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Broadband reference hydrophones for ultrasound

For a safe application of this diagnostic technique, the sound generated by medical  ultrasound equipment must be known. For the calibration of sound  pressure sensors (hydrophones), a novel reference hydrophone has been  developed at PTB which – due to its high measuring bandwidth – is particularly   suited to measure the short sound pulses of medical devices.

Core piece of the new reference hydrophone is the thin foil supported by the red ring with an electrode pair (0.2 mm) in the centre.

Correct measurement of the short – and thus – broadband pulses which are emitted by diagnostic transducer heads requires hydrophones with a transfer  function which is, in the ideal case, as flat as possible over a large frequency range. Non-ideal transfer functions can be compensated for with the so-called impulse deconvolution method which will be a part of future standards for hydrophone measurements. This requires, however, calibration data whose magnitude and phase are known in a large frequency range. This is why several bodies are working intensively on an extension of the frequency range of hydrophone calibration.

At PTB, special broadband reference membrane hydrophones have been developed and manufactured. They are characterised by a small diameter of the receiver electrode pair (200 μm) which is applied to the upper and lower side of a thin piezoelectric foil of only 9 μm in thickness. In combination with a particular broadband differential amplifier electronics, this construction results in a significantly larger useable frequency range than obtainable from all currently commercially available hydrophones. Up to 40 MHz, the frequency response is extremely flat, and the thickness-mode resonance typical of hydrophones does not occur until approx. 105 MHz for this foil thickness. However, for calibrations beyond 70 MHz, no primary standards have been available so far worldwide, so calibration data here rely on model assumptions or extrapolations.

An important auxiliary means easy to use is now available for the extension and verification of calibration procedures. Hydrophones of this instrument type can, of course, also be advantageously used directly for exposimetry on the medical product, in addition to their use as a secondary standard.

Contact at PTB:

Phone: +49-531-592-0