Applies to- hydrophones employing piezoelectric sensor elements, designed to measure the pulsed and continuous-wave ultrasonic fields generated by ultrasonic equipment;- hydrophones used for measurements made in water and in the frequency range between 15 MHz and 40 MHz; - hydrophones with or without an integral amplifier;- hydrophones with a circular piezoelectrically active element.Specifies- relevant hydrophone characteristics;- methods of determining directional response and hydrophone sensitivity based on relative or comparative measurements;and describes- absolute hydrophone calibration methods.Recommendations and references to accepted literature are made for the various relative and absolute calibration methods in the frequency range covered by this International Standard.