Chapter 4 — PHYSICS OF SOUND 



describe many biologic noises. All Sonar Tech- 

 nicians should review these tapes and films 

 periodically to maintain proficiency in their 

 recognition of sounds. 



UNDERWATER SOUND TRANSMISSION 

 LOSSES 



loss is only half that of a sphere, or 3 db each 

 time the distance is doubled. Spreading loss 

 at close range is very high, but beyond about 

 2000 yards the loss becomes less significant. 



Absorption 



Now that you know something of the theory 

 of sound, you are ready to take a closer look 

 at what happens to an underwater sound pulse. 



To gain the full benefit of echo ranging sonar 

 equipment, you must be able to transmit an 

 underwater sound pulse and recognize the re- 

 turning echo from a target. Detection of the 

 echo depends on its quality and relative strength, 

 compared with the strength and character of 

 other sounds that tend to mask it. 



Sonar Technicians must know (1) what can 

 weaken sound as it travels through water, (2) what 

 conditions in the sea determine the path and 

 speed of sound, and (3) what objects affect the 

 strength and character of the echo. 



When a sound wave travels through water, it 

 encounters elements that reduce its strength. 

 Any signal strength lost in this manner is known 

 as a transmission loss. 



TRANSMISSION LOSSES 



As a sound pulse travels outward from its 

 source, it becomes more and more weakened. 

 Much of its energy is lost because of sea 

 conditions and distance. Three factors directly 

 related to sound transmission losses are diver- 

 gence, absorption, and scattering. The latter two 

 are referred to as attenuation loss, and are 

 dependent on transmission frequency. Divergence 

 loss is independent of frequency. 



Divergence 



When a sound wave is projected from a 

 point source, it assumes a spherical shape, 

 spreading equally in all directions. This spread- 

 ing is called divergence, and the further the 

 wave travels, the more energy it loses. The 

 energy lost by a sound wave due to spherical 

 divergence is inversely proportional to the square 

 of the distance from its source, or 6 db each 

 time the range doubles. 



In shallow water areas, the surface and 

 bottom are boundaries that limit the vertical 

 divergence of the sound wave. Consequently, 

 the expanding wavefront Is cylindrical, rather 

 than spherical, in shape. Cylindrical spreading 



In the topic on sound waves, you learned 

 how a sound pulse moves through water. The 

 repeated compressions and rarefactions of the 

 sound wave cause the water molecules to move 

 back and forth, thus passing the sound wave 

 along. An old saying goes: "You can't get some- 

 thing for nothing." In our illustrative case, 

 energy is lost (in the form of heat) by the sound 

 pulse in its efforts to compress the water. 

 Energy lost to the medium in this manner is 

 called absorption loss. 



Scattering 



Besides losses caused by divergence and 

 absorption, a sound wave loses energy due 

 to the composition of the medium through which 

 it passes. Composition of the sea naturally 

 varies from place to place, and from time to 

 time. In general, however, sea water contains 

 large amounts of minute particles of foreign 

 matter and many kinds of marine life of all 

 shapes and sizes. Each time the sound wave 

 meets one of these particles, a small amount 

 of the sound is reflected away from its direc- 

 tion of movement and is lost. The reflection 

 losses to the water are known as scattering 

 losses. Some of the scattered energy is re- 

 flected back to the sonar receiver and is then 

 called volume reverberation (discussed later 

 in this section). 



REFLECTIONS 



When a sound wave strikes the boundary 

 between two mediums of different densities, 

 the wave will be reflected, just as light is 

 reflected by a mirror. Some of the energy 

 will be lost, but most of it will be reflected 

 at an angle equal to the angle of incidence. 

 The angle of incidence is the angle, with respect 

 to the perpendicular, at which the wave strikes 

 the boundary. According to the physics law of 

 regular reflection (reflection from a smooth 

 surface) the angle of reflection equals the angle 

 of incidence. 



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