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FACTORS AFFECTING THE SOUND BEAM

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sound wave. The frequency (in hertz) of the sound wave is the number of wavelengths that occur every second. FACTORS AFFECTING THE SOUND BEAM The particular sound waves of interest to the sonar operator are the waves that leave the sonar transducer in the form of a beam and go out into the water in search of a submarine. If the sound beam finds a target, it will return in the form of an echo. The use of sonar equipment depends on the presence and the recognition of an echo from a target. Detection of the echo depends on the quality and relative strength (loudness) of the echo compared to the strength and character of other sounds, since they tend to mask or cover it. The sonar operator should know what factors can weaken the sound beam as it travels through water, what factors in the seawater determine the path and speed of the sound beam, and what factors affect the strength and character of the echo. Any signal strength lost during the beam’s travel through the water is known as “transmission loss.” Some of the factors determining transmission loss are discussed in the following paragraphs. Absorption and Scattering Some of the sound energy emitted by the source will be absorbed while passing through the water. The amount absorbed this way depends on the sea state. Absorption is high when winds are great enough to produce whitecaps and cause a concentration of bubbles in the surface layer of the water. In areas of wakes and strong currents, such as riptides, the loss of sound energy is greater. Therefore, echo ranging through wakes and riptides is difficult because of the combined effect of false echoes, high reverberations, and increased absorp- tion. Absorption is greater at higher frequencies than at lower frequencies. Sound waves are weakened when they reach a region of seawater that contains foreign matter, such as seaweed, silt, animal life, or air bubbles. This foreign matter scatters the sound beam and causes loss of sound energy. The practical result of scattering is to reduce echo strength, especially at long range. Reflection Echoes occur when the sound beam hits an object or a boundary region between transmission mediums in such a manner as to reflect the sound or to throw it back to its origin. Reflection of sound waves sometimes happens when a wave strikes a medium of different density from that through which it has been traveling. This will occur in cases where the two mediums are of sufficiently different densities, and the wave strikes at a large angle. This happens because the sound wave travels at different speeds through the two different densities. For example, a sound wave traveling through seawater is almost entirely reflected at the boundary of the water and air. The speed of sound in seawater is about four times greater than the speed of sound in air, and the density of water is more than 800 times greater than that of air. Therefore, practically all of the sound beam will be reflected downward from the sea surface. Similarly, when a sound wave traveling through the seawater strikes a solid object like a submarine, the difference in the density and the sound velocity in the two mediums is such that all but a small amount of the sound beam will be reflected. That portion of the beam that strikes surfaces of the submarine perpendicular to the beam will be reflected directly back to the origin as an echo. In calm seas, most of the sound energy that strikes the water surface from below will be reflected back down into the sea. A scattering effect occurs as the sea gets progressively rougher. In these circumstances, part of any sound striking the surface is lost in the air, and part is reflected in scattering directions in the sea. In water less than 600 feet deep, the sound may also be reflected off the bottom. Other factors being equal, the transmission loss will be least over a smooth, sandy bottom and greatest over soft mud. Over rough and rocky bottoms, the sound is scattered, resulting in strong bottom reverberations. Reverberation When sound waves echo and re-echo in a large hall, the sound reverberates. Reverberations are multiple reflections. Lightning is an example of this from nature. When lightning discharges, it causes a quick, sharp sound; but by the time the sound of the thunder is heard, it is usually drawn out into a prolonged roar by reverberations. 4-2



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