Comparative Study of Resonance Frequency of Circular Piezo Microphone Based on Photoacoustic, Laser Speckle, and Piezoelectric Effects

Abstract

Resonance frequency of a circular piezoelectric microphone was measured by two types of methods which were named as photoacoustic and speckle method. In the photoacoustic method; acoustic waves were obtained due to the optical excitation and resulting thermally induced mechanical distortion was measured by the piezoelectric layer of microphone. Photoacoustic effect is a type of optical excitation, which creates an acoustic wave due to the absorbed light energy which causes thermal expansion of material. In contrast to the photoacoustic method, speckle method was conducted by electrical excitation of the piezoelectric layer which results in mechanical distortion and this distortion was detected by using changes in laser speckle pattern. Measurements were taken from 6 different piezo microphones in 3 different diameters of 15 mm, 35 mm, and 50 mm and the same thickness of brass plate in the frequency range of 0-11 kHz. As a conclusion, it is found that the resonance frequencies of same diameter microphones determined by the photoacoustic method are close but different with the results of the speckle method. It is believed that the differences in the results are caused by differences of excitation/detection mechanism for same microphones and shape, material parameter differences between microphones for different microphones.