Analysis on spatial difference of ocean ambient noise spectrum in the northern South China Sea

To study the spatial difference of ocean ambient noise in offshore China and reveal its forming reasons, the difference between the noise level (NL) of two stations is compared based on long-term observation data collected from two stations in the same season. Considering the influence of underwater acoustic channels and the characteristics of noise sources, the transmission loss level (TL) from the port ship noise sources to the measuring station is analyzed and showed a strong correlation between the transmission loss difference (TLD) and noise level difference (NLD) at the band of 50~500 Hz. At the same time, the analysis of ship noise source level (SL) presents a difference about 20 dB between the two ports, corresponding to the variation of the ship density before and during the fishing moratorium. These prove at this frequency band the ambient noise is closely related to ship noise sources, and the noise difference between the two stations results from the transmission loss and the ship noise sources.