Numerical and experimental pressure determination in the very near field of a piezoelectric transducer

Measurements in the very near field of piezoelectric
transducers are fundamental for many ultrasonic problems. In such cases also the
transducer vibrations should be known to perform mathematical models of radiated
beams. Acoustic pressure measurements near to the transducer surface can give
the necessary information. The pressure of the radiated wave at the transducer
surface corresponds to its normal vibration velocity multiplied by the [ampersand]rho;c
value of the medium. However, this is valid only for the central wave, when the
edge wave of the transducer can be ignored. On the other hand, pressure
measurements on and very near to the transducer surface are not possible because
of the voltage leakage between the electronic transmitter and the PVDF
hydrophone used in such measurements. By means of a numerical model, central and
edge waves were found for a plane PZT transducer 7.5mm in radius, with the
applied 2.7MHz voltage pulse composed of 3 cycles. Two types of boundary
conditions of Dirichlet and Neumann were considered showing a negligible
difference in the case of short pulses. Basing on numerical and experimental
results, practical conditions were determined which make it possible to carry
out pressure measurements in the very near field of the transducer, and hence to
determine the transducer vibrations which are important for modeling ultrasonic
pulse beams.