Abstract: The composition of noises and their characters in ultrasonic echo signals are analyzed. Considering the distinctness of distribution between defects signals and noises, a time frequency neighborhood adaptive de-noise method based on wavelet packet transform is presented. According to the noise level and the variance in a neighborhood region, the smooth density to the center point value is adjusted adaptively, which approaches to an adaptive de-noise process. For no parameter or reference signal is needed, this method is robust. Experiments with both simulated signals and measured signals have been carried out, and the result show that this method can improve the signal noise ratio and the distinguishability between signals of different defects classes, and suppress energy attenuation as well as signal distortion efficiently.

Key words: Adaptive de-noise, Signal distortion, Time frequency neighborhood, Ultrasonic inspection