Piezoelectric cellular PP films with enhanced performance for low frequency ultrasound

Piezoelectric cellular PP films are a relatively new type of electromechanical transducer material. They have strong application potential in air-borne ultrasonic transducers because of their low acoustic impedance and high piezoelectric activity. Cellular PP transducers with varied structure in the density range between 251 and 606 kg/m3 are processed. They show low acoustic impedances between 0.024 and 0.027 MRayls and strong piezoelectric activity with coefficients up to 800 pm/V. Acoustic measurements revealed a relative bandwidth of 35% at −6 dB (resonance frequency around 150 kHz) and an acoustic transmit sensitivity of 6 mPa/V at 30 cm far on principal axis for circular samples of 16 mm diameter. The vibratory pattern of the transducer films were determined below, at and above the resonance frequencies. At frequencies below the resonance, the transducer acts like an ideal piston. In addition, we demonstrate that the transducer samples can be operated at frequencies close to either the first or the second resonance. To do this, fabric samples were contactless inspected in through transmission mode at 140 kHz and 400 kHz.