Broadband ferroelectret transducers

Ferroelectrets can be significantly improved in their piezoelectric activity by DC-biasing of the properly expanded cellular films. Experiments with polypropylene films have resulted in piezoelectric d₃₃-coefficients of up to 750 pC/N in the near-ultrasonic range. Stacks of up to 3 layers of ferroelectrets exhibit d₃₃-values in excess of 2000 pC/N. Ultrasonic senders built with such ferroelectrets generate high sound pressures, the levels depending on transducer size, applied AC-voltage, and distance from source. Unbiased receivers consisting of stacks of 4 or 5 films exhibit sensitivities of about 10 mV/Pa in the near ultrasonic range.