Measurement of sound pressure sensitivity of high-frequency microphone using multiple harmonics in intense ultrasonic beams

Intense ultrasonic beams suffer progressive waveform distortion due to the nonlinearity of the air, causing numerous harmonics to be generated in the beam. Since these harmonic pressures can be theoretically predicted with sufficient accuracy by the Khokhlov-Zabolotskaya-Kuznetsov model, it is possible to obtain the pressure sensitivity of a microphone at high frequencies. To generate intense ultrasonic waves, a planar aperture source of 118 mm in diameter and with resonant frequencies of 40 and 63 kHz was used. A specialized microphone developed and tested for receiving high-frequency ultrasonic waves was fabricated from a single cellular polypropylene (CPP) sheet of 50 mm in thickness and of a sensing area approximately 13 mm(2). Using at least the first nine harmonics, i.e., those in the range of 40 to 567 kHz, it was demonstrated in this study that the frequency response of the CPP microphone has a sensitivity of -70 to -80 dB re. 1 V/Pa at frequencies below 400 kHz.