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A Few Specific Sources of Noise
Cavitation—the tearing apart of a fluid when the negative pressure (dilatation) becomes sufficiently large. This process causes the formation of bubbles and the radiation of sound (Urick, 1975). Cavitation imposes an upper limit to the maximum acoustic power output of a sonar system. For example, at 3 kHz at shallow depths, Urick indicates that the cavitation threshold is slightly more than 1 atm = 1.013 bar = 1.013 × 1011 µPa = 220 dB re 1 µPa. Some cavitation can be tolerated so that the maximum levels can be a factor of 2 to 3 greater than the threshold, suggesting a maximum level of slightly more than 230 dB re 1 µPa. One reason for constructing arrays of sources is to create higher equivalent source levels along the array main beam in the far field than could be achieved by a compact source because of the limitations imposed by cavitation.
Microseisms—naturally occurring noise created by the nonlinear interaction of oppositely propagating ocean surface waves. Oppositely propagating waves give rise to a standing wave pattern that radiates sound with twice the frequency of that of the interacting surface waves. Microseisms are the dominant natural noise source in the space- and time-averaged ocean noise spectra below 5-10 Hz. Seismologists created the term microseisms because they also are the dominant source of noise in high-quality, on-land seismometer measurements; however, their source mechanism is unrelated to seismic processes in the solid earth. The Wenz curves (Plate 1) list “Seismic Background” above “Surface Waves—Second-Order Pressure Effects,” but it is now known that the latter are the dominant source of prevailing ocean noise. Earthquakes and other tectonic processes contribute only intermittently.
Sonic Boom—a wave that is generated continuously by an object traveling faster than the speed of sound in the atmosphere. A sonic boom starts as a nonlinear shock wave with discontinuous jumps in pressure and fluid density. Because of dissipation and absorption, it eventually evolves into a linear acoustic wave at some distance from the source region. Its temporal character depends on the shape and size of the supersonic object, its speed, and its trajectory. The leading wavefront of a sonic boom is much like the bow wave of a surface ship, which is being “towed” along by the moving object. The sonic boom is a transient with respect to a receiver not traveling with the same velocity as the supersonic object creating the boom.
Thermal Noise—the pressure fluctuations associated with the thermal agitation of the ocean medium itself. It is what is left over when all other noise sources are removed and so provides the lowest bound for noise levels in the ocean. Thermal noise dictates the shape and level of ambient noise spectra above 50-100 kHz (depending on sea state; see Plate 1).