Quasi-dispersion of air-coupled ultrasonic signal for angle-dependent reception

In many practical applications of air-coupled ultrasonic inspection, the accurate direction of the wavefronts of the ultrasound in air is unknown. Thereby, the wave may reach the transducer at an oblique incidence angle. It may lead to misinterpretation of the waveform of the measured wave. This work investigated the quasi-dispersion of planar-wave signal affected by the receiving angle. An analytical model represented as a simplified model implementation was proposed to provide a theoretical prediction of the received signal. A numerical multiphysics model established by COMSOL Multiphysics® was used to demonstrate the signal reception by wavefield visualization. Simulational signals were in excellent agreement with the signals from analytical model. Finally, experiments with the similar configurations of transducers used in analytical and simulational models were conducted. The results confirmed that quasi-dispersion of the air-coupled ultrasonic signal which is highly dependent on incidence angle and wavelength.