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Per Octave

1 10 100 1,000 10,000 100,000 Frequency (Hz)

Figure 6.1 General spectrum of deep-sea noise showing five frequency bands of differing spectral slopes. The slopes are given in decibels (dB) per octave of frequency. (Urick, 1983; Principles of Underwater Sound, 3rd edn; reproduced with permission of McGraw-Hill Publishing Company.)

1 10 100 1,000 10,000 100,000 Frequency (Hz)

Figure 6.1 General spectrum of deep-sea noise showing five frequency bands of differing spectral slopes. The slopes are given in decibels (dB) per octave of frequency. (Urick, 1983; Principles of Underwater Sound, 3rd edn; reproduced with permission of McGraw-Hill Publishing Company.)

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 etal., 1948) having a slope of —5 to —6 dB octave-1 (about —17 dB decade-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 +6 dB octave-1.

For some prediction purposes, average representative ambient-noise spectra for different conditions are adequate. Such average working curves are shown in Figure 6.2 for different conditions of shipping and wind speed. These curves are adapted from Wenz (1962); consequently, they are often referred to as Wenz curves. In the infrasonic region below 20 Hz, only a single line is drawn. The ambient-noise spectrum at any location and time is approximated by selecting the appropriate shipping and wind curves and connecting them at intermediate frequencies. When more than one source of noise is present (e.g. shipping and wind noise), the effective noise background is obtained by summing the intensities of the contributing sources. When

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