Sensitivity measurements of piezoelectric polymer hydrophones from 0.2–2 MHz using a broadband-pulse technique

It is widely recognized that the sensitivity of hydrophones used to measure medical diagnostic ultrasound fields should be uniform over several octaves above the center frequency (i.e., above the mean of the upper and lower −3 dB frequencies in the transmitted acoustic-pressure spectrum). However, a bandwidth extending to at least ten times below the diagnostic pulse-center frequency is needed for accurate (error ≈5%) measurement of the peak rarefactional pressure. Since at present it is not common for manufacturers of medical-use hydrophones to provide sensitivity information below 1–2 MHz, a study was undertaken to determine these low-frequency sensitivities. The technique uses broadband, plane-wave pressure pulses generated by electrical short-pulse excitation of a thick piezoelectric ceramic disk. The hydrophone response is calculated from measurements of the source transducer and hydrophone-voltage waveforms. The frequency responses of both needle-type and spot-poled membrane polymer hydrophones were measured using this technique. The spot-poled membrane hydrophones had −3-dB bandwidths extending below 0.2 MHz, the lower limit for the calibration technique. The needle-type hydrophones studied, however, all exhibited a response roll-off of greater than 3 dB in the frequency range studied. Therefore, given the above bandwidth criterion as a function of diagnostic pulse-center frequency, the sensitivity to at least 0.2 MHz should be established for diagnostic-use hydrophones, because a uniform response below 1 MHz cannot be assumed.