274 



SOUND 



256. Waves. While the disturbance which travels out 

 from a sounding body is commonly called a wave, it is by no 

 means like the type of wave best known 

 X <*** to us, namely, the water wave. 



If a . closely coiled heavy wire is sus- 

 V^ pended as in Figure 173 and the weight is 



drawn down and then released, the coil 

 will assume the appearance shown ; there 

 is clearly an overcrowding or conden- 

 sation in some places, and a spreading 

 out or rarefaction in other places. The 

 pulse of condensation and rarefaction 

 which travels the length of the wire is 

 called a wave, although it bears little or 

 no resemblance to the familiar water wave. 

 Sound waves are similar to the waves 

 formed in the stretched coil. 



Sound waves may be said to consist of a 

 series of condensations and rarefactions, 

 and the distance between two consecutive 

 condensations and rarefactions may be de- 

 / fined as the wave length. 



f 257. How One Sounding Body produces 



A Sound in Another Body. In Section 255 



*B we saw that any object when disturbed 



FIG. 173. Waves in a vibrates in a manner peculiar to itself, 



coiled wire. 



its natural period, a long-roped ham- 

 mock vibrating slowly and a short-roped hammock vibrating 

 rapidly. From observation we learn that it requires but little 

 force to cause a body to vibrate in its natural period. If a 

 sounding body is near a body which has the same period as 

 itself, the pulses of air produced by the sounding will, although 

 imperceptibly small, set the second body into motion and cause 



