Two efficient methods for measuring hydrophone frequency response in the 100 kHz to 2 MHz range

Abstract

As new medical applications of ultrasound emerge with operating frequenciesin the hundreds of kilohertz to low megahertz region, it becomes more important to haveconvenient calibration methods for hydrophones in this frequency range. Furthermore,short diagnostic ultrasound pulses affected by finite amplitude distortion require that thehydrophone frequency response be known well below the center frequency. Nationalstandards laboratories can provide accurate calibration data at these frequencies, but thetwo methods now employed, laser interferometry and three-transducer reciprocity, are bothsingle-frequency techniques, and they can be time-consuming procedures. Therefore, twoefficient methods for generating a wideband acoustic pressure spectrum have beenimplemented to cover this frequency range. In one method a high-voltage pulse generatorwas used to excite a thick piezoelectric ceramic disk, producing a plane-wave acousticpressure transient <1 µs in duration with peak amplitude of about 40 kPa. In the othertechnique, time delay spectrometry (TDS), a purpose-built 1–3 piezoelectric compositesource transducer weakly focused at 20 cm was swept over the 0–2 MHz range. Itstransmitting voltage response at 1 MHz was 11 kPa/V. The broadband pulse technique hasthe advantage of being simpler to implement, but TDS has a much greater signal-to-noiseratio because of the frequency-swept narrowband filter employed.