A simplified technique for calibration of the response of a miniature hydrophone as a function of frequency was evaluated experimentally for reproducibility and for its uncertainty in calibration of hydrophones intended for intensity measurements in broadband medical ultrasound fields. In this hydrophone calibration technique, the known total acoustic power passing through a plane lying normal to an ultrasound beam is compared with the surface integral of the intensity over that plane. Broadband source transducers calibrated by the National Bureau of Standards were driven in a narrow-band tone-burst mode to calibrate the hydrophone at multiple discrete frequencies. A least squares polynomial fit to the discrete values provided a continuous estimate curve of the hydrophone intensity response factor (Kf2) as a function of frequency. Good reproducibility of this multipoint technique was demonstrated by an independent, but similar, calibration of the hydrophone by the Bureau of Radiological Health. This hydrophone was then employed to measure acoustic intensities and power in broadband fields of typical pulse echo systems. If the variation in Kf2 is adequately small over the frequency spectrum of the measured pulse and the wave pressure is not so high as to be producing large nonlinear effects, a ±30% uncertainty in SPTA intensity can be achieved by use of a single Kf2 value determined either at the center frequency of the pulse or as the mean of the Kf2 values at the center and half-power frequencies of the spectrum of the pulse. In those cases where the variability of Kf2 exceeds ±1.5 dB over a one-octave range centered about the center frequency of the pulse’s spectrum, use of an effective Kf2 value determined by a deconvolution technique is recommended.
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Jones et al. (1981) studied this question.