| The Line spectrum components in ship-radiated noise not only have the characteristics of high energy intensity and high stability,but also contain a large number of inherent characteristics of ship.The Line spectrum can also reflect the essential characteristics of target ship.Therefore,the accurate extraction of line spectrum has important practical value for target classification and recognition.The line spectrum of ship-radiated noise is mainly composed of low frequency line spectrum and medium-high frequency modulated characteristic line spectrum.The low frequency line spectrum is not beneficial to the high-quality feature extraction due to the interference of marine environment noise and some low frequency noise.However,The medium-high frequency line spectrum is affected by the blade frequency and axis frequency of propeller,forming the modulation characteristic line spectrum,which can avoid the environmental noise interference and realize the effective extraction of the modulation line spectrum and the feature quantity.Ship-radiated noise has obviously non-Gaussian and non-stationary characteristics.For the extraction of modulation characteristic line spectrum,the classical DEMON spectrum analysis method is consulted.However,it is found that the band-pass filtering and square detection methods used in this method have a large number of high frequency harmonic components and spectrum aliasing phenomena.Moreover,when extracting line spectrum by power spectrum analysis,the noise suppression ability is poor and the extracted line spectrum is poor.Harmonic characteristics are not obvious and other shortcomings.For finding the solutions of above problems,this paper deeply studies the wavelet analysis and high-order spectral analysis methods,and proposes an improved modulation feature extraction method,which mainly includes the following four aspects:1.The basic characteristics of ship-radiated noise and its background noise are studied in detail.The components of ship-radiated noise and its generating mechanism are discussed.The simulation model of ship-radiated noise is established according to its acoustic characteristics.2.The wavelet analysis method is studied in detail,and the complex wavelet demodulation method is used to filter and demodulate the ship-radiated noise signal at the same time,which realizes adaptive filtering and effectively suppresses the aliasing of high frequency harmonic components and spectrum.3.The principle of higher-order statistics is studied in detail.The principle and characteristics of bispectrum and 1(1/2)dimension spectrum are discussed in depth.The modulation envelope is analyzed by using 1(1/2)dimension spectrum with better performance,and a better line spectrum extraction effect is obtained.4.The wavelet demodulation and 1(1/2)-dimensional spectral analysis method are used to construct the ship-radiated noise modulation line spectrum extraction model with low SNR.The experimental analysis is carried out by using the simulated signal and the measured data respectively,and the validity and universality of the method are verified. |
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