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Hearing aids are generally equipped with electret microphones. These transducers are of simple design, can be miniaturized, and have good electro-acoustic properties. Recently, a new type of electret microphone, based on piezoelectrets, was introduced. Piezoelectrets consist of a closed-cell foam, often of polypropylene. The foam is poled by a corona discharge and thus assumes piezoelectric properties. Microphones have been built with such piezoelectrets with sensitivities of about 2.5 mV/Pa which can be increased to 15 mV/Pa by stacking a number of films. Also, directional microphones based on this principle have been designed. Piezoelectret microphones are of even simpler design than electret microphones and also have good electro-acoustic properties. Introduction For several decades, most hearing aids have been equipped with electret condenser microphones (ECMs). Recently, silicon condenser microphones have also been considered for use in hearing aids, but ECMs are still the dominating type. The reasons for the predominance of ECMs are their good electroacoustic properties and that they can be miniaturized [1]. Silicon microphones offer similar properties, although their ENL is somewhat higher. They have the additional advantage that they may be exposed to temperatures up to 260°C for short periods of time without degradation. Thus, they can withstand the heat produced during reflow soldering processes and may be used as SMDs on printed circuit boards. Nevertheless, ECMs are still the microphones of choice in present-day hearing aids. Some years ago, a modified type of electret microphone, utilizing a charged cellular polymer such as polypropylene (PP), was introduced and characterized [2,3]. Since the charged, cellular polymer assumes piezoelectric properties, these transducers are now generally referred to as “piezoelectret” microphones. They also have advantageous properties but are of even simpler construction than the classical ECMs. For this reason, such microphones may be suitable for use in hearing aids. A discussion of their properties, and a comparison with ECMs, is therefore of interest and will be presented in this review. A comparison of piezoelectret microphones with silicon microphones has been given before [4]. Electret microphones The schematic design of an ECM is shown in Fig. 1 (left). It consists of an electret which is mostly a polytetrafluoroethylene (PTFE) or polyfluoroethylenepropylene (FEP) film of about 12 μm thickness. This film is cemented to a back electrode and then charged by a corona discharge or a similar process to a surface potential of about 200 V. A membrane, often a metallized polymer film, is mounted opposite the electret, but separated from it by a shallow air gap [1]. These microphones can be miniaturized down to dimensions of about 2 to 3 mm. A few typical transducers of this kind are shown in Fig. 1 (right). The electro-acoustic properties of electret microphones are described by the following data [1]: Their frequency responses are relatively flat in the range extending from 20 Hz to 20 kHz and they have high sensitivities of about 10 to 20 mV/Pa. Their equivalent noise levels are generally below 25 dB(A) and their dynamic range, as determined by nonlinear distortion, extends up to 140 dB sound pressure level (SPL). The vibration sensitivities are about -60 dBV/g. They can also be exposed to mechanical and thermal shocks and to humidities in accordance with IEC standards. Piezoelectret microphones Piezoelectrets (also referred to as ferroelectrets) are a new group of charged polymer foams often consisting of cellular polypropylene (PP) [5-8]. A cross section of such a piezoelectret film is shown in the upper part of Fig. 2 (left). For the films to assume piezoelectric properties they have to be electrostatically charged, which is preferably achieved by a corona process, see Fig. 2 (right). The charge distribution so obtained is schematically illustrated in Fig. 2 (left, bottom). The voids with their positive and negative C6.1 S E N S O R + T E S T C o n f e r e n c e s 2 0 1 1 S E N S O R P r o c e e d i n g s 4 6 3 charges at the air/polymer-interfaces possess large dipole moments which induce image charges on the metal electrodes. Application of an external force changes the dimension of the voids, which in turn changes their dipole moments and thus the image charges. This is the origin of the piezoelectric activity. An advantage of PP piezoelectrets is that their piezoelectric and mechanical properties can be easily adapted to specific applications by pressure expansion processes, which change the thickness and Young’s modulus of the films [9-11]. Further advantages of piezoelectret films are their high piezoelectric d33-coefficients, low weight, high flexibility, mechanical stability, availability in large areas, and low material cost [7]. Figure 1: Schematic representation and operating principle of the electret microphone (left) and view of various electret microphones (right).

C6.1 - Hearing Aid Microphones: From Electret to Piezoelectret Transducers