Chapter 12 — THE GOVERNING FUNDAMENTALS OF METEOROLOGY 



209.25 



Figure 12-24. — Sound wave travel. 



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 in a substance. 

 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 

 chapter 16 of this manual. In fresh water of 

 65° F, sound velocity is approximately 4,790 

 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 4,800 

 fps in sea water of 39° F. 



Wavelength 



Wavelength for any sound wave train is the 

 distance between any two sucessive compressions 



or rarefactions. Wavelength can be computed by 

 using the formula for wavelength previously 

 mentioned in the section on optical phenomena. 

 Wavelength is also dependent on frequency; 

 therefore, when the frequency is low, the sound 

 waves will be long; when it is high, the waves 

 will be short. 



CHARACTERISTICS OF SOUND 



Basic characteristics of sound are speed, 

 frequency, wavelength, pitch, and intensity. The 

 first three of these characteristics have been 

 discussed previously in this chapter. 



Pitch 



Pitch depends on the frequency, or number 

 of cycles per second, which the detector 

 receives. 



An object that vibrates many times per 

 second produces a sound with a high pitch, 

 as in the instance of a whistle. The slower 

 vibrations of the heavier wires within a piano 

 cause a low-pitched sound. 



Intensity 



Intensity and loudness often are mistaken 

 as having the same meaning. Although they are 

 related, they actually are not the same. Inten- 

 sity is a measure of a sound's energy. Loudness 

 is the effect of intensity on an individual, in the 

 same manner that pitch is the effect of frequency. 

 Increasing the intensity causes an increase in 

 loudness, but not in a direct proportion. To 

 double the loudness of a sound requires about 

 a tenfold increase in the sound's intensity. The 

 unit of measure of sound intensity is the 

 decibel (db), which is a measure of the intensity 

 of a sound compared with an established 

 standard. The intensity levels of some common 

 noises are: a whisper, 10 to 20 db; heavy street 

 traffic, 70 to 80 db; thunder, 110 db. 



Doppler 



You probably are familiar with the changing 

 pitch of a train whistle as the train passes 

 near you at high speed. As the train approaches, 

 the whistle has a high frequency. As the train 

 goes by, the frequency drops abruptly and 

 becomes a long, drawn-out sound. The apparent 



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