High-Sensitivity Piezoelectric Microphones Based on Stacked Cellular Polymer ﬁlms (L) Joachim Hillenbranda) and Gerhard M

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High-sensitivity piezoelectric microphones based on stacked cellular polymer films (L) Joachim Hillenbranda) and Gerhard M. Sesslerb) Department of Telecommunications, Darmstadt University of Technology, Merckstrasse 25, 64283 Darmstadt, Germany ͑Received 30 July 2004; revised 1 September 2004; accepted 7 September 2004͒ Improvements of the sensitivity of piezoelectric microphones based on charged cellular polymer films are reported. The improvements are achieved by ͑1͒ an increase of the piezoelectric ͑ ͒ d33-coefficient of the cellular polypropylene films by pressure expansion and 2 stacking of the films. Microphones consisting of a single film of such material have sensitivities of about 2 mV/Pa at 1 kHz, independent of size, while for a microphone with five stacked films a sensitivity of 10.5 mV/Pa was measured. The equivalent noise level is about 37 dB͑A͒ for the single-film transducer and 26 dB͑A͒ for the stacked version. Advantages of these new piezoelectric transducers include their simple design, low cost, and small weight, as well as a large range of shapes and sizes possible. © 2004 Acoustical Society of America. ͓DOI: 10.1121/1.1810272͔ PACS numbers: 43.38.Fx, 43.38.Ar, 43.38.Kb ͓AJZ͔ Pages: 3267–3270

I. INTRODUCTION pere, Finland͒. The film is usually charged on its surface by Cellular polypropylene ͑PP͒, after appropriate electrical a corona discharge. Due to the ensuing electric field in the charging, is highly piezoelectric.1–3 In particular, the piezo- interior of the film, discharges occur in the voids and charging as shown in the lower part of the figure is achieved. A electric d33-coefficient of this material reaches values of about 150 pC/N in the audio frequency range and is thus charge distribution of this kind in a nonhomogeneous mate- 2 about five times as high as that of polyvinylidenefluoride rial causes the piezoelectric effect. ͑PVDF͒, the best conventional piezoelectric polymer. It was All piezoelectric films used in the present microphones therefore suggested to use charged cellular PP in electro- were made of a commercial cellular PP film ͑VHD40 by acoustic and electromechanical transducers. In particular, Treophan, Neunkirchen, Germany͒. Expansion of these films implementations of loudspeakers,4 microphones5,6 and is achieved by a pressure treatment consisting in the appli- hydrophones6 have been described in the literature. Although cation of an increased gas pressure for some time, followed of considerably simpler design than conventional transduc- by pressure reduction to atmosphere. This results in an in- ers, the previously implemented new devices did not yet crease of the thickness of the lenslike voids which originally reach the electro-acoustic performance of older capacitive extends up to about 5 ␮m and approximately up to about 10 and piezoelectric systems. ␮m after expansion. According to the Paschen law, the in- Recently, the piezoelectric activity of cellular PP has creased thickness after expansion lowers the electric field been significantly increased by thickness-expansion of the required for breakdown. Thus, for a given voltage, more polymer.7–11 Microphones with such improved films, show- voids experience stronger breakdown, causing an increase of ing sensitivities of about 2.2 mV/Pa at 1 kHz, have already the piezoelectric coefficient. Since metallization after the ex- been described by the present authors.12 The use of film pansion causes shrinking of the film, a second expansion stacks in such microphones, not yet implemented experimen- after metallization softens the material and thus results in 10 tally, is a method to further enhance the sensitivity of these another increase of d33 . transducers. This suggests to build advanced microphones The frequency response of the d33-coefficient of such and to examine their electro-acoustic properties. samples was measured, utilizing the inverse piezoelectric ef- In the present letter, the implementation and character- fect, by sinusoidal electrical excitation and interferometric ization of such microphones are described. In particular, the measurement of the resulting surface deflection. A typical cellular films are briefly specified in Sec. II, the microphone response of an expanded sample is shown in Fig. 2. In the design and measuring methods are outlined in Sec. III, audio frequency range, d33-coefficients of about 420 pC/N electro-acoustic measurements on the new systems are re- were found. These values are larger than those previously ported in Sec. IV, and the properties of these microphones reported for nonexpanded samples by a factor of 3. The slow are discussed in Sec. V. decrease of d33 up to about 30 kHz is due to an increase of Young’s modulus, while the resonance at approximately 140 II. CELLULAR PP kHz is determined by Young’s modulus and the mass of the The upper part of Fig. 1 shows a SEM photograph of the film.9 ͑ cross sectional area of a cellular film HS01 by VTT, Tam- The pressure dependence of d33 was determined quasis- tatically by measuring the generated charge upon pressure 9 a͒Electronic mail: [email protected] application. Typical results show that up to pressures of a b͒ Electronic mail: [email protected] few kPa d33 increases slightly. This indicates that the stress–

Downloaded 22 Sep 2011 to 222.66.175.219. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp FIG. 3. Frequency response of cellular PP microphones with single film and a stack of five films, determined by a comparison method in an acoustic coupler. Films of about 55 ␮m thickness ͑VHD40͒ were used. FIG. 1. SEM photograph of cross section of cellular PP film ͑HS01͒ of 70 ␮m thickness ͑top͒ and schematic view of charge distribution in this mate- sensitivity of a microphone with n layers should be n times rial ͑bottom͒. as high as that of a single-film transducer, while its capacitance and resonance frequency ͑see Sec. V͒ decrease by a strain relationship of the cellular films is not exactly linear in factor of n. this pressure range, as expected for cellular materials.13 Electroacoustic measurements were carried out by plac- ing the microphone in an acoustic coupler with a volume of III. MICROPHONE DESIGN AND MEASURING 0.4 cm3. A 1/8-in. condenser microphone ͑B&K 4138͒, also METHODS extending into the coupler, serves as a reference. The sound The expanded cellular VHD40 films were used to con- pressure in the coupler is generated with a small electro- struct piezoelectric microphones.6,12 These transducers con- dynamic speaker in a separate cavity coupled by a 4.5-cm- sist simply of a piece of the cellular material of 0.3 cm2 size long metallic pipe into the measuring coupler. that is metallized on both sides. For a film thickness of 55 Electronic data recording and processing is carried out ␮m, the capacitance of the microphone is 8 pF. Shielding with the above-mentioned audio analyzer, which allows the requires the mounting of the film in a small housing. The evaluation of the frequency response of the sensitivity, its microphone output is fed directly into a preamplifier of unity amplitude dependence, the total harmonic distortion, and the gain ͑B&K 2669͒14 and its output into an audio analyzer noise spectrum of the microphone and its amplifier. ͑R&S UPD͒. In the stacked microphones, the single film is substituted IV. EXPERIMENTAL RESULTS by a stack of films metallized on both sides which are glued The measured frequency responses of microphones with on top of each other. Since the sound pressure acts on all one and five films of cellular PP films are shown in Fig. 3. As films and since the films are electrically connected in series, seen from the figure, the open-circuit sensitivity of the five- the output voltages of all layers add up and the open circuit film microphone is about 10.5 mV/Pa at 1 kHz and thus, as expected ͑see above͒, almost five times larger than the 2.2 mV/Pa of the single-film microphone at this frequency. Both responses decrease by about 1 dB from 20 Hz to 1 kHz, as expected from the frequency response of the d33-coefficient shown in Fig. 2. The ripples seen at 2 kHz and above are due to the fact that the dimensions of the pressure chamber with its connector are comparable to a quarter wavelength at these frequencies. To test the linearity of the microphones, the dependence of the sensitivity on applied sound pressure was examined. The data ͑not shown in this letter͒ indicates that there is a 3% sensitivity increase up to 3.2 kPa ͑164 dB SPL͒. This result is in qualitative agreement with the increase of d33 with pressure, discussed above, and thus an indication of nonlinearities of the stress–strain relationship. The total harmonic distortion ͑THD͒ of the cellular microphone is also related to this nonlinearity. As measure- FIG. 2. Interferometrically measured d33-coefficient for expanded cellular PP film ͑VHD40͒ of 55 ␮m thickness. ments show, THD increases approximately proportionally to

3268 J. Acoust. Soc. Am., Vol. 116, No. 6, December 2004 J. Hillenbrand and G. M. Sessler: Letters to the Editor

Downloaded 22 Sep 2011 to 222.66.175.219. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp sound pressure and is less than 1% at 164 dB SPL. This very film. Efforts are underway to produce cellular films of poly- small distortion originates probably to some part from the mers with better charge stability than that of the presently loudspeaker used in these experiments. The part generated used PP types. by the microphone is again due to the small nonlinearities of Additional features of the cellular microphones are their the stress–strain relationship. low harmonic distortion and their high resonance frequen- The A-weighted noise voltages of the single-film trans- cies. As Fig. 2 shows, the resonance frequency of a single- ducer and the five-film stack transducer, combined with a film microphone is expected to be at about 140 kHz. For a preamplifier, are 3.0 and 4.2 ␮V, respectively. From these stack of n films, the resonance frequency decreases by a values, total equivalent noise levels ͑ENLs͒ of 37 and 26 factor of n since the mass of the system increases and the dB͑A͒, respectively, are obtained. The noise corresponds stiffness decreases by this factor. This suggests a resonance closely to that of the preamplifier, as specified by the frequency of 28 kHz for the five-film transducer. manufacturer.14 The improvement of the ENL for the stack In addition to these features, the cellular PP micro- microphone by 11 dB is due to the increase of the sensitivity phones have a simple design. The transducers consist essen- ͑14 dB͒, reduced by the increase of the preamplifier noise tially only of one or several pieces of charged and metallized which is mainly due to the lowering of the capacitance cellular films, equipped with suitable backing and shielding. (Ϫ3 dB). No miniature air gaps, as in electret microphones, have to be maintained. Cellular microphones can therefore be manufac- tured at very low cost. These features make such micro- V. DISCUSSION AND CONCLUSIONS phones very suitable for a wide range of applications. The sensitivity M of the single-film microphone de- Because of the ambiguous meaning of the term ‘‘cellular 15 microphones’’ and in view of the correlation of electret and scribed above is related to the d33-coefficient by piezoelectric properties in cellular films, we suggest calling ͑s ϩ␧s ͒ ϭ 1 2 ͑ ͒ these transducers ‘‘piezoelectret microphones.’’ M d33 ␧␧ , 1 0

where s1 and s2 are the combined thicknesses of all solid or ␧ ␧ gas parts of the cellular film, respectively, and o and are ACKNOWLEDGMENTS the absolute and relative permittivities, respectively. From Eq. ͑1͒ the observed sensitivity of 2.2 mV/Pa is obtained for The authors are grateful to Dr. Xiaoqing Zhang for ϭ ␮ ϭ ␮ ͑ preparation of the expanded cellular films, to Treofan for s1 26 m, s2 30 m calculated from the density and to- ͒ ␧ϭ supplying the original films, and to the Deutsche Fors- tal thickness of the film , and 2.35 by substituting d33 ϭ475 pC/N, which is close to values actually measured in- chungsgemeinschaft and the Volkswagen Foundation for fi- terferometrically ͑cf. Fig. 2͒. nancial support. The measured sensitivity of 10.5 mV/Pa for a five-film microphone is very high for a piezoelectric microphone and is comparable with sensitivities of electret condenser micro- 1 J. Lekkala, R. Poramo, K. Nyholm, and T. Kaikkonen, ‘‘EMF force phones. Even higher sensitivities may be possible by further sensor–a flexible electret film for physiological applications,’’ Med. Biol. Eng. Comput. 34,67–68͑1996͒. increasing the d33-coefficients which can be achieved by in- 2 G. M. Sessler and J. Hillenbrand, ‘‘Electromechanical response of cellular creasing the charge density and by decreasing Young’s electret films,’’ Appl. Phys. Lett. 75, 3405–3407 ͑1999͒. 10 modulus of the cellular films. The sensitivity may also be 3 S. Bauer, R. Gerhard-Multhaupt, and G. M. Sessler, ‘‘Ferroelectrets: Soft improved by increasing the number n of piezoelectric films Electroactive Foams for Transducers,’’ Phys. Today 57,37–43͑February in the stack microphone. Since such an increase lowers the 2004͒. 4 M. Antila, T. Muurinen, J. Linjama, and H. Nyka¨nen, ‘‘Measurement capacitance of the device, stray capacitances and the input methods of flat panel electromechanical film loudspeakers,’’ Active 97, capacitance of the preamplifier have an adverse effect on the 607–618 ͑1997͒. sensitivity. For this reason, the sensitivity of the present ex- 5 H. Nyka¨nen, M. Antila, J. Kataja, J. Lekkala, and S. Uosukainen, ‘‘Active perimental design will not gain very much by increasing n control of sound based on utilizing EMFI-technology,’’ Active 99,1159– 1170 ͑1999͒. beyond 5. However, systems with reduced stray capacitance 6 R. Kressmann, ‘‘New piezoelectric polymer for air-borne and water-borne and/or with larger transducer area will show improved sen- sound transducers,’’ J. Acoust. Soc. Am. 109, 1412–1416 ͑2001͒. sitivities for nϾ5. The eventual limit will be reached when 7 M. Paajanen, M. Wegener, and R. Gerhard-Multhaupt, ‘‘Understanding the film stack capacitance becomes smaller than the input the role of the gas in the voids during corona charging of cellular electret films—a way to enhance their piezoelectricity,’’ J. Phys. D 34, 2482–2488 capacitance of the preamplifier. ͑2001͒. Equally important is the equivalent noise level which is 8 J. Hillenbrand, X. Zhang, J. Zhang, and G. M. Sessler, ‘‘Pressure-treated at 37 and 26 dB͑A͒ for the single- and five-film micro- cellular polypropylene with large piezoelectric coefficients,’’ 2003 Annual Report, Conf. Electric. Insul. and Diel. Phenom. ͑2003͒, pp. 40–43. phones, respectively. Particularly the latter value is again 9 16 X. Zhang, J. Hillenbrand, and G. M. Sessler, ‘‘Piezoelectric d33-coefficient comparable with that for typical electret microphones and of cellular polypropylene subjected to expansion by pressure treatment,’’ is much better than that of previous cellular microphones ͓52 Appl. Phys. Lett. 85, 1226–1228 ͑2004͒. dB͑A͔͒.6 10 X. Zhang, J. Hillenbrand, and G. M. Sessler, ‘‘Improvement of piezoelec- A drawback of the present cellular microphones is their tric activity of cellular polymers by a double-expansion process,’’ J. Phys. D 37, 2146–2150 ͑2004͒. decrease of sensitivity at temperatures in excess of 60 °C due 11 M. Wegener, W. Wirges, R. Gerhard-Multhaupt, M. Dansachmu¨ller, R. to the instability of the electret charges in the cellular PP Schwo¨diauer, S. Bauer-Gogonea, S. Bauer, M. Paajanen, H. Minkkinen,

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Downloaded 22 Sep 2011 to 222.66.175.219. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp and J. Raukola, ‘‘Controlled inflation of voids in cellular polymer ferro- Cambridge, 1997͒. electrets: Optimizing electromechanical transducer properties,’’ Appl. 14 See data sheet ‘‘B&K microphone preamplifier type 2669,’’ Phys. Lett. 84, 392–394 ͑2004͒. www.bksv.com/pdf/bp1422.pdf 12 J. Hillenbrand and G. M. Sessler, ‘‘New piezoelectric transducers based on 15 J. Hillenbrand and G. M. Sessler, ‘‘Piezoelectricity in cellular electret expanded cellular polymer electrets,’’ in Proceed. of ICA 2004, The 18th films,’’ IEEE Trans. Dielectr. Electr. Insul. 7, 537–542 ͑2000͒. Internat. Congress on Acoustics, Kyoto, Japan, 2004, Vol. I, 349-352. 16 G. M. Sessler, ‘‘Microphone,’’ in Encyclopedia of Science & Technology, 13 L. J. Gibson and M. F. Ashby, Cellular Solids, 2nd ed. ͑Cambridge U.P., 9th ed., Vol. 11 ͑McGraw-Hill, New York, 2002͒, pp. 88–95.

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