High-sensitivity piezoelectric microphones based on stacked cellular polymer films

Improvements of the sensitivity of piezoelectric microphones based on charged cellular polymer films are reported. The improvements are achieved by (1) an increase of the piezoelectric d(33)-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 dB(A) for the single-film transducer and 26 dB(A) 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. (C) 2004 Acoustical Society of America.