a sensor, or an actuator [9, 10 ].

B. Coating material The two pieces of coating material used on both sides of

piezo film were the conductive macromolecular compound (CMC), indium tin oxide (ITO), or carbon nanotubes (CNTs). We will consider some of the properties of each of them [6].

1) Conductive macromolecular compound (CMC) One of the coating materials available is conductive

macromolecular compound. The frequency characteristics of the film speaker with the coating material CMC is shown to have several disadvantages as follows;

Figure 4. Film speaker crumpled in any shape.

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Page 3: [IEEE 2011 IEEE First International Conference on Consumer Electronics - Berlin (ICCE-Berlin) - Berlin, Germany (2011.09.6-2011.09.8)] 2011 IEEE International Conference on Consumer

a) The conductive characteristics of the film speaker shows some limitations in the audio frequency ranges (20-20 kilohertz).

b) The performance is not good enough in some audio

frequency ranges (20-20 kilohertz). c) It is difficult to achieve a uniform thickness when it is

coated on the both surfaces of piezo film.

d) The sound pressure level (SPL) of a film speaker is not high enough to generate a high quality of sound.

e) The frequency response shows that the

characteristics of the SPL are not useful in the frequency range below 400 hertz.

2) Indium tin oxide (ITO) The second coating material often used is indium tin oxide

(ITO, or tin-doped indium oxide). It is a solid solution composed of indium oxide (In2O3) and tin oxide (SnO2). The typical constitutive ratio is 90% In2O3 and 10% SnO2 by weight. It is transparent and colorless in thin layers while in bulk form it is yellow to grey. In the infrared region of the spectrum it is a metal-like mirror [11 ]. It has high electrical conductivity and high optical transparency. However, the coating material ITOs are brittle and need a high processing temperature [12, 13]. Indium tin oxide is one of the most widely used transparent conducting oxides since it has good properties including electrical conductivity and relative ease with which it can be deposited as a thin film. But there are several main disadvantages with ITO. Because of high cost and limited supply of indium, the fragility and lack of flexibility of ITO layers, and the costly layer deposition requiring a vacuum, alternatives are being sought. It was found to be fragile due to mechanical pressure caused by small vibrations of the film. Continuous mechanical vibrations due to the operating signal of input voltage have been found to crack the ITO. There have been several trials to use other materials to replace ITO. The alternative coating materials are carbon nanotube (CNT) conductive coatings as a prospective replacement [14].

3) Carbon nanotube (CNT) as a new coating material for film speakers

We have considered some of the limitations of CMT and ITO, and we propose to replace them with carbon nanotubes (CNTs). We have introduced CNT as a new coating material for developing a two-dimensional film speaker system. Since their discovery in 1991, CNT have been studied as a key material for many engineering applications. They are used because of their strength and electrical properties [5, 14].

.

Figure 5. Piezo film in roll form.

Lin at al. found that very thin carbon nanotube films, fed by ac current, could produce sound waves. Their thin film speakers are flexible, stretchable, and transparent[4]. We have substituted CNT for CMC and ITO to improve the operating performance of the system. The speaker system we have designed shows outstanding advantages, such as light weight, flexibility, wide ranges of operating frequencies, and low impedance values, and stability.

IV. SYSTEM DESIGNED AND ITS PERFORMANCE

Fig. 5 shows the piezo film to be used as part of a 2-dimensional film speaker. On both sides of the film, coating materials are laminated. It can be tailored to almost any shapes as a building block. Fig. 6 shows two examples of products manufactured from film speakers. We have shown these products as commercially available ones. For an aesthetic point of view, we have designed our film speakers in the form of (a) two standing sign boards and (b) two triangular sails of a sailing ship. These could be put on a table for usage.

(a) (b)

Figure 6. Manufactured products. (a) Two standing sign boards. (b) Two triangular sails on a sailing ship.

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Page 4: [IEEE 2011 IEEE First International Conference on Consumer Electronics - Berlin (ICCE-Berlin) - Berlin, Germany (2011.09.6-2011.09.8)] 2011 IEEE International Conference on Consumer

Sound pressure level (SPL) is the local pressure deviation from the average atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in the air and a hydrophone in water. The SI unit for sound pressure p is the pascal (Pa). SPL is a logarithmic measure of the effective sound pressure of a sound relative to a reference value. It is measured in decibels (dB) above a standard reference level. The commonly used "zero" reference sound pressure in air is 20 micropascal rms, which is usually considered the threshold of human hearing (at 1 kilohertz). SPL is defined by,

)1(.dBlog20log10 10

2

10 ���

����

�⋅=�

��

����

�⋅=

ref

rms

ref

rms

pp

ppSPL

In (1), pref is the reference sound pressure and prms is the rms sound pressure being measured [15]. Shown in Fig. 7, is the SPL values in decibel to show the frequency characteristics of the film speaker using CNT and CMC in the frequency range of 1000-1800 hertz. It is clear that the SPL (in decibel) of CNTs is better than that of CMC [6]. SPL of CNT becomes bigger as the value of surface impedance becomes smaller. This means that by choosing proper values of surface impedance, the output of a film speaker can be obtained. For example, SPL 72 decibel is usually considered to be a good quality of sound. In this case, the proper surface impedance of 50 ohms per square would be a good choice of CNT.

The proposed carbon nanotube-based two-dimensional film speaker showed excellent acoustic responses over a wider frequency. Compared to the traditional three-dimensional speakers, the designed film speaker has the advantages of being extremely thin, flexible and lightweight. It is very stable and shows low surface impedances. We have shown that the proper surface impedance values of CNTs were 50-1000 ohms per square. The proposed thin film speaker would make a significant impact on computer screens, portable audio systems, and other industrial applications.

Figure 7. SPL (in dB) frequency characteristics of a film speaker based on CNT and CMC for coatings in the frequency range of 1000-18000 hertz.

ACKNOWLEDGMENT The authors wish to express their thanks to In Suk Park at

FILS Co., Ltd. for his assistance in the experiments and important suggestions for the preparation of this work. Without his help, this work would have been impossible to complete.

This work was supported by Hansei University.

REFERENCES [1] W. H. Preece, “Royal. Soc. Proc.,” London, vol. 30, pp. 408-411, 1879-

1880. [2] H. D. Arnold and I. B. Crandall, “Phys. Rev.”, 1917, vol. 10, pp. 22-38. [3] G. Gautschi, “Piezoelectric sensorics: force, strain, pressure,

acceleration and acoustic emission sensors, materials and amplifiers”, Springer, 2002.

[4] X. Lin et al., “Flexible, stretchable, transparent carbon nanotube thin film speakers,” Nano Letters, 2008, vol. 8, no.12, pp. 4539-4545.

[5] K. Suzuki et al., “Study of carbon-nanotube web thermoacoustic loud speakers,” Japanese J. of App. Phy. Vol. 50 , Jan. 2011, 01BJ10.

[6] J. J. Kang and K. Um, “Carbon nanotube as a new coating material for developing two dimensional speaker systems,” ACN 2011, CCIS 199, in press.

[7] MSIUSA.com, “Piezo film sensors”, Technical manual, Measurement Specialties, Inc., 1999.

[8] G. Gautschi, “Piezoelectric sensors”, Springer 2002, pp.1-45. [9] R.L. Clark, R.A. Burdisso, C.R. Fuller, “Design approaches for shaping

polyvinylidene fluoride sensors in active structural acoustic control(ASAC), J. Intell. Master. Syst. Struct. Vol. 4, 1993, pp. 354-365.

[10] C. Guigou, C.R. Fuller, “Adaptive feedforward and feedback methods for active/passive sound radiation control using smart foam,” J. Acoust. Soc. Am 104, 1998, pp. 226-231.

[11] http://en.wikipedia.org/wiki/Indium_tin_oxide. [12] D.R. Caims et al., “Strain-dependent electrical resistance of tin-doped

indium oxide on polymer substrate,” Appl. Phys. Lett.76, 2000, pp. 1425-1427.

[13] X. Yu, R. Rajamni, K. Stelson, T. Cui, “Carbon nanotube-based transparent thin film acoustic actuators and sensors. sensors and actuator ,” A 132, 2006, pp.626-631.

[14] http://www.rdmag.com/News/2011/04/Materials-Researchers-find-replacement-for-rare-material-indium-tin-oxid/. Retrieved, April 2011.

[15] D. A. Bies and C.H. Hansen, Engineering Noise Control. Spon Press, 3rd ed., 2003.

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