Broadband PVDF membrane hydrophone for comparisons of hydrophone calibration methods up to 140 MHz

A PVDF membrane hydrophone has been constructed in particular for comparisons of broadband ultrasound hydrophone calibration methods and of the results obtained by different laboratories. Intercomparisons have to accompany the efforts currently undertaken to enhance the calibration frequency ranges and to implement the extension from the determination of amplitude-only to complex-valued calibration data. It can be expected that such hydrophone data will be used much more frequently in the future for exposure measurements on medical ultrasound equipment, in particular for the detection of nonlinearly distorted waveforms. The hydrophone design chosen has a foil thickness of 9 microm and an electrode diameter of 210 microm. A broadband differential preamplifier (-3 dB roll-off frequency: 95 MHz) is integrated to achieve a high signal-to-noise ratio over a broad frequency range (e.g., 26 dB-30 dB in the range 50 MHz to 140 MHz for measurements of nonlinearly distorted pulses). The hydrophone response was characterized by means of a primary interferometric calibration technique, by substitution calibration using time-delay spectrometry, and by complex broadband pulse calibration using nonlinear sound propagation. The results show a flat frequency response up to 40 MHz (maximum variations below +/-0.6 dB) and a thickness mode resonance at about 105 MHz. They indicate a useable bandwidth up to 140 MHz. The effective diameter as derived from directional response measurements is 240 microm at frequencies beyond 15 MHz.