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Piezoelectric transducer

The piezoelectric transducer may be used to produce ultrasonic waves. Basically it is a crystal which can change its thickness due to an induced strain when a potential difference is applied between the two faces (Figure 1(a)) or acquire a potential difference between the faces when a stress is placed between them (Figure 1(b)).

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).


 
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© Keith Gibbs