2012 Volume 53 Issue 2 Pages 342-348
In order to monitor acoustic emission (AE) in a noisy environment, an AE monitoring system with a real-time noise reduction function using spectral subtraction (SS) was developed. First, the improvement in the S/N ratio and distortion of the waveform after the process were compare to those in ε-filter and wavelet shrinkage methods. The improvement in the S/N ratio of the waveform processed by SS constantly increased and was observed to be independent of the S/N ratio of the wave before processing. The distortion of the waveform processed by SS was less than the distortions of the waves processed using ε-filter and by wavelet shrinkage. Next, the effect of two parameters in the SS process — frame number (fn) and over-subtraction factor (α) — on the noise reduction performance was studied. The S/N ratio of the signals processed by the SS technique improved with decreasing fn and with increasing α. However, the processed waveform was distorted when α was large. It is necessary to set the value of fn to 16 or less and α to 5 or less so that SS showed an advantage to reduce noise with low waveform distortion.
Cylindrical wave AE signals produced by the Hsu-Nielsen source (pencil lead breaking) were monitored with the developed system in an environment with artificial noise. The developed system was able to acquire AE signals with a sampling frequency of up to 25 MHz. The noise was reduced and AE signals with S/N ratio of 0 dB before the process could be detected. The S/N ratio of the AE signal was improved by approximately 10 dB using the SS technique.
