Effects of diffraction on the sensitivity of needle-type ultrasonic receivers

Abstract

The low-frequency sensitivity of piezoelectric receivers usually is assumed to be represented accurately by the −3-dB cutoff frequency fco=(2πRiCi)−1, where Ri is the loading (e.g., amplifier) resistance and Ci is the total capacitance. For typical needle-type hydrophones such as used in medical ultrasound exposimetry, fco is less than 50 kHz. However, theoretical studies have shown that diffraction effects at the needle tip can cause a low-frequency rolloff in sensitivity at frequencies much higher than that predicted by this simple electrical model. To examine this effect, broadband frequency response measurements of several needle-type hydrophones were made in the frequency range 0.2–2 MHz. The active sensor material was polyvinylidene fluoride, and needle diameters ranged from a few tenths of a millimeter to approximately 1 mm. In all cases the sensitivity decreased with decreasing frequency, with the −3 dB points all lying above 400 kHz. Such behavior calls the use of these devices into question when accurate knowledge of the pressure waveform is required, particularly with regard to measuring the peak rarefactional pressure in pulsed waveforms displaying significant finite amplitude distortion.