Marine Physical Laboratory, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0238, USA.
J Acoust Soc Am. 2013 Aug;134(2):950-8. doi: 10.1121/1.4812270.
Acoustic attenuation in seawater usually has little effect on the spatial statistics of ambient noise in the ocean. This expectation does not hold, however, at higher frequencies, above 10 kHz, and extreme depths, in excess of 6 km, an operating regime that is within the capabilities of the most recently developed acoustic instrument platforms. To quantify the effects of attenuation, theoretical models for the vertical directionality and the spatial coherence of wind-generated ambient noise are developed in this paper, based on a uniform distribution of surface sources above a semi-infinite, homogeneous ocean. Since there are no bottom reflections, all the noise is downward traveling; and the angular width of the directional density function becomes progressively narrower with increasing frequency because sound from the more distant sources experiences greater attenuation than acoustic arrivals from overhead. This narrowing of the noise lobe modifies the spatial coherence, shifting the zeros in the horizontal (vertical) coherence function to higher (lower) frequencies. In addition, the attenuation modifies the amplitudes of the higher-order oscillations in the horizontal and vertical coherence functions, tending to suppress the former and enhance the latter. These effects are large enough to be detectable with the latest deep-diving sensor technology.
在海水中，声波衰减通常对海洋环境噪声的空间统计特性影响不大。然而，在高频（超过 10 kHz）和极端深度（超过 6 km）时，这种预期并不成立，而这些正是最新开发的声纳仪器平台的工作范围。为了量化衰减的影响，本文基于半无限、均匀海洋上方表面声源的均匀分布，针对风生环境噪声的垂直指向性和空间相干性，开发了理论模型。由于不存在海底反射，所有噪声都是向下传播的；随着频率的增加，指向性密度函数的角宽度逐渐变窄，因为来自更远声源的声音比来自头顶的声音经历更大的衰减。这种噪声瓣的变窄会改变空间相干性，将水平（垂直）相干函数中的零点移到更高（更低）的频率。此外，衰减会改变水平和垂直相干函数中高阶振荡的幅度，倾向于抑制前者并增强后者。这些影响之大，足以用最新的深海传感器技术检测到。
