ABSTRACT
Ambient noise is the prevailing, unwanted background of sound at a particular location in the ocean at a given time of the year. It does not include transient sounds such as the noise of nearby ships and marine organisms, or of passing rain showers. It is the background of noise, typical of the time, location and depth against which a signal must be detected. Ambient noise also excludes all forms of self-noise, such as the noise of current flow around the sonar. Thus, ambient noise is the residual sound level remaining after all identifiable, transient-noise sources have been removed (Urick, 1983: chapter 7). Levels of noise sources are commonly specified as the root-mean-square (rms) sound pressure level in a 1-Hz band (referred to as “spectrum level”). Different units indicate how the noise levels were derived (see Pierce, 1989: chapter 2, for an in-depth discussion). Units commonly encountered in the technical literature include: spectrum level, dB re 1µPa; dB re 1µPa2 Hz−1; and dB re µPaHz1/2 (where “re” is an abbreviation for “relative to”). The discussion by Carey (1995) regarding the potential for confusion when using decibels to estimate spectral quantities is relevant here.
Figure 6.1 is a hypothetical example of the spectrum of ambient noise in the open ocean (Urick, 1983: chapter 7). This spectrum is composed of segments of different slope, each exhibiting a different behavior. A number of frequency bands in the spectrum can be associated with readily identifiable noise sources. Five such frequency bands are indicated in Figure 6.1. Band I, lying below 1Hz, is associated with noise of hydrostatic origin (tides and waves) or with seismic activity (Kibblewhite and Ewans, 1985). Valid measurements in this band (and in Band II) are extremely difficult to make because of the self-noise of the hydrophone and its supporting structure caused by currents (e.g. cable strumming). Band II is characterized by a spectral slope of −8 to −10dBoctave−1 (about −30dBdecade−1). The most probable source of noise in deep water appears to be oceanic turbulence.
In Band III, the ambient noise spectrum flattens out and the noise appears to be dominated by distant shipping traffic. Band IV contains the Knudsen spectra (Knudsen et al., 1948) having a slope of −5 to −6dBoctave−1 (about −17 dBdecade−1) in which the noise originates at the sea surface near the point of measurement. Band V is dominated by thermal noise originating in the molecular motion of the sea and is uniquely characterized by a positive spectrum having a slope of +6dBoctave−1.
