Comparative study of transducers for air-borne sound based on normal and irradiation cross-linked polypropylene piezoelectret films

Abstract

In this paper, a comparative study of normal polypropylene (PP) and irradiation cross-linked polypropylene (IXPP) piezoelectret films is reported. The PP and IXPP piezoelectret films were prepared by a gas-pressure expansion process and hot-pressing process followed corona charging, respectively. The quasi-static piezoelectric d 33 coefficients and thicknesses of fabricated PP and IXPP films are around 380 and 700 pC/N, and 70 and 140 μm, respectively. One of the advantages of IXPP is thicker films with higher piezoelectric coefficient d33 are more easily to be obtained compared with normal PP. The dynamic piezoelectric d33 values, measured by an acoustic method at 1 kHz, are 300 and 550 pC/N for the studied PP and IXPP films, respectively. The characteristic acoustic impedance determined from dielectric spectroscopy measurements for IXPP film is only 3×104 kg/(m2s), about equal to that of the PP film of 2.5×104 kg/(m2s), which results in a good match to air. The sound pressure level (SPL) of an actuator of 3.14 cm2 area based on IXPP film, driven with 100 V rms at 10 kHz and tested 20 cm away from the transducer, is about 68 dB, while for PP, it is 65 dB under the same conditions. The sensitivity of a microphone made of IXPP film is −48 dB (3.8 mV/Pa) at 1 kHz, which is approximately four times that of a PP film microphone (0.9 mV/Pa at 1 kHz). Compared with normal PP, the IXPP film-based transducers show higher transmitting and receiving sensitivities. This is primarily due to larger piezoelectric d 33 coefficients and thickness of the IXPP piezoelectret films comparing with those of PP.