couplant is a material (usually liquid) that facilitates the transmission
of ultrasonic energy from the transducer into the test specimen.
Couplant is generally necessary because the acoustic impedance
mismatch between air and solids (i.e. such as the test specimen) is
large. Therefore, nearly all of the energy is reflected and
very little is transmitted into the test material. The couplant
displaces the air and makes it possible to get more sound energy
into the test specimen so that a usable ultrasonic signal can
be obtained. In contact ultrasonic testing a thin film of oil,
glycerin or water is generally used between the transducer and
the test surface.
scanning over the part or making precise measurements, an immersion
technique is often used. In immersion ultrasonic testing both
the transducer and the part are immersed in the couplant, which
is typically water. This method of coupling makes it easier to
maintain consistent coupling while moving and manipulating the
transducer and/or the part.