Aviation Electronics Technician 1 - Historic Naval Ships Association

A similar case often arises in connection withsonar. Sound waves often strike small objects in thesea, such as fish or air bubbles. These small objectscause the waves to scatter. Each object produces asmall echo, which may return to the transducer. Thereflections of sound waves from the sea surface andthe sea bottom also create echoes. The combinedechoes from all these disturbances are called“reverberations.” Since they are reflected fromvarious ranges, the y seem to be a continuous sound.Reverberations from nearby points may be so loudthat they interfere with the returning echo from atarget.There are three main types of reverberation, orbackward scattering of the sound wave. They are asfollows:1. There is reverberation from the mass of water.Causes of this type of reverberation are notcompletely known, although fish and other objectscontribute to it.2. There is reverberation from the surface. Thisis most intense immediately after the sonartransmission; it then decreases rapidly. The intensityof the reverberation increases markedly withincreased roughness of the sea surface.3. There is reverberation from the bottom. Inshallow water, this type of reverberation is the mostintense of the three, especially over rocky and roughbottoms.DivergenceJust as the beam from a searchlight spreads outand becomes weaker with distance, so does sound.The farther the target is from the sonar transducer, theweaker the sound waves will be when they reach it.This is known as spreading or divergence.RefractionIf there were no temperature differences in thewater, the sound beam would travel in a straight line.This happens because the speed of sound would beroughly the same at all depths. The sound beamwould spread and become weaker at a relativelyconstant rate.Unfortunately, the speed of sound is not constantat all depths. The speed of sound in seawaterincreases from 4,700 feet per second to 5,300 feet persecond as the temperature increases from 30°F to85°F. Salinity and pressure effects on sound speedare not as extreme as the large effects produced bytemperature changes in the sea. Because of thevarying temperature differences in the sea, the soundbeam does not travel in a straight line, but followscurved paths. This results in bending, splitting, anddistorting of the sound beam.When the sound beam is bent, it is said to berefracted. A sound beam is refracted when it passesfrom a medium of a given temperature into a mediumwith a different temperature. An example of this is asound beam traveling from an area of warm water intoan layer of cold water. The sound beam will bendaway from the area of higher temperature (highersound velocity) toward the lower temperature (lowersound velocity).As a result of refraction, the range at which asubmarine can be detected by sound may be reducedto less than 1,000 yards, and this range may changesharply with changing submarine depth.Speed of the Sound BeamAs mentioned previously, sound travels muchfaster in seawater than in the atmosphere. Near sealevel, sound travels through the atmosphere atapproximately 1,080 feet per second. In seawater,that same sound beam will travel at approximately4,700 to 5,300 feet per second.There are three main characteristics of seawaterthat affect the speed of the sound wave travelingthrough it. These characteristics are as follows:1. Salinity (the amount of salt in the water)2. Pressure (caused by increased depth)3. Temperature (the effect of which is calculatedin terms of slopes, or gradients)There is a high mineral content in seawater. Thedensity of seawater is approximately 64 pounds percubic foot, while fresh water has a density of about62.4 pounds per cubic foot. This difference is causedby the salt in the seawater. Salt content in seawater iscalled the salinity of water.The overall effect of increasing the salinity is anincrease in the speed of the sound beam in the water.This means that as the sound travels through water ofvarying salinity, it travels faster through the waterwith more salt content. Such a change in salinity isconsiderable at the mouth of a river emptying into thesea. Elsewhere, the difference in salinity is too small4-3