Estimation of Nonlinear Acoustic Parameter Using Bispectral Analysis.

Abstract

The fact that material degradation can be calculated by measuring nonlinear acoustic effect has been proposed by previous studies. They reported that the most conventional method to measure nonlinear acoustic effect is to measure the absolute magnitude of the fundamental and 2nd order harmonic frequency component in the propagated ultrasonic wave. For this aim, power spectral analysis technique has been used widely. However, there is a fatal disadvantage in using power spectrum in that the gaussian additive noise superimposed in the wave signal remains in the power spectrum domain, Moreover, the magnitude of the 2nd order harmonic frequency component generated by nonlinear effect is so small that it may be suppressed by the noise remaining in the power spectrum domain. This paper proposes the bispectrum analysis method to overcome this problem. In order to evaluate the performance of our proposed bispectrum method, several sham signals with different SNR (Signal-to-Noise Ratio) were generated in a computer based on the nonlinear wave signal model, and the nonlinear parameter β was estimated in both methods of power spectrum and bispectrum to compare the accuracy. Simulation results showed that the bispectrum method was more stable and could estimate closer nonlinear parameter to the true value than the power spectrum method. Also, in order to verify that our method is applicable to the actual signal the bispectrum method was applied to the actual experimental signals that were obtained from the transmission test in several SS41 specimens with different degrees of degradation.