ARE CURRENT HYDROPHONE LOW-FREQUENCY RESPONSE STANDARDS ACCEPTABLE FOR MEASURING MECHANICAL CAVITATION INDEXES/

The purpose of this paper is to determine the error introduced by ultr asonic hydrophones used to measure current or proposed Mechanical (MI) and Cavitation (CI) Indices, assuming that the hydrophones meet bandw idth specifications contained in US and IEC measurement standards. The se indices are based on the peak rarefactional pressure, p(r). Since t he portion of the pressure waveform where p(r) occurs is dominated by low frequency components, attention was placed on the low frequency hy drophone response specifications. Both simulated and actual diagnostic pressure pulses (with center frequency f(c)) were subjected to single -pole high-pass filtering for a range of -3 dB cut-off frequencies (f( a)). The error in the indices introduced by this filtration was evalua ted. At both f(a)=0.5 f(c) (the US requirement) and f(a)=0.86 f(c) (ca lculated from the IEC -6 dB bandwidth specification at 0.5 f(c)), resu lts showed that errors exceeding -30% could be expected. Furthermore, to reduce errors to less than 5%, the low frequency hydrophone respons e should extend at least an order of magnitude below the center freque ncy of the pressure wave. For example, for a 3.5 MHz transducer, the h ydrophone should have a lower cut-off frequency of less than 350 kHz, which at present constitutes a challenge because of the lack of commer ical hydrophones calibrated below 1 MHz.