Ferroelectrets (or piezoelectrets) are a new group of polymers consisting of closed-cell foam films, often of polypropylene (PP), as depicted in Fig. 1 (top). The films are poled such that a charge distribution as shown in Fig. 1 (bottom) is generated . This makes the films piezoelectric. Advantages of such ferroelectret films are their high piezoelectric coefficients, the availability in large areas, and their low costs .
Because of their small elastic modulus, ferroelectrets are easily compressed by sound waves. This allows one to construct high-quality microphones of simple design. An air gap, as in condenser microphones, is not required. With stacked films the microphone open-circuit sensitivity increases proportional to the number of films. A piezoelectret microphone with four films, as shown in Fig. 2, has a sensitivity of about 10 mV/Pa and an equivalent noise level of 27 dB(A) , comparable to electret microphones.
Recently, piezoelectret films were made of layers of the fluoropolymers FEP and PTFE. These are more temperature resistant than the PP films and can be used up to 90°C [4,5]. The frequency response of a microphone based on such films is shown in Fig. 3. Advantages of the piezoelectret microphones are low harmonic distortion, low equivalent noise level, and, for stacked systems, large sensitivity. Due to the absence of an air gap, they are also of much simpler construction than conventional electret microphones.
•  G. M. Sessler and J. Hillenbrand, Appl. Phys. Letters 75, 3405-3407 (1999).
•  S. Bauer, R. Gerhard-Multhaupt, and G. M. Sessler, Physics Today 57, No. 2, 37-43 (2004).
•  J. Hillenbrand and G. M. Sessler, J.Acoust Soc. Am. 116, 3267-3270 (2004); IEEE Trans. DEI 13, 973-978 (2006).
•  Z. Hu and H. von Seggern, J. Appl. Phys. 99, 024102 (2006); R. A. C. Altafim et al, IEEE Trans. DEI 13, 979-985 (2006).
•  X. Zhang, J. Hillenbrand, and G. M. Sessler, J. Appl. Phys. 101, 054114 (2007).
•  J. Hillenbrand and G. M. Sessler, Ferroelectrics 472, 77-89 (2014)
•  X. Zhang, G. M. Sessler, Y. Xue, and X. Ma, J. Phys. D: Appl. Phys. 49,205502 (2016).