Miniature Ferroelectret Microphone Design and Performance Evaluation Using Laser Excitation

Miniature microphones suitable for measurements of ultrasonic wave field scans in air are expensive, or lack sensitivity or do not cover the range beyond 100 kHz. Essential, that they are too large for such fields measurements. The use of ferroelectret (FE) film is proposed to construct a miniature, needle-style 0.5 mm diameter sensitive element ultrasonic microphone. FE has an acoustic impedance much closer to that of air compared to other alternatives, and is low cost and easy to process. Performance of the microphone was evaluated by measuring the sensitivity area map, directivity, AC response and calibrating the absolute sensitivity. Another novel contribution here, is that the sensitivity map was obtained by scanning the focused beam of a laser diode over the microphone surface, producing thermoelastic ultrasound excitation. The electroacoustic response of the microphone served as a sensitivity indicator at a scan spot. Micrometer scale granularity of the FE sensitivity was revealed in the sensitivity map images. It was also demonstrated that the relative AC response of the microphone can be obtained using pulsed laser beam thermoelastic excitation of the whole microphone surface with a laser diode. The absolute sensitivity calibration was done using the hybrid three transducer reciprocity technique. A large aperture, air coupled transducer beam was focused onto the microphone surface, using the parabolic off-axis mirror. This measurement validated the laser AC response measurements. FE microphone performance was compared to bi-axially stretched PVDF (Polyvinylidene difluoride) microphone of the same construction.