Ultrasonic Testing Level 1 Practice Test

Non-contact transducers face an air gap between the transducer and the test surface, which creates a large impedance mismatch at the boundary. Most of the acoustic energy is reflected at this interface, so only a small portion enters the material. That results in weaker received echoes from flaws, i.e., lower signal strength. Since the noise level of the system stays roughly the same, the weaker signal leads to a lower signal-to-noise ratio. Piezoelectric contact transducers use a coupling medium with impedance closer to the material, allowing more energy to transmit and better SNR. So the statement describing decreased signal strength and decreased SNR best captures the typical behavior of non-contact transducers.