AEROGRAPHER'S MATE 3 & 2 



AMPLITUDE 



TRANSVERSE WAVES 



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LONGITUDINAL WAVES 



Figure 12-23. — Characteristics of transverse and longitudinal waves. 



209.24 



the maximum displacement of the particles, is 

 labeled on each type wave; in the transverse 

 wave, it is half the distance measured vertically 

 from crest to trough. Amplitude is determined 

 by the energy of the wave. 



SOUND WAVES 



compressions are represented by dark rings. 

 As the sound waves spread out, their energy 

 simultaneously spreads through an increasingly 

 large area. Thus the wave energy per unit area 

 becomes weaker as distance increases. This 

 loss of energy due to distance is called spreading 

 loss. 



Sound waves are longitudinal or compres- 

 sion waves, set up by some vibrating object. 

 In its forward movement, the vibrating object 

 pushes the water particles lying against it, 

 producing an area of high pressure, or com- 

 pression. 



On the backward movement of the vibrating 

 object the water particles return to the area 

 from which they were displaced during 

 compression and travel beyond, producing an 

 area of low pressure, or a rarefaction. The 

 compression moves outward by pushing the water 

 particles immediately in front of the com- 

 pressed particles. The rarefaction follows the 

 compression, transferring the pull produced by 

 the backward movement to the particles 

 immediately ahead. The next forward movement 

 of the vibrating object produces another com- 

 pression and so on. In figure 12-24, the 



Frequency 



The frequency of a sound wave is the number 

 of vibrations per second produced by the sound 

 source. A source, for example, may transmit 

 on a frequency of 5 kHz, or 5,000 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 themselves 

 have very little actual movement. The wave, 

 however, may travel great distances at a high 

 rate of speed. 



Density 



Earlier in this chapter, density has been 

 defined as mass per unit volume, and its effect 

 on the transmission of light has been explained. 



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