A new method of hydrophone calibration using the KZK time domain finite difference algorithm

This work describes a novel absolute calibration technique that is capable to provide the frequency response of hydrophones over a wide, 100 MHz bandwidth. This bandwidth is significantly larger than the one in which the hydrophones are routinely calibrated at present. The developed technique takes into account nonlinear propagation phenomena. More specifically, the hydrophone calibration was determined by comparing the differences between the experimental results and those obtained from modeling finite amplitude wave propagation. The simulation model was derived from the time domain algorithm that solves the nonlinear KZK equation of acoustic wave propagation, and accounts for the effects of medium nonlinearity, diffraction and absorption. Focused circular transducers were used as acoustic sources in all the experiments and simulations. The KZK time-domain algorithm was initially validated using the known hydrophone calibration in the frequency range from 2 to 20 MHz. The calibration of several hydrophones was then carried out from 5 MHz to 100 MHz. The calibration results were independently validated and the overall uncertainty determined. The results were typically within the measurement uncertainty of the independent calibrations from 5 to 40 MHz. These calibrations provided the information needed to develop a reliable method that allows a rapid determination of the ultrasound hydrophone compliance with the specification recommendations. The recommendations were published by the American Institute of Ultrasound in Medicine and the National Electronic Manufacturer's Association (AIUM/NEMA) to improve assessment of potential bioeffects by calculation of the Mechanical (MI) and Thermal Indices (TI). The developed technique is also capable to provide phase information of the hydrophone in the considered frequency range. Combined phase and amplitude spectrum information was used to remove the influence of hydrophone's frequency response on the measured acoustic field. The results of the measurements obtained using limited bandwidth hydrophones, i.e. hydrophones that did not comply with the newest AIUM/NEMA requirements, were successfully improved. Thus, the developed technique can provide acoustic pressure-time waveforms which are virtually unbiased by the hydrophone response. Fundamental limitations of the technique were also determined and are discussed together with the suggestions for further refining of the developed approach.