COUPLANT

As stated earlier the amount of sound reflected
at an interface is governed
by the relative acoustic
impedances of the two mediums across the interface.

Air has a very low acoustic impedance compared to Perspex,
and as a result the majority of the sound will be reflected back into the probe if air is present in between the probe and the test piece.

Therefore, when ultrasonic testing
it is important to exclude
all air and this is generally achieved
by using oil, grease or water as “Couplant”.

In a diving mode sea water will act as the couplant. Some typical values for acoustic
impedance are given below