Stacked piezoelectret microphones of simple design and high sensitivity

Pressure-chamber and free-field measurements of high sensitivity piezoelectret microphones of two different designs are reported. High sensitivity is achieved by (1) an increase of the piezoelectric d₃₃ -coefficient of the cellular polypropylene films by a pressure expansion procedure and (2) stacking of the films. The pressure expansion results in large d₃₃-coefficients of up to 600 pC/N in the audio frequency range. Microphones consisting either of cemented stacks or edge-clamped but otherwise loose and non-cemented stacks of up to six expanded films were built. For all investigated microphones, sensitivities of about 2.3 mV/Pa per film were found, resulting in an open circuit sensitivity of 15 mV/Pa at 1 kHz for a six-film-microphone. The equivalent noise level is about 37 dB(A) for a single-film transducer and 26 dB(A) for a five-film microphone. Distortion is less than 1 % at a sound pressure level (SPL) of 164 dB. Advantages of the new piezoelectric transducers include their simple design, consisting essentially only of the cellular film or films and electrical shielding. Other advantages are low cost, small weight, as well as a large range of shapes and sizes possible with such microphones