Phase velocity evaluation of two-layered gypsums by using wavelet transform

A non-destructive method to evaluate the phase velocity in two-layered gypsums using the wavelet transform of surface waves is proposed. Model tests were performed using gypsum composed of two layers. A piezoelectric actuator with frequencies ranging from 150 Hz to 5 kHz was selected as a harmonic source. The surface waves were measured with two accelerometers. Wavelet transform analyses were performed to obtain the dispersion curves from the measured responses. The experimental results show that the near-field effects can be neglected if the distance between the two receivers is greater than three times the wavelength. A simple inversion method using a weighted factor based on the normal distribution is proposed. The inversion shows that the predicted phase velocity agrees reasonably well with the measured phase velocity when the wavelength influence factor is 0. 2. The propagation depth of surface waves is from 0. 41 to 0. 67 times the wavelength. The range of wavelength varying with the phase velocity in a dispersion curve matches well with that estimated using the inversion method. This study shows that the simple wavelet transform can effectively produce the phase velocity of the two-layered gypsums.