Understanding Acoustic Emission Testing- Reading 1 Part B-A

PZT:- If the p.d or the stress is changing the resulting effect also changes. Therefore if
an alternating potential difference with a frequency equal to the resonant frequency of
the crystal is applied across it the crystal will oscillate. A number of crystalline
materials show this effect – examples of these are quartz, barium titanate, lithium
sulphate, lead metaniobate, lead zirconate titanate (PZT) and polyvinylidine difluoride.
Piezoelectric transducers can act as both as a transmitter and a detector of vibrations.
However there are certain conditions. The crystal must stop vibrating as soon as the
alternating potential difference is switched off so that they can detect the reflected
pulse. For this reason a piece of damping material with an acoustic impedance the
same as that of the crystal is mounted at the back of the crystal. (See Figure 2).The
transducer is made with a crystal that has a thickness of one half of the
wavelength of the ultrasound, resonating at its fundamental frequency. A layer of
gel is needed between the transducer and the body to get good acoustic coupling (see
acoustic impedance).
Charlie Chong/ Fion Zhang
http://www.schoolphysics.co.uk/age16-19/Medical%20physics/text/Piezoelectric_transducer/index.html