Marine Physical Laboratory, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0238, USA.
J Acoust Soc Am. 2012 Apr;131(4):2643-57. doi: 10.1121/1.3688506.
Observations of ambient noise in the ocean are generally band limited, because of the natural spectral shape of the noise or the restricted bandwidth of the detection system. Either way, the noise may be regarded as white noise to which a band-limiting filter has been applied. An analysis of the two-point cross-correlation function of such filtered noise is presented for two cases, isotropic and surface-generated noise. The most pronounced effects occur with high-pass and bandpass filters when the low-frequency cut-off falls well above the first few zeros in the coherence function. In this situation, the sensor separation is very many times the longest acoustic wavelength (associated with the lowest frequency) in the passband. The filtering then produces sharp pulses at correlation delays equal to the numerical value of the acoustic travel time between the sensors. Although these pulses are narrow, they have a finite width, within which a fine structure appears in the form of multiple rapid oscillations, due to the differentiating action of the filter. The number of such oscillations increases as the low-frequency roll-off of the filter becomes steeper. This fine structure is evident in several recently published experimental determinations of the cross-correlation function of band-limited ocean ambient noise.
海洋环境噪声的观测通常是带限的，这是由于噪声的自然谱形或探测系统的带宽限制。无论哪种情况，噪声都可以视为已应用带限滤波器的白噪声。本文针对各向同性噪声和表面产生噪声这两种情况，对这种滤波噪声的两点互相关函数进行了分析。当低频截止频率远高于相干函数中的前几个零点时，高通和带通滤波器会产生最显著的影响。在这种情况下，传感器之间的距离是通带中最长声波长（与最低频率相关）的许多倍。然后，滤波会在相关延迟等于传感器之间声传播时间的数值处产生尖锐的脉冲。尽管这些脉冲很窄，但它们具有一定的宽度，在该宽度内，由于滤波器的微分作用，会出现多次快速振荡的精细结构。随着滤波器的低频滚降变得更加陡峭，这种精细结构的数量会增加。在最近发表的几项关于带限海洋环境噪声互相关函数的实验测定中，都可以明显看出这种精细结构。
