See also: Fluid-flow principles Laplacian Navier-Stokes equation Wave equation Where p is the acoustic pressure and the differential operator ∇ 2, known in mathematics as the laplacian, is, in many underwater acoustics applications, given in either spherical or cylindrical coordinates. See also: Gulf Stream Kuroshio Oceanography The cold and warm eddies that are spun off from these currents are present in abundance and significantly affect acoustic propagation. Major ocean currents, such as the Gulf Stream and Kuroshio, have major effects on acoustics. Shallow water is even more variable due to tides, freshwater mixing, and interactions with the seafloor. All these layers depend on the season and the geographical location, and there is considerable local variation, depending on winds, cloud cover, atmospheric stability, and so on. Below that, most seawater reaches a constant temperature. Winds mix the upper layer, giving rise to a layer of water of approximately constant temperature, below which is a region called the thermocline. As the temperature of the upper ocean increases, so does the sound speed. Solar heating of the upper ocean has one of the most important effects on sound propagation. When agreement is not satisfactory, either the acoustic theory or the ocean model must be modified. But by limiting the theory to the most salient features, and through reasonable approximations, some solutions can be obtained that are in agreement with experimental results. It is impossible to include all of these effects in the equations of acoustics. The dynamics of the sea are very complex, and one driven by solar heating, winds, atmospheric forces, bathymetry, major ocean currents, turbulence, and the Earth's rotation. Generally the environmental parameter that dominates acoustic processes in oceans is the temperature, because it varies both spatially and temporally. The ratio of intensities after traversing a distance r is given by Eq. The absorption coefficient α is the exponential loss in intensity of an acoustic signal of a given frequency per meter of its path. Absorption has been measured in the laboratory and at sea ( Fig. ![]() At about 65 kHz magnesium sulfate dominates absorption, and boric acid is important near 1 kHz. The pressure variation caused by an acoustic wave changes the ionic balance and, during the passage of the pressure-varying acoustic field, it cannot return to the same equilibrium, and energy is given up. These molecules are normally in equilibrium with their ionic constituents. Over the frequency range from about 100 Hz (cycles per second) to 100 kHz, absorption is dominated by the reactions of two molecules, magnesium sulfate (MgSO 4) and boric acid. Absorption is the loss of energy due to internal causes, such as viscosity. Sound has a remarkably low loss of energy in seawater, and it is that property above all others that allows it to be used in research and application.
0 Comments
Leave a Reply. |