Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

Matching Layer (Wear Plate)
For immersion, angle beam, and delay line transducers the wear plate has
the additional purpose of serving as an acoustic transformer between the
high acoustic impedance of the active element and the water, the wedge
or the delay line all of which are of lower acoustic impedance.
This is accomplished by selecting a
matching layer that is ¼ λ
wavelength thick and of the desired
acoustic impedance (the active
element is nominally ½ λ wavelength).
The choice of the wear surface
thickness is based upon the idea of
superposition that allows waves
generated by the active element to be
in phase with the wave reverberating
in the matching layer as shown in
Figure (4).