NOTE 1 Although some physiotherapy medical applications of medical ultrasound are developing which operate in the frequency range 40 kHz to 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 [4].

NOTE 2 Calibration methods for underwater acoustics hydrophones applicable in the frequency range from 200 Hz to 1 MHz are available in IEC 60565‑1[2], and for frequencies from 0,01 Hz to several kilohertz in IEC 60565‑2[3].

NOTE 3 Some hydrophones can have non-circular active elements, arising from slight deviations from a circular structure caused, for example, by electrode structure; or, conversely, the active elements can actually be squares. It is important in these cases to pay special attention to the directional response and to the effective radii of the active element through various axes of rotation.