Term Paper on "Kinsler Defines Acoustics"

Term Paper 5 pages (1423 words) Sources: 1+

[EXCERPT] . . . .

(Pacific). In case of rough or "back of the envelope" SNR calculations, ambient noise lvel (NL) is deducted from the sound intensity level with this equation:

SNR = SIL - NL

When the reading is higher than 0 dB (decibels), the signal can be detected and separated from the background noise. If less than 0 dB, it is inaudible. (Pacific) Detection of signals in noise is called signal processing.

The basic concept is more complex in actual application. First, the ambient noise field (NL) of the ocean is very variable according to time, location and frequency. Effects can be seasonal, (Pacific) such as the presence or absence of a storm track that brings in loud wave noise by the hour, as the passing of a hip. Second, the properties of propagation of water column vary widely according to location and depend on location, physical oceanographic properties, local bathymetry and bottom properties. On this account, more sophisticated numerical models have been developed in the last many decades in providing improved predictions of acoustic environmental properties. And lastly, natural sounds from sources such as marine mammals and earthquakes have important variability I the source level, which makes the calculation of signal-to-noise ratio even harder to perform.

The SOFAR or deep sound channel has its significance in underwater acoustics. SOFAR means Sound Fixing And Ranging. The acronym was formed when a channel in the deep ocean was discovered within which the acoustic energy from a small explosive charge could. travel long distances. Small explosives are usually deployed there by downed aviators. Hydrophones may be used to locate the source of the charge in searching for and rescuing pilots who have fallen out into the sea.

Channeling of wave sound happens because there is a minimum in the vertical sound speed profile in the ocean that is caused by changes in the density of that water column. (Pacific). This density is affected by water temperature, depth pressure and the ocean's salt concentration level. The speed of sound in water or the ocean changes because of changes in temperature and pressure and salinity only has a minor effect. (Pacific)

The speed of sound goes down with the temperature and then increases with that of depth pressure.

The minimum speed of sound at the channel axis is the outcome of higher temperatures on the surface of the ocean as well as higher pressures towards the ocean's bottom. Water temperature on the surface remains warm. But as depth increases, temperature and sound speeds decrease.

At a bottom of the permanent thermocline or below it, water temperature is uniform. Here, the pressure of the water column due to the depth takes over. The sound speeds then increase on account of increase in pressure. (Pacific) The deep sound channel axis is between 600 and 1,200 m below the sea surface at low and middle latitudes. This is deepest in the subtropics and come to the surface in high latitudes, where sound propagates in the surface layer. (Pacific)

Sound waves can be "trapped" in the deep sound channel and propagate in long distances. This is because these waves go through little attenuation beyond that, and this is due to geometric scattering or spread as well as minor volume scattering in water.

This becomes simple to think if we imagine the water column as a layer cake with different densities of water piled on top of one another. Sound waves refract as they cross the layers at different densities. The refraction of these sound waves from higher velocities above and below the sound channel axis reflects the sound back towards that axis. Sound energy is thus refracted towards the axis of the sound channel away from the surface and the bottom of the water or ocean.

Sound propagation in the deep sound channel does not attenuate as quickly as bottom or surface-interacting paths because propagating waves do not interact with either the sea surface or sea floor.

Bibliography

Acoustical Society of America, The. Acoustics and You, 2002

Federation of American Scientists (FSA). Underwater Acoustics. Military Analysis

3. Ford, John. An Ocean of Sound: an Exploration of Underwater Acoustics, 2001

4. Kinsler, L. E, et al. What is acoustics? Fundamentals of Acoustics. third edition.

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