SOUND . 417 



Compare the auditorium of your school room with these diagrams. 



How does it fulfill the requirements for producing the best 

 results? 



How is the room heated? 



Do the hot-air currents interfere with the sound waves? 



Speed of Sound. We are all familiar with the fact that we do 

 not hear thunder as soon as we see a flash of lightning. This is 

 because it takes time for sound to travel. Sound travels in air at 

 32 F. (0 C.) 1088 feet per second, and the speed increases two feet 

 for every degree of rise in temperature Centigrade, or a little over 

 one foot for each degree Fahrenheit. 



Sound is produced by some vibrating body which causes the air 

 to vibrate. Sound is transmitted through a series of condensations 

 and rarefactions. The place where the rarefaction is greatest is 

 called a node. The condensation gradually grows less and less until 

 the waves die out. 



Sound waves producing two unlike tones cause beats or places 

 where one wave annihilates another, resulting in silence. This pro- 

 duces a wavy or throbbing sensation. The ding-dong of a bell and 

 the tremulo are good examples. 



The beat is caused by the meeting of the waves, first at condensation. 

 Since one wave is longer than the other they gradually come to a place where 

 condensation of one wave and rarefaction of the other take place at the same 

 time, thus producing a momentary silence. 



Noise, Sound and Tone. Any instrument which will cause the 

 air to vibrate will produce noise, sound or tone. Noise is the result 

 of irregular vibrations of the air. Sound is the result of regular 

 vibrations of the air. 



Experiment. Construct a disc with a series of holes as shown in the illus- 

 tration 308a. 



Arrange the holes at equal distances apart with the inside row containing 

 24 holes, the next 27, 30, 32, 37, 40, 45, and 48. The last row of holes should 

 be made at uneven distances apart. Attach the disc to a rotating motor or 

 other rotating device and blow through each set of holes in succession with 

 a piece of glass tubing. The outer row of holes will produce uneven vibrations, 

 causing noise. The inner rows of holes will produce even vibrations, resulting 

 in sound and also tone. Tone is produced in the same way as sound, but has 



