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RADIO ACOUSTIC RANGING 



621 



vibrating 150 times per second, the frequency of the sound wave produced will be 

 150 cycles per second. The wave length is the distance between particles in the same 

 phase. It is determined by the ratio of the wave velocity (v) to the frequency (/) ; for 

 example, if the velocity of sound is 1,500 meters per second and if the frequency is 

 150 cycles per second, the wave length is 10 meters. The intensity of a sound wave is 

 defined as the energy (the capacity to do work) transmitted through a unit area in a 

 unit of time. Intensity is usually measured in terms of energy, in ergs transmitted per 

 second through 1 square centimeter of surface perpendicular to the forward motion. 

 The intensity of sound nmst not be confused with loudness, because mtensity is purely 



WAVE LENGTH 



CONDENSATION 



Figure 121. — Sinusoidal sound wave. 



a rate-of-energy term, while loudness depends on frequency as well as intensity. Unless 

 the sound is of a frequency perceptible to the human ear, it is not loud, although its 

 intensity may be great. 



A sound wave transmitted through an elastic medium will travel in a straight path 

 until it encounters a change in physical characteristics of the medium, where its direction 

 will be changed by either reflection, refraction, diffraction, or a combination of these. 

 At any point in a medium where sounds of identical frequency and wave form arrive 

 from difl^erent directions there will be interference. This may result in a decrease or 

 an increase in intensity at this point between the limits of zero and the sum of their 

 intensities, depending on the relation of the phases and the values of the intensities 

 of the two waves. 



When a sound is transmitted thi'ough a medium from one point to another its energy 

 is attenuated. In practice this attenuation is due to the summation of all acoustic 

 energy losses which take place. The failure of part of the sound energy generated at 

 the source to reach the reception point may be due to transmission of energy to points 

 other than that of reception, and due to the conversion of acoustic energy into heat. 

 Some of the generated acoustic energy may fail to reach the point of reception because 

 of lack of directivity of the source; simple refraction, reflection, diffraction, and inter- 

 ference; and diffuse refraction and reflection. In any case some of the acoustic energy 

 will be lost during transmission because of the viscosity of the medium. The amount 

 of energy reaching the point of reception will be decreased bj^ turbulence, aeration, and 

 suspension of foreign matter. 



For long-distance transmission of sound, water is probably the best medium. 

 It has homogeneous physical properties which are relatively stable as compared with 



