Hillenbrand et al.: JASA Express Letters

[http://dx.doi.org/10.1121/1.4826915]

Published Online 5 November 2013

Electret microphones with stiff diaphragms J. Hillenbrand, S. Haberzettl, and G. M. Sessler Institute for Telecommunications Technology, Darmstadt University of Technology, Merckstrasse 25, D-64283 Darmstadt, Germany [email protected], [email protected], [email protected]

Abstract: Electret microphones with stiff plates instead of flexible diaphragms are described. The stiff plate and a backplate, separated by a soft cellular polymer spacer ring, yield a capacitance that is varied by the incoming sound wave; thus a voltage change in the plates is induced. Various such plate microphones were built and characterized. Sensitivities of more than 10 mV/Pa and equivalent noise levels of 23 dB(A) are obtained. An analytical model for the sensitivity of plate microphones was developed. Advantages of the plate microphones are high mechanical robustness, low harmonic distortion, and no risk of membrane collapse. C 2013 Acoustical Society of America V

PACS numbers: 43.38.p, 43.38.Bs, 43.38.Kb [JL] Date Received: July 23, 2013 Date Accepted: October 9, 2013

1. Introduction Most commercial pressure microphones utilize thin diaphragms. Mathematically speaking, such diaphragms are membranes or plates. Membrane vibrations are exclusively controlled by tensional forces, whereas plate vibrations are also influenced by bending forces. In conventional condenser,1 electret,1,2 and dynamic microphones,1 metal foils or metallized polymer films of approximately 2–10 lm thickness are used as diaphragms while micro-electro-mechanical systems (MEMS) microphones, often also called silicon microphones,3 are based on diaphragms in the sub-lm range, consisting of silicon or gallium arsenide or their chemical compounds. For lower frequencies, all of these diaphragms are deflected by the sound pressure in such a way that the maximum deflection is located in the center of the diaphragm, while closer to the border of the diaphragm the deflection diminishes. Sensor designs with diaphragms much thicker than 10 lm have also been described in the past.4 The thick membrane contacts the backplate during operation, yielding a constant membrane deflection and sensitivity over the area of the transducer. Some years ago, microphones based on charged cellular polymers, later on referred to as piezoelectret microphones5,6 were introduced that are based on ferroelectret7,8 or, sometimes also called, piezoelectret films.9 In these microphones, the sound pressure acts, for lower frequencies, uniformly on one or both sides of the piezoelectret films; this results in a uniform thickness change everywhere on the films. It can be shown theoretically,10 that such a parallel movement of the electrodes generates lower harmonic distortion than the membrane-like movements of the diaphragm of conventional microphones. Experimentally, the very low harmonic distortion of piezoelectret microphones of less than 1% at 164 dB sound pressure level was shown several years ago.6 Along with piezoelectret based microphones also accelerometers were developed and investigated. In these piezoelectret accelerometers,11 the bonding of the stiff seismic mass and the piezoelectret film ensures a parallel movement of the two electrodes. The disadvantage of these polypropylene based piezoelectret accelerometers, their insufficient temperature stability, was solved some time later by replacing the piezoelectret films by an uncharged cellular ring, a charged fluorinated ethylene propylene

J. Acoust. Soc. Am. 134 (6), December 2013

C 2013 Acoustical Society of America V

EL499

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 138.5.159.110 On: Mon, 13 Apr 2015 06:18:21

Hillenbrand et al.: JASA Express Letters

[http://dx.doi.org/10.1121/1.4826915]

Published Online 5 November 2013

(FEP) film, and an air gap. In these electret accelerometers,12 the stiff seismic mass and the stiff backplate are the two electrodes of the sensor which are separated by the soft cellular ring. This design ensures the parallel movement of the electrodes. These accelerometers were taken as the basis of a new type of electret microphone with a stiff plate instead of a flexible membrane. The original height of the seismic mass cylinder was reduced, and a stiff plate was obtained. Aluminum was chosen as plate material because it yields high stiffness and low weight plates. Certainly, plates made of titanium, fiber-reinforced composites or structured and stiffened plates, already used in MEMS directional microphones,13 will be more stable than Al plates with the same weight or lighter for the same stability. Then the unwanted vibration sensitivity of the plate microphone is reduced. In the present microphone design, the plate is positioned in an opening of the sensor housing, and opening and plate are covered with a polymer membrane. This construction ensures both that the sound pressure can reach the plate without significant damping and that the plate microphone is mechanically robust and waterproof. Various such plate microphones with different air gap thicknesses, electret voltages, and membrane materials were built and investigated. In the following, the design of these plate microphones is discussed in detail. and measurements with the microphones are presented. 2. Microphone design The present plate microphone design is based on a previous piezoelectret accelerometer design.14 As shown in Fig. 1 (left), the plate microphone consists of two housing parts. Housing part 2 is open on its lower side, and the opening is covered with a slightly stretched polymer membrane. Inside the housing, a small printed circuit board (PCB) with a junction field-effect transistor (JFET) and two resistors as well as the actual transducer is located. The transducer, illustrated in an enlarged view in Fig. 1 (right), is composed of an electret film, a back plate, a 500 lm thick aluminum plate with a diameter of 7 mm, a cellular polypropylene (PP) ring, which acts as a spring element, and the membrane. The two housing parts are connected by a thread that allows one to continuously adjust the total length of the housing. By turning the two housing parts relative to each other, the static force of the membrane onto the aluminum plate can be continuously adjusted, and the cellular PP ring changes its thickness. The plate is held in its equilibrium position by this force as well as the counterforce generated by the cellular ring on the other plate side. No additional forces are generated by the enclosed volume of the air gap for slow changes of the equilibrium positions because the gap is not completely airproof, and the pressure inside therefore remains constant. For the relatively fast, periodical vibrations of the membrane, however, restoring forces of the air are generated that are

Fig. 1. (Color online) Cross section of a plate microphone (left). The housing consists of two parts that are connected by a thread. Magnified cross section of the part of the microphone around the Al plate (right).

EL500 J. Acoust. Soc. Am. 134 (6), December 2013

Hillenbrand et al.: Electret microphones with stiff diaphragms

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 138.5.159.110 On: Mon, 13 Apr 2015 06:18:21

Hillenbrand et al.: JASA Express Letters

[http://dx.doi.org/10.1121/1.4826915]

Published Online 5 November 2013

comparable to the forces of the cellular ring and have to be taken into account. More details about the operation of the microphone are presented at the beginning of Sec. 4. 3. Cellular polypropylene ring An essential part of the present plate microphones and the previous electret accelerometers are spacer rings made of cellular polymers.15 Such materials, made of PP, with air volume to polymer ratios of about 50:50, and a closed cell structure with lens-like air voids, are orders of magnitude softer than the base material PP.15 Because of the lens-like air voids and the low stiffness, cellular PP can be rendered highly piezoelectric by corona charging.7–9 In the past, pressure expansion methods were developed16,17 to increase the piezoelectric d33-coefficients mainly by decreasing Young’s modulus of the cellular films. These methods not only affect the size of Young’s modulus but also its frequency response. Because corona charging has only little or no influence on the mechanical properties of the cellular films, the results of former pressure expansion studies with PP piezoelectrets, relating to the elastic moduli, can be directly used to obtain optimized cellular ring materials for the plate microphones. Measurements of Young’s modulus of cellular PP were made by using an interferometer.18 For a typical expanded PP film, a slight increase of Young’s modulus from 0.8 to about 1 MPa in the frequency range from 100 Hz to 50 kHz was found. Besides their small Young’s modulus, which is adjustable by pressure expansion methods, the relatively flat frequency response of the modulus is a special feature of the expanded cellular PP materials. 4. Model calculations In the microphone, depicted in Fig. 1, backplate, air gap, FEP film, and its metallization form a capacitor. An incoming sound wave generates a periodical pressure difference between both membrane surfaces because the pressure inside the microphone remains constant. This pressure difference compresses and expands the soft cellular PP ring and periodically changes the height of the air gap and the capacitance of the above mentioned condenser. Restoring forces are generated both by the elastic properties of the cellular ring and due to the adiabatic volume change of the air enclosed inside the cellular ring. Because the FEP-electret film is charged, the periodical capacitance variation generates a voltage across the condenser electrodes connected to the FET on the PCB. The FET, electrically shielded by the sensor housing, is used as first stage of an impedance converter or an amplifier. The gate capacitance of the FET and various parasitic capacitances reduce the open circuit output voltage. This reduction and thus the voltage at the gate can be calculated by means of a capacitive voltage divider if all capacitances are known. Gate voltage and output voltage of the microphone are related by the amplification factor of the circuit, consisting of the FET and some internal and external resistors. Therefore the microphone output voltage can be calculated if the open circuit voltage is known. In the following, an analytical model for the open circuit sensitivity of a plate microphone will be presented. The symbols used are given in brackets in Fig. 1 (right). After corona charging, the electret charges with area density r are located, to a good approximation, directly at the surface of the FEP film. If backplate and Al plate are temporarily short-circuited, the electric field inside the air gap EA can be written as19 EA ¼

rtE ; e0 ðetA þ tE Þ

(1)

where tE and tA are the thicknesses of FEP electret film and air gap, respectively, e0 is the vacuum permittivity, and e is the relative permittivity of the FEP film.

J. Acoust. Soc. Am. 134 (6), December 2013

Hillenbrand et al.: Electret microphones with stiff diaphragms EL501

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 138.5.159.110 On: Mon, 13 Apr 2015 06:18:21

Hillenbrand et al.: JASA Express Letters

[http://dx.doi.org/10.1121/1.4826915]

Published Online 5 November 2013

If the air gap thickness tA changes by an infinitesimally small value dtA under open-circuit conditions, the electric field given by Eq. (1) remains constant and a small voltage dV will be generated, dV ¼ EA dtA ¼

rtE  dtA : e0 ðetA þ tE Þ

(2)

The mass of the Al plate mD, the cellular polymer ring with thickness tP, area AP, and Young’s modulus Y (spring 1), the enclosed air in the gap between back plate and electret film with thickness tA and area AA (spring 2), and the tensioned membrane (spring 3) can be considered as a mass-spring-system with three springs mechanically in parallel. It can be shown that restoring forces due to the tension of the membrane (spring 3) are smaller than the forces due to the cellular ring (spring 1) and due to the enclosed air (spring 2) and can therefore be neglected in the following formulas. For very low membrane tension and very low forces on the non-glued cellular ring, its two actual contact areas are smaller than the geometric areas due to the relatively rough surfaces. Thus for very low forces, two irregular air gaps exist at the interfaces of the ring that are compacted and finally disappear in the case of increasing forces. Formally, these irregular air gaps are non-linear springs mechanically in series with the cellular ring spring 1. Because these two springs are only relevant for very low forces and because their analytical description is impossible, they will be neglected in the following formulas, however, will be qualitatively discussed in Sec. 5. The damping of the system is dominated by the viscoelastic properties of the cellular polymer ring and will not be treated in the model. The stiffnesses of springs 1 and 2 can be calculated by taking into account the definition of Young’s modulus of the cellular ring and by considering an adiabatic change of the enclosed air volume, respectively. A small pressure change dp at the membrane with the effective area AM changes the thickness of the cellular ring and the thickness of the air gap by the same value dtA because both springs are coupled by the stiff Al plate. Therefore both spring stiffnesses have to be added and one has   Y  AP cp0 AA AM dp ¼ þ (3) dtA ; tP tA where p0 is the atmospheric pressure and c the ratio of specific heats. Note that the effective membrane area AM is larger than the plate area and smaller than the geometric membrane area. By inserting Eq. (3) into Eq. (2), the voltage sensitivity SV of the microphone, well below resonance, can be obtained as  1 dV rtE Y  AP cp0 AA  SV ¼ ¼ þ  AM : (4) tP tA dp e0 ðetA þ tE Þ A direct measurement of the surface charge density r is difficult. However, r can be replaced by the electret voltage (or surface potential) VE by means of r ¼ e0eVE/tE.19 The electret voltage can be directly controlled by the corona grid voltage during charging and can be easily measured with an electrostatic voltmeter. It can be shown that the electret voltage in an electret transducer is equivalent to an external dc voltage of the same magnitude applied to the electrodes of a capacitive transducer. The voltage sensitivity in Eq. (4) can then be written as  1 eVE Y  AP cp0 AA SV ¼  þ  AM : (5) ðetA þ tE Þ tP tA The resonance frequency fres ¼ xres =2p of the plate microphone corresponds to the resonance frequency of a mass-spring-system with two springs and can thus be written as

EL502 J. Acoust. Soc. Am. 134 (6), December 2013

Hillenbrand et al.: Electret microphones with stiff diaphragms

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 138.5.159.110 On: Mon, 13 Apr 2015 06:18:21

Hillenbrand et al.: JASA Express Letters

[http://dx.doi.org/10.1121/1.4826915]

fres

1 ¼ 2p

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ffi 1 Y  AP cp0 AA : þ tP tA mD

Published Online 5 November 2013

(6)

Insertion of Eq. (6) into Eq. (5) yields the voltage sensitivity SV ¼

eVE AM  ðetA þ tE Þ x2res mD

(7)

as a function of the resonance frequency of the microphone. 5. Measurements and discussion Various plate microphones with different electret voltages and membrane materials were built and investigated.18 Measurements were performed with different membrane tensions and thus different static pressures on the cellular ring, which can be adjusted by varying the length of the housing by adjustment of the thread. Free-field measurements were carried out with the plate microphones in an anechoic chamber. A dynamic loudspeaker at a distance of 1.5 m from the plate microphones was used as sound source. The reference sound pressure level was determined with a 1/2-inch condenser microphone (B&K 4191), a preamplifier (B&K 2669) and a measuring amplifier (B&K 2636). In Fig. 2 (left), four frequency responses of the sensitivity for a plate microphone with a 25 lm thick FEP membrane and an electret film charged to 500 V are shown. The highest sensitivity of about 11 mV/Pa was obtained with the lowest FEP membrane tension and thus the lowest static pressure on the cellular ring. The measurement does not reveal a clear resonance frequency. For the following measurements, the membrane tension and static pressure on the ring were successively increased. For the second highest membrane tension, a fairly flat frequency response up to 10 kHz with sensitivities between 4 and 5 mV/Pa was measured. Such characteristics are adequate for many potential applications of the plate microphone. For the highest static ring pressure, a sensitivity of about 3.5 mV/Pa and a clearly visible resonance frequency of approximately 9 kHz were found. In Fig. 2 (right), frequency responses and calculated sensitivities for a plate microphone with a 40 lm thick PP membrane and charged to 410 V are presented. In contrast to the measurements of Fig. 2 (left), all responses in Fig. 2 (right) show a clear resonance peak. This is probably due to the thicker and somewhat harder PP membrane used in the latter microphone. Measured sensitivities from about 4.5 to

Fig. 2. (Color online) Sensitivities of plate microphones for different static pressures on the cellular PP ring. A 25 lm thick FEP membrane was used (left). A 40 lm thick PP membrane was used (right). The solid lines are sensitivities calculated from the corresponding resonance frequency.

J. Acoust. Soc. Am. 134 (6), December 2013

Hillenbrand et al.: Electret microphones with stiff diaphragms EL503

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 138.5.159.110 On: Mon, 13 Apr 2015 06:18:21

Hillenbrand et al.: JASA Express Letters

[http://dx.doi.org/10.1121/1.4826915]

Published Online 5 November 2013

0.6 mV/Pa and resonance frequencies from 8.5 to 20 kHz can be extracted from Fig. 2 (right). The solid lines in the figure represent sensitivities calculated by Eq. (7) with the experimentally determined resonance frequencies. Measured and calculated sensitivities are in good agreement, hence, plate microphone sensitivities are well described by Eq. (7). This equation is based on Eq. (6), which yields the resonance frequency of the microphone as a function of its total spring constant [term in brackets in Eqs. (5) and (6)]. The measurements in Fig. 2 (right) show, that increasing the membrane tension increases the resonance frequency and, according to Eq. (6), increases also the total spring constant of the microphone. Because the portion of the total spring constant, which is due to the air, is only insignificantly influenced by the membrane tension (tA may slightly depend on it), the term Y  AP =tP in Eqs. (5) and (6) has to increase when the membrane tension and the pressure on the cellular ring is increased. Because the other geometrical parameters AP and tP, similar to tA, do not significantly depend on the membrane tension, the observed resonance frequency variation has to be explained by a pressure dependent Young’s modulus Y, or, in other words, by a nonlinear mechanical behavior of the cellular ring. To illustrate this, for each of the four calculated sensitivity levels shown in Fig. 2 (right), the corresponding Young’s modulus was calculated by Eq. (5) and written next to the level line. A Young’s modulus range from 0.29 to 5.9 MPa is thus necessary to obtain by Eq. (5) sensitivity values from 3.4 to 0.6 mV/Pa. As mentioned in the preceding text, Eq. (5) does not hold for very low membrane tensions when irregular air gaps exist at the surfaces of the cellular ring. Such air gaps significantly reduce the total spring constant of the microphone and can explain the apparent Young’s modulus value of 0.29 MPa. The next larger Young’s modulus of 1.2 MPa is a typical value for expanded cellular PP measured for low deflections (cf. Sec. 3). The values of 2.6 and 5.9 MPa may then be explained by a strong non-linear behavior of the cellular ring material. Besides a high and flat frequency response of the sensitivity, the noise produced by the particular microphone is of importance. Thus such measurements were performed, and they showed that the noise of the plate microphones is dominated by the noise of the integrated FET. A-weighted RMS noise voltages of about 3 lV were measured. For the microphones presented in the preceding text, with sensitivities of 11 and 4.5 mV/Pa, equivalent noise levels (ENL) of 23 and 30 dB(A), respectively, were obtained. In particular, the value of 23 dB(A) is low and adequate for most applications. So far, no total harmonic distortion measurements have been performed with the present plate microphones. However, plate microphones with cellular rings and piezoelectret microphones, with measured total harmonic distortions of less than 1% at 164 dB,6 are mechanically comparable with the difference that the sound pressure acts in the plate microphones on a cellular polymer area about four times smaller than that for the piezoelectret microphones. Thus a total harmonic distortion of less than 1% at 152 dB, still a good value, can be expected for a plate microphone with a relatively high membrane tension. Then no irregular air gaps exist at the surfaces of the cellular ring that could produce some additional distortion. An obvious disadvantage of the plate microphones is their large vibration sensitivity. For some applications this may be irrelevant. If not, the suppression of vibration signals can be obtained by using an additional accelerometer, or an identical second plate microphone with closed housing, and subtract both sensor signals according to active noise cancellation procedures. 6. Conclusions In the present paper, plate microphones are introduced. These microphones are based on the electret principle. However, contrary to conventional electret and also other microphone types, a stiff plate and a soft cellular polymer spacer ring instead of a flexible diaphragm are used as pressure sensitive elements. Thus the main advantages of the plate microphones, compared to conventional electret microphones, are better mechanical robustness and waterproofness. Further features of the plate microphones are

EL504 J. Acoust. Soc. Am. 134 (6), December 2013

Hillenbrand et al.: Electret microphones with stiff diaphragms

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 138.5.159.110 On: Mon, 13 Apr 2015 06:18:21

Hillenbrand et al.: JASA Express Letters

[http://dx.doi.org/10.1121/1.4826915]

Published Online 5 November 2013

low total harmonic distortion and relatively high sensitivity because no membrane collapse can occur and thus large electret voltages are possible. If necessary, the large vibration sensitivity of the plate microphones can be suppressed by adding an identical but soundproofed sensor and subtracting the two output signals. Various plate microphones were built and investigated. So far, a sensitivity of more than 10 mV/Pa for a low resonance frequency microphone and about 5 mV/Pa for a microphone with fairly flat response up to 20 kHz could be realized. The lowest equivalent noise level measured was 23 dB(A). An analytical model was developed that describes the open circuit voltage sensitivity and the resonance frequency of a plate microphone by mechanical and electrical parameters such as the air gap thickness, Young’s modulus of the cellular ring, and the surface potential of the electret film. Reasonable agreement between experimental data and model calculations was found. Acknowledgments The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) and by the Hessische Ministerium f€ ur Wissenschaft und Kunst. References and links 1

J. Eargle, The Microphone Book, 3rd ed. (Focal Press–Elsevier, Oxford, 2012). G. M. Sessler and J. E. West, “Self-biased condenser microphone with high capacitance,” J. Acoust. Soc. Am. 34, 1787–1788 (1962). 3 G. M. Sessler, “Silicon microphones,” J. Audio Eng. Soc. 44, 16–22 (1996). 4 J. E. West, I. Busch-Vishniac, G. A. Harshfield, and T. G. Pickering, “Foil electret transducer for blood pressure monitoring,” J. Acoust. Soc. Am. 74, 680–686 (1983). 5 J. Hillenbrand and G. M. Sessler, “High-sensitivity piezoelectric microphones based on stacked cellular polymer films,” J. Acoust. Soc. Am. 116, 3267–3270 (2004). 6 J. Hillenbrand and G. M. Sessler, “Stacked piezoelectret microphones of simple design and high sensitivity,” IEEE Trans. Dielectr. Electr. Insul. 13, 973–978 (2006). 7 S. Bauer, R. Gerhard-Multhaupt, and G. M. Sessler, “Ferroelectrets: Soft electroactive foams for transducers,” Phys. Today 57(2), 37–43 (2004). 8 R. Gerhard-Multhaupt, “Less can be more—holes in polymers lead to a new paradigm of piezoelectric materials for electret transducers,” IEEE Trans. Dielectr. Electr. Insul. 9(5), 850–859 (2002). 9 Z. Hu and H. von Seggern, “Breakdown-induced polarization buildup in porous fluoropolymer sandwiches: A thermally stable piezoelectret,” J. Appl. Phys. 99, 024102 (2006). 10 M. Pedersen, W. Olthuis, and P. Bergveld, “Harmonic distortion in silicon condenser microphones,” J. Acoust. Soc. Am. 102(3), 1582–1587 (1997). 11 J. Hillenbrand, M. Kodejska, Y. Garcin, H. v. Seggern, and G. M. Sessler, “High-sensitivity ferroelectret-film accelerometers,” IEEE Trans. Dielectr. Electr. Insul. 17, 1021–1027 (2010). 12 J. Hillenbrand S. Haberzettl, T. Motz, and G. M. Sessler, “Electret accelerometers: Physics and dynamic characterization,” J. Acoust. Soc. Am. 129(6), 3682–3689 (2011). 13 R. N. Miles, Q. Su, W. Cui, M. Shetye, F. L. Degertekin, B. Bicen, C. Garcia, S. Jones, and N. Hall, “A low-noise differential microphone inspired by the ears of the parasitoid fly Ormia ochracea,” J. Acoust. Soc. Am. 125(4), 2013–2026 (2009). 14 J. Hillenbrand, S. Haberzettl, T. Motz, and G. M. Sessler, “Voltage sensitivity of electret- and piezoelectret-accelerometers,” in Proceedings of the 14th International Symposium on Electrets, Montpellier, France (2011), pp. 219–220. 15 L. J. Gibson and M. F. Ashby, Cellular Solids: Structure and Properties, 2nd ed. (Cambridge University Press, Cambridge, 1999). 16 M. Wegener, W. Wirges, J. Fohlmeister, B. Tiersch, and R. Gerhard-Multhaupt, “Two-step inflation of cellular polypropylene films: Void-thickness increase and enhanced electromechanical properties,” J. Phys. D 37, 623–627 (2004). 17 X. Zhang, J. Hillenbrand, and G. M. Sessler, “Improvement of piezoelectric activity of cellular polymers using a double-expansion process,” J. Phys. D 37, 2146–2150 (2004). 18 J. Hillenbrand, S. Haberzettl, and G. M. Sessler, “Electret microphones with stiff plates as membranes,” in Proceedings of the International Conference on Acoustics AIA-DAGA, Merano, Italy (2013), pp. 897–898. 19 G. M. Sessler (editor), Electrets, 3rd ed. (Laplacian Press, Morgan Hill, CA, 1999), Vol. 1. 2

J. Acoust. Soc. Am. 134 (6), December 2013

Hillenbrand et al.: Electret microphones with stiff diaphragms EL505

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 138.5.159.110 On: Mon, 13 Apr 2015 06:18:21

Electret microphones with stiff diaphragms.

Electret microphones with stiff plates instead of flexible diaphragms are described. The stiff plate and a backplate, separated by a soft cellular pol...
393KB Sizes 0 Downloads 7 Views