Acoustic impedance matching for axial wave radiation from cylindrical polyvinylidene fluoride film air ultrasonic transducer mounted inside of concentric hollow cylinder

Abstract

It is well known that radial vibration of cylindrical polyvinylidene fluoride (PVDF) film excites acoustic waves towards radial direction in 360-deg range. When the length of cylindrical film is equal to or less than half the wavelength, the excited wave towards the axial direction becomes stronger than the radial wave. In this work, a cylindrical PVDF transducer was concentrically mounted inside of a hollow cylinder, and it was observed that the smaller the gap between film surface and inner wall surface of the pipe, the more the axial wave was excited. In order to explain this phenomenon, radiation impedance for axial wave excitation was calculated, and it was found that (1) radiation impedance increases with decreasing gap; (2) energy flow from transducer to axial wave increases with decreasing gap and becomes maximum at impedance matched condition; and (3) for the radiation impedance larger than optimum matched condition, the output decreases with increasing radiation impedance (decreasing gap) and the resonance frequency of cylindrical transducer shifts to the higher side due to loading of imaginary part of too high radiation impedance. All these results were successfully observed by experiments at 85-kHz burst mode. This principle can be applied to a new class of ultrasonic air transducer.