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Broadband Reference Hydrophones for Ultrasound

04.09.2006

Membrane hydrophones developed and manufactured at PTB allow extremely broadband acoustic pressure measurements and the comparison of extended hydrophone calibration techniques

Hydrophone measurements are fundamental for the determination and declaration of the acoustic output of medical ultrasound equipment. For the correct measurement of broadband pulses as emitted by diagnostic transducer heads, hydrophones providing a flat transfer function over a very large frequency range are necessary. Also for the impulse deconvolution method used for the compensation of non-ideal frequency responses, which can be expected to become part of future hydrophone measurement standards, calibration data are needed over a broad frequency range and in amplitude and phase. Therefore, efforts are currently undertaken by several calibration laboratories to extend the hydrophone calibration frequency ranges.

To be able to compare the different calibration techniques recently developed and extended at PTB with each other and internationally with methods of other metrology institutes, special broadband reference hydrophones with small receiver elements (electrode diameter: 200 µm) were constructed and manufactured in the Ultrasonics Working Group. By using a very thin piezoelectric foil (thickness: 9 µm) in combination with particularly broadband differential amplifier electronics, a significantly larger useable frequency range than obtainable from currently commercially available hydrophones could be achieved. The frequency response is extremely flat up to 40 MHz, the thickness-mode resonance is located at about 105 MHz, and the useable bandwidth can be expected to span even beyond 140 MHz (Figure 1). However, calibration data above 70 MHz are not primarily traceable up to now. Especially for the extension and verification of the calibration techniques, however, an important tool is available now. Besides the application as secondary standards, hydrophones of this type can of course be used directly for advanced exposure measurements on medical devices.

PTB broadband membrane hydrophone and respective complex-valued frequency response

Figure 1: PTB broadband membrane hydrophone and respective complex-valued frequency response

Contact person:

Volker Wilkens, FB. 1.6, AG 1.62, E-Mail: Volker.Wilkens@ptb.de