SOUND 415 



the cliff and are sent back, producing the same sound you made. 

 Water is a good reflector of sound. People in balloons can easily 

 hear sound reflected from water. 



Clouds will reflect sound. This is shown by firing a cannon. 

 If the sky is clear a large gun produces a sharp report, but if there 

 are clouds in the sky the report of the gun will be heard as an echo 

 similar to thunder " rolling." In fact, the rolling sound of thunder 

 is due to the sound waves rebounding from the clouds. 



Whispering galleries are examples of sound waves rebounding 

 from some point where they have concentrated, usually in a concave 

 surface. Many of the rotundas of the capitol buildings in the 

 United States are examples of whispering galleries, but one must 

 stand in the place where the sound waves are reflected to a point 

 or focus. 



Sound Waves in a Room. People often experience difficulty in 

 hearing a speaker in some auditoriums. Sound waves travel in 

 straight lines, unless stopped by draperies, screens, etc. Ventilating 

 flues, hot air registers, steam radiators and air currents interfere 

 with sound waves. A speaker who is located some little distance 

 from the listeners, with a hot-air floor register in front of him, will 

 have difficulty in being heard, since the sound waves are interfered 

 with by the hot air traveling upward toward the ceiling. Some of 

 the sound waves will be reflected back, others upward, and a small 

 number will reach the listeners. 



Hot air rising from footlights in front of a speaker will interfere 

 with sound waves. Frequently singers prolonging a note seem to 

 be producing an uneven tone because of the unevenness of the sound 

 waves passing through the heated currents of air which rise from the 

 floor between the singer and the listener. It seems necessary, then, 

 in an auditorium to have the air evenly heated. In some halls 

 wires have been strung to improve the resonance of the room. This, 

 however, appears to be practically useless. 



Sound waves die on soft or dead surfaces such as carpets, cur- 

 tains and drapery. Velvet is the best material for exhausting sound 

 waves. Metal mirrors and polished plate glass reflect sound waves. 

 The best way to improve an auditorium in which voices are not 

 well heard is so to construct walls and ceilings that the sound waves 



