Amplitude and phase calibration of hydrophones up to 70 MHz using broadband pulse excitation and an optical reference hydrophone

A substitution calibration technique for piezoelectric ultrasonic hydrophones is presented that uses an optical multilayer hydrophone as the reference receiver. Broadband nonlinearly distorted focused pulses are first measured with the reference hydrophone and then with the hydrophone to be calibrated. By Fourier transformation of the time wave forms and division of the frequency spectra, the complex-valued frequency response of the hydrophone under test is obtained in a broad frequency range in a very fast and efficient way and with high frequency resolution. The results obtained for a membrane hydrophone and a needle-type hydrophone are compared with those obtained by independent calibration techniques such as primary calibration using optical interferometry and secondary calibration using time-delay spectrometry, and good agreement is found. The calibration data obtained are apt to improve the results of ultrasound exposure measurements using broadband voltage-to-pressure conversion; This is demonstrated for standard pulse parameter determination from exemplar exposure measurements on a commercial diagnostic ultrasound machine. For the membrane hydrophone, the evaluation method commonly used leads to an overestimation of the positive peak pressure by up to 50%, an underestimation of the rarefactional peak pressure by up to 11% and an overestimation of the pulse intensity integral by up to 28%. (C) 2004 Acoustical Society of America.