This International Standard is applicable to:- hydrophones used for measurements made in water and in the ultrasonic frequency range up to 40 MHz; NOTE - Although some physiotherapy medical applications of medical ultrasound are developing which operate in the frequency range 40 kHz - 100 kHz, the primary frequency range of diagnostic imaging remains above 2 MHz. It has recently been established that, even in the latter case, the hydrophone response at substantially lower frequencies can influence measurements made of key acoustic parameters [1]. hydrophones employing circular piezoelectric sensor elements, designed to measure the pulsed and continuous-wave