Piezoelectret microphones: A new and promising group of transducers

Abstract

Piezoelectret microphones, first described a few years ago, are transducers based on the strong longitudinal piezoelectric effect of charged cellular polymers. Such microphones have recently been improved in two respects: Firstly, an expansion process was used to increase the piezoelectric d33 coefficients of cellular polypropylene (PP) films in the audio frequency range up to 600 pC/N and, secondly, stacking of several films was applied to increase the microphone sensitivity. Transducers with six films now show open-circuit sensitivities of up to 15 mV/Pa, comparable to that of electret microphones. Other characteristics of piezoelectret microphones are their low equivalent noise level of about 26 dB(A) and the very small total harmonic distortion of less than 0.1% at 140 dB SPL. The piezoelectric activity of the PP films and the microphone sensitivities are stable at room temperature but start to decay above 50<th>°C. Recently, directional piezoelectret microphones with various directional characteristics have been designed. Major advantages of piezoelectret microphones are their simple design, their low harmonic distortion, and their wide frequency range extending into the ultrasonic region.