Ultrasound waves are generated by applying a short voltage pulse, approximately less than 5Șs, across a
Piezoelectric (PZT) material. The voltage pulse generates a very wide frequency band relative to the PZT material,
which causes the PZT material to mechanically vibrate at its designed resonant frequency. The generated ultrasound
energy is then transferred from the PZT material into the surrounding medium. For example, if the PZT material is
formed into a thin disc shape, then most of the generated ultrasound energy will travel from the upper and lower
surfaces of the disc. The PZT disc is polarized by the manufacturer during the production process, such that the
positive and negative poles are electrostatically coated with a thin layer of silver deposit to allow for easy electrical
connections. The negative pole can be grounded to the transducer’s metallic packaging to facilitate simple
packaging, as seen in Figure 1 (a). When a voltage pulse (Vpulse) is applied across the poles of the PZT material, the
mechanical vibration is stretched over time as it slowly decreases in magnitude; a behavior similar to the ringingeffect of a metal bell when stroked. For most applications, the ringing-effect is an undesirable outcome since the
reflected ultrasound can arrive before the ringing is adequately damped if the material is thin. Therefore, techniques
such as dampening-material, or backing-material, is applied to the back side of the PZT disc to greatly reduce the
number of resonant cycles produced. The backing-material, however, also dampens the signal amplitude of the
generated ultrasound wave, which is an undesired outcome. With adequate dampening, or smaller numbers of cycles
produced, we can achieve higher measurement resolution in order to test thinner material, or distinguish finer details
of the material under test.