The accompanying modulation feature of ship-radiating line spectrums due to the dynamic fluctuating ocean surface

Aiming at the accompanying modulation of ship-radiating line spectrums discovered in experiments,a continuous wave propagating model is established by using parabolic-equation method.The effects of wind speed,sourcereceiver range and source depth on the frequency interval and intensity of accompanying modulation line spectrum are revealed.The numerical simulation shows that the frequency interval from the carrier wave is determined by the swell wave owning a stable frequency.The intensity of the accompanying modulation line spectrum increases with the wind speed.The variation trend of the intensity of the accompanying line spectrum with range is basically the same as that of the carrier line spectrum.The intensities of the up-shifted and down-shifted line spectrum are in the same level and about 10 dB lower than the carrier component when the average source depth is within 3 m.However,the intensities of the two components are quite different and about 20 dB lower than the carrier component when the source depth is deeper except the depth where the transmission loss is high.The long window time-frequency analysis of ship-radiating noise data shows that the frequency interval from the carrier wave is about 0.1 Hz,and the intensities of the accompanying modulation line spectrums are about 10 dB lower than the carrier component.The accompanying modulation features of ship-radiating noise caused by dynamic fluctuating ocean surface are of great significance to the underwater target classification.