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Ultrasonics

Working Group 1.62

Hydrophones

Hydrophones are used in all areas of ultrasonic measuring techniques to measure sound pressure and sound pressure variations in space and time. The checking of relevant acoustic data of medical ultrasonic equipment in particular has considerably gained in importance after the adoption of the IEC 61157 measuring instructions and their application as a national standard.

A hydrophone is an acousto - electrical transducer. It converts the alternating sound pressure acting on the hydrophone into a proportional alternating voltage.The reception sensitivity M = Uo / p must be determined by calibration of the arrangement. Above all piezo-electric materials are used as acousto-electrical transducers, but also procedures making use of other physical effects.

Two hydrophone types are used whose designs differ substantially. The classical type is the needle-type hydrophone. In the past, it was equipped with a transducer element made of piezoceramics; today it is in the majority of cases provided with piezo-foil (PVDF and Co-polymers). Piezoelectric material (ceramics or foil) is glued on a "needle point", the needle (damping element) being frequently used as electrode supply lead. The damping element is to dampen the incident sound wave to such an extent that the effect of the reflection from the element's rear side on the piezoelectric material can be neglected.

An alternative design of a hydrophone was possible only after piezo-foils had become available: the membrane-type hydrophone. Here, the piezo-foil is put on a frame, and only the area provided for sound reception is polarized and equipped with electrodes. This construction provides a hydrophone type which in fact comes very close to the "ideal" hydrophone at low frequencies. At these frequencies, the thin foil (25 µm) is almost transparent to sound waves. Commercial hydrophone diameters (sensitive area) are between 1 mm and 0,075 mm, the foils being between 25 mm and 9 mm thick. The effective frequency range follows from the design and the foil thickness.

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