Non-contact transducers generally exhibit

• A. Lower signal strength and lower SNR
• B. Higher signal strength and higher SNR
• C. No change in signal strength or SNR
• D. Lower signal strength but higher SNR

Answer: (A)

Non-contact transducers face a big energy transfer challenge because of the air gap between the transducer and the test piece. Air and solids have very different acoustic impedances, so most of the ultrasonic energy is reflected at the boundary rather than transmitted into the material. Only a small portion gets into the piece, so the transmitted signal is weaker. That weaker energy also produces weaker echoes, which lowers the signal-to-noise ratio since the noise level stays roughly the same while the signal drops. Because of this impedance mismatch and the resulting attenuation, non-contact setups typically exhibit both lower signal strength and lower SNR, compared with properly coupled contact transducers.