Study on Ultrasonic Signal De-noising Based on Lifting Wavelet Transform with Improved Threshold Function

In ultrasonic testing of welding flaws,the reliability and quality of tests are considerably affected by noise and spurious signals.Thus,signal de-noising and increasing of the signal to noise ratio are a key to successful application of ultrasonic NDT and NDE.In this paper,the theory of lifting wavelet transform is introduced,and the principle of wavelet threshold de-noising method is analysed.Then on the basis of soft and hard threshold function,a lifting wavelet de-noising method based on improved threshold function is put forward.Finally,the experimental studies of signal de-noising are carried out for ultrasonic echo-signals of welding flaws,and the performance of de-noising is compared and analysed from two aspects: signal to noise ratio and root-mean-square error.Results show that this method has overcome the disadvantages of soft and hard threshold methods in some degree,and obtained a better de-noising performance and a higher signal-noise ratio.Moreover,lifting wavelet transform is of flexible design and fast computation with a simple programme,thus,it has a good application prospect in real-time signal de-noising.