Feasibility of pulse-echo thickness measurements in air with a laterally displaced receiver

Air-coupled ultrasound (ACU) is an attractive option in non-destructive testing when the target of inspection is affected by the coupling liquid, or when the target is too big to be immersed in the coupling liquid. The challenge with ACU is the huge impedance mismatch between the air and the target. In pulse-echo measurements this mismatch causes a huge difference in level between the first reflection, from the air-target interface, and the tail, from multiple reflections inside the target. This can cause the first reflection to mask the tail signal. An analytical model for pulse-echo and through-transmission measurements has been developed by using the angular spectrum method. This model was verified against measurements in a water tank in a through-transmission setup. The model was then used to model echoes from a steel plate in air. These studies show that moving the receiving transducer laterally away from the acoustic axis of the transmitter will reduce the level of the first reflection more than the level of the tail of the signal. Hence, to avoid masking the signals from the interior of the plate by the strong reflection from the air-steel interface, the receiving transducer should be placed off the acoustic axis of the transmitter. Displacing the receiver from the transmitter acoustical axis represents a new challenge, as dispersion in the plate could potentially make the measured spectra difficult to interpret. Our studies show that the dispersion does not interefere with the resonance frequencies, as the spectral peaks are insensitive to the start of the time window the spectra are calculated from. This makes the computation of the thickness robust and simple, as the half wave resonance frequencies can be used.