Piezoelectret-based hydrophone: an alternative device for vibro-acoustography

Piezoelectric polymers are highly desirable for ultrasound applications since their low acoustic impedance is much closer to the impedances of air and water than those of traditional piezoceramics. However, the piezoelectric effect observed in these poled polymers is limited to a few dozen picocoulombs per newton and requires very low-noise amplification. A different class of polymeric material known as piezoelectrets, presents piezoelectric effect in the same order of magnitude as those found on piezoceramics. This new class of materials has been explored in a wide range of applications such as flexible keyboards and airborne ultrasound transducers. Based on these polymers, we present here a new transducer for vibro-acoustography (VA), which is an ultrasonic image technique employed in medical diagnosis or material analysis based on ultrasound scattered by a target object. A calibration was carried out using a standard hydrophone, which is normally employed in VA. The average sensitivity of the transducer in the continuous wave mode was 1.712 mV Pa−1(−182.7 dB re 1 V Pa−1) with a maximum sensitivity of 18.25 mV Pa−1 (−154.8 dB re 1 V Pa−1) at its resonance frequency around 40 kHz. Subsequently, measurements in the burst mode and of directional sensitivity were taken and are presented here together with VA images obtained from a chicken bone and a metal sphere.