Wavelet Transform Signal Processing Applied to Ultrasonics

The wavelet transform (WT) provides a new tool for processing transient signals and it can be considered as an alternative to the classical Short-Time Fourier Transform (STFT) for describing the time-frequency evolution of such signals. The purpose of the present paper is to provide an overview of the applicability of the wavelet transform to ultrasonic signal analysis. The WT will be briefly introduced with emphasis on the aspects which make it suitable for our applications. In particular the WT has been utilized to enhance the ultrasonic signal detection in presence of background noise, and the application of this technique for flaw detection will be presented. Improvements in detection were quantified using steel samples of different thickness and with simulated flaws. Furthermore, the self-adjusting window structure of the WT results in a time-scale representation of signals which can display the temporal variation of the spectral components with varying resolution. This property is extremely useful in the study of dispersive wave propagation, in particular for extracting the dispersion relation of the Pseudo-Lamb wave velocity in thin coatings. Numerical simulations and experimental results will be presented.