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

3.9: Electromagnetic Acoustic Transducers (EMATs)
As discussed on the previous page, one of the essential features of ultrasonic
measurements is mechanical coupling between the transducer and the solid
whose properties or structure are to be studied. This coupling is generally
achieved in one of two ways. In immersion measurements, energy is coupled
between the transducer and sample by placing both objects in a tank filled
with a fluid, generally water. In contact measurements, the transducer is
pressed directly against the sample, and coupling is achieved by the
presence of a thin fluid layer inserted between the two. When shear waves
are to be transmitted, the fluid is generally selected to have a significant
viscosity.