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 octave s 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-cente r frequency is needed for accurate (error approximate to 5%) measurement of the peak rarefactional pressure. Since at present it is not common for man ufacturers of medical-use hydrophones to provide sensitivity information be low 1-2 MHz, a study was undertaken to determine these low-frequency sensit ivities. 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 tra nsducer and hydrophone-voltage waveforms. The frequency responses of both n eedle-type and spot-poled membrane polymer hydrophones were measured using this technique. The spot-poled membrane hydrophones had -3-dB bandwidths ex tending below 0.2 MHz, the lower limit for the calibration technique. The n eedle-type hydrophones studied, however, all exhibited a response roll-off of greater than 3 dB in the frequency range studied. Therefore, given the a bove 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 I MHz cannot be assumed. [S0001-4966(99)01002-4].