Generation of high power unipolar pulse with a piezocomposite transducer

We attempt to optimise the electrical excitation of a piezoelectric transducer in order to generate high power unipolar pressure pulses. In this method the electrical excitation voltage is a ramp with a duration equal to the time of flight T of ultrasonic waves through the transducer thickness. Moreover, the backing material and the transducer have equal acoustical impedances. In this case the resulting pressure waveform is a strictly positive pulse with a duration of 2/spl times/T. The choice of piezocomposite materials agrees well with the condition on the backing impedance. Indeed, the low acoustical impedance of such materials easily permits one to build appropriate backing materials. Low voltage (32 V) and high voltage trials (3.8 kV) were successively conducted using different transducers. In both cases a 1-3 piezocomposite material was used for the building of the transducers, which consisted of plates with a diameter of 20 mm. This material had an acoustical impedance of 9.7 MRayl. The transducer used in low voltage essays was 1.7 mm-thick, while a 2.4 mm-thick transducer was used for high voltage trials. A material with a 9 MRayl impedance backed both transducers. In both trials pressure waveforms were close to the predicted shape. During high voltage essays a 3 MPa pressure pulse was obtained. A simple inversion of the excitation voltage leads to the generation of strictly negative pressure pulses.