Underwater acoustics: principles and applications

Kinsler defines acoustics as the science of sound: the generation, transmission and reception of energy in the form of vibrational waves in matter. This covers a large range of disciplines and problems, including noise control, vibration and structural acoustics, and underwater acoustics. Underwater acoustics is using acoustic energy to detect objects in the oceans or sea beds - underwater - just like using radar to detect objects in the air. Acoustics systems guide underwater vessels, such as submarines, through ocean depths in the pursuit (Acoustical Society of America 2002).

We know that sound is transmitted in very long distances, even hundreds of miles, through the wave environment, which makes sound a very important tool for both commercial and military purposes. (ASA) Acoustics signals detect the presence and location of commercially useful fish, map the ocean floor to establish the safest paths for supertankers, explore the earth's geological formations and discover oil deposits in the ocean floor. (ASA) These are the basic reasons for advancing this new and very useful branch of acoustics, which is seen as growing in through the next decade in discovering and using the sea to man's full benefit. The study was slowed down since the end of the Cold War.

Propagation means spread, and sound waves propagate by means of alternating compressions and rarefactions, which in turn, are detected by the ears or a receiver as changes in pressure. (Pacific Marine Environmental Laboratory 2002). We know that the basic components of a sound wave are amplitude, wavelength, and frequency. Amplitude is proportional to the maximum distance a vibrating particle is displaced from the background environment, while the wavelength is that distance between two successive compressions, or the distance the wave travels in a single cycle of vibration (Pacific). And its frequency is the rate of oscillation of the vibration of the waves particles, such as from high to low and again to high, which is measured in cycles per second or in Hertz (Hz). Increased frequency is perceived by the human ear as a high or higher-pitched sound, and an increased amplitude as a louder sound. (Pacific). The average sounds we hear are those with frequencies between 20 and 20,000 Hz. Sounds below 20 Hz are infrasonic, while those above 20,000 Hz are ultrasonic. (Pacific) Vibrations propagate at different speeds: in water, the speed of a sound is approximately 1,500 meters per second; in the air, the speed is approximately 340 meters per second only. Proportionately, a 20-Hz water sound will be 75 meters long while a 20-Hz air sound, only 17 meters long. We know that water and other liquids have lower compressibility, which leads us to assume that a linear method of measurement is sufficient. But the ocean's temperature and salinity vary greatly, leaving no uniform medium for assumptions. Add to this the uncontrollable waves on the ocean's surface and the irregular rocks and non-solid deposits at the bottom of the ocean. It is incorrect to assume that the ocean is homogeneous, therefore, that is, that sound speed is the same everywhere. This is why research is ongoing on the variations of sound as it propagates through the random ocean.

Anything that occupies the volume of the ocean can spread or scatter sound. (Ford 2001) and this is intercepted by a device used in oceanography, called the Acoustic Doppler Current Profiler of ADCP. It detects currents according to the volume scattering strength of the sound. Sound speed by scattering from sediments, on the other hand, is determined by the roughness of the surface of the sediment and the density variations and the sediment's sound speed. (Ford). Scattering also occurs near the air and water interface, where both the roughness of the surface and the bubble near the interface may contribute to and determine scattering strength.

A particular acoustic signal is detectable in the ocean depends on a factor of the level of the signal of interest in relation to the background noise level of the ocean. (Pacific). This level of noise in the ocean environment is called ambient noise and is usually expressed as signal to noise ratio or SNR.

Using the SNR, any value greater than 1 means that the signal is detectable above the noise. A number below 1 means that the signal is lost or "buried" in the noise background. (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.