The thickness of the piezoelectric transducer (Tt
) and the velocity of sound in the given piezoelectric transducer
(ȣt
) determine the transducer’s resonant frequency (f), as described by equation (1) [2]. The wave length (Ȝ) of the
ultrasound signal is inversely proportional to the resonant frequency and dependant on the velocity of sound in the
material under test (ȣ
m), as shown in equation (2) [2]. In order to protect the exposed PZT surface from prolonged
physical damage, a wear-plate is interfaced to the front face of the PZT. In addition, the wear-plate has an important
role in acting as a matching-layer between the PZT material and the material under test. The recommended thickness
of the wear-plate is ¼ Ȝ calculated from equation 2, and has an optimum acoustic impedance Zw, expressed in units
of MRayls =106
Kg/(s.m2
), which lies between the acoustic impedance of the transducer Zt
and the acoustic
impedance of the material under test Zm, as described in equation (3)[2].