A similar case often arises in connection with sonar. Sound waves often strike small objects in the sea, such as fish or air bubbles. These small objects cause the waves to scatter. Each object produces a small echo, which may return to the transducer. The reflections of sound waves from the sea surface and the sea bottom also create echoes. The combined echoes from all these disturbances are called “reverberations.” Since they are reflected from various ranges, the y seem to be a continuous sound. Reverberations from nearby points may be so loud that they interfere with the returning echo from a target. There are three main types of reverberation, or backward scattering of the sound wave. They are as follows: 1. There is reverberation from the mass of water. Causes of this type of reverberation are not completely known, although fish and other objects contribute to it. 2. There is reverberation from the surface. This is most intense immediately after the sonar transmission; it then decreases rapidly. The intensity of the reverberation increases markedly with increased roughness of the sea surface. 3. There is reverberation from the bottom. In shallow water, this type of reverberation is the most intense of the three, especially over rocky and rough bottoms. Divergence Just as the beam from a searchlight spreads out and becomes weaker with distance, so does sound. The farther the target is from the sonar transducer, the weaker the sound waves will be when they reach it. This is known as spreading or divergence. Refraction If there were no temperature differences in the water, the sound beam would travel in a straight line. This happens because the speed of sound would be roughly the same at all depths. The sound beam would spread and become weaker at a relatively constant rate. Unfortunately, the speed of sound is not constant at all depths. The speed of sound in seawater increases from 4,700 feet per second to 5,300 feet per second as the temperature increases from 30°F to 85°F. Salinity and pressure effects on sound speed are not as extreme as the large effects produced by temperature changes in the sea. Because of the varying temperature differences in the sea, the sound beam does not travel in a straight line, but follows curved paths. This results in bending, splitting, and distorting of the sound beam. When the sound beam is bent, it is said to be refracted. A sound beam is refracted when it passes from a medium of a given temperature into a medium with a different temperature. An example of this is a sound beam traveling from an area of warm water into an layer of cold water.    The sound beam will bend away from the area of higher temperature (higher sound velocity) toward the lower temperature (lower sound velocity). As a result of refraction, the range at which a submarine can be detected by sound may be reduced to less than 1,000 yards, and this range may change sharply with changing submarine depth. Speed of the Sound Beam As mentioned previously, sound travels much faster in seawater than in the atmosphere. Near sea level, sound travels through the atmosphere at approximately 1,080 feet per second. In seawater, that same sound beam will travel at approximately 4,700 to 5,300 feet per second. There are three main characteristics of seawater that affect the speed of the sound wave traveling through 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 calculated in terms of slopes, or gradients) There is a high mineral content in seawater. The density of seawater is approximately 64 pounds per cubic foot, while fresh water has a density of about 62.4 pounds per cubic foot. This difference is caused by the salt in the seawater. Salt content in seawater is called the salinity of water. The overall effect of increasing the salinity is an increase in the speed of the sound beam in the water. This means that as the sound travels through water of varying salinity, it travels faster through the water with more salt content. Such a change in salinity is considerable at the mouth of a river emptying into the sea. Elsewhere, the difference in salinity is too small 4-3