Chapter 4— PHYSICS OF SOUND 



COMPRESSION 



RAREFACTION 



71.21 

 Figure 4-5.— The sound wave. 



by dark rings. As the sound waves spread out, 

 their energy simultaneously spreads through an 

 increasingly large area. Thus the wave energy 

 becomes weaker as distance increases. 



Another way of representing the actions of 

 a sound wave is illustrated in figure 4-5. Com- 

 pressions are shown as hills above the reference 

 line, and rarefactions as valleys below it. The 

 wavelength is the distance from one point on a 

 wave to the next point of similar compression. 

 The change occurring in compression and rare- 

 faction in the space of 1 wavelength is called a 

 cycle. 



Frequency 



The frequency of a sound wave is the number 

 of vibrations per second produced by the sound 

 source. A sonar transducer, for example, may 

 transmit on a frequency of 5 kHz, or 5000 

 vibrations per second. Motion is imparted to 

 the sound wave by the back-and-forth movement 

 of the particles of the medium, in effect passing 

 the wave along, although the particles them- 

 selves have very little actual movement. The 

 wave, however, may travel great distances at 

 a high rate of speed. 



Density 



Perhaps you've heard people speak of a heavy 

 fog as being "thick as pea soup." This murky 

 condition would be a dense fog caused by the 

 atmosphere being filled with small particles of 

 water called vapor condensation. A fog-filled 

 atmosphere is heavier (because of the weight 

 of the water particles in it) than a clear atmos- 

 phere. The measure for this "thickness" of a 



substance is called density, and is defined as 

 "weight per unit volume." In the study of general 

 physics, density of any substance is a com- 

 parison of its weight to the weight of an equal 

 volume of pure water. Because of the salt 

 content (salinity) of sea water, it has a density 

 greater than that of fresh water. 



Density is also an indication of the sound 

 transmission characteristics of a substance, or 

 medium. When a sound wave passes through a 

 medium, it is transmitted from particle to 

 particle. If the particles are loosely packed 

 (as they are in fresh water as compared with 

 sea water), they have a greater distance to 

 move to transmit the sound energy. In so doing 

 time is consumed, and the overall result is a 

 slower speed of sound in a less dense medium. 



Density and elasticity are the basic factors 

 that determine sound velocity. The formula for 

 determining velocity is: 



c.\/-|- 



where c equals velocity, E equals the medium's 

 elasticity, and p equals the medium's density. 

 Variations in the basic velocity of sound in the 

 sea are caused by changes in water tempera- 

 ture, pressure, and density, as will be seen in 

 the section on sound propagation. In fresh water 

 of 65°F, sound velocity is approximately 4790 

 feet per second (fps). In sea water, velocity 

 depends on pressure and temperature in addition 

 to salinity. For all practical purposes, you can 

 assume that sound travels at a speed of 4800 

 fps in sea water of 39°F. 



Wavelength 



If a sonar transducer vibrates at the rate of 

 25,000 vibrations per second, and if the tempera- 

 ture is 39°F, the first wave will be 4800 feet 

 away at the end of the first second. Between the 

 transducer and the front of this wave there will 

 be 25,000 compressions. Thus, the wavelength, 

 that is, the distance between points of similar 

 compression, must be 4800 + 25,000 or 0.192 foot, 

 because there are 25,000 compressions extending 

 through a distance of 4800 feet. The wavelength 

 always can be found if the frequency and the 

 velocity are known. Formula: 



Wavelength = 



Velocity . 

 Frequency 



37 



