18 



THE OCEAN OP AIR 



while the bell is ringing, the sound will be found to 

 grow fainter and fainter as more air is removed until 

 it can hardly be heard. When air is again let into the 

 bell jar the ringing becomes louder. If you have the 

 materials you will find this experiment interesting. 

 Figure 33 will show you how to set up the apparatus. 

 When sound is reflected from a surface such as a 

 cliff, a building, or a wall, we hear an echo. Echoes 

 may be disturbing in large auditoriums unless the 

 shape of the walls and ceiling is so planned that the 

 echoes which are produced will not be annoying. 



Fin. 33 



Oftentimes echoes may be prevented by hanging cur- 

 tains over the walls that reflect the sound. 



Some sounds are pleasing to the ear while others 

 are harsh and displeasing. If a sounding body is send- 

 ing out a regular vibration such as the one shown at 

 a in Figure 34, the human ear recognizes it as a musi- 

 cal tone. But if the vibration is irregular as shown at 

 b, we hear what we commonly think of as noise. The 

 only difference between musical tones and noises is 

 in the evenness of the vibration an even vibration, 

 pleasing tones ; an uneven vibration, noise. 



FTC. 34 



Sound waves are used to make flying safer. Mail 

 planes and other commercial planes must frequently 

 fly in foggy and stormy weather. A device known as 

 the sonic altimeter has recently been added to the 

 equipment of airplanes to make such flying less 

 hazardous. A sound wave is started by a shrill whistle 

 on the plane and is directed toward the earth. The 

 time taken for the sound wave to reach the earth and 

 to be reflected to the plane is registered by the sonic 

 altimeter. The pilot knows the velocity of sound in 

 air at any given temperature ; hence, if he knows the 



time taken by the sound he can quickly determine his 

 altitude. 



Sound waves are used to determine the depth of the 

 ocean. In much the same manner as the aviator may 

 tell his altitude by use of sound, ocean depths may be 

 measured. Sound waves are sent from the surface of 

 the water toward the bottom of the ocean where they 

 are reflected as an echo from a hill or building. The 

 time required for the sound to reach the bottom and 

 be reflected to the surface is noted. The velocity of 

 sound in water is known and thus the depth of the 

 ocean at that point may be easily found. 



What is the nature of musical tones? These make 

 possible the great variety of music which can be pro- 

 duced by instruments. 



In the first place it is possible for a musical tone 

 to be played loudly or softly, thus permitting the pro- 

 duction of many effects with an instrument. Again, 

 musical tones differ in their highness or lowness, or 

 as the musician says, their "pitch." Some tones are 

 very high and others are very low. Knowing that 

 musical tones are produced by vibrating bodies, it is 

 not difficult to see why they vary in pitch. When a 

 body is making only a few vibrations per second it 

 produces a low tone or one that has a low pitch, 

 while one that is making many vibrations each sec^ 

 ond produces a high note or one that has a high pitch. 

 Middle C on the piano has 259 vibrations per second 

 while the next C above has 517. These tones are eight 

 notes or one octave apart and are tuned to the inter- 

 national pitch. 



The octave of any tone makes twice as many vibra- 

 tions per second as it does. Thus middle C of a tuning 

 fork, 256; C above, 512. Figure 35 shows the scale 

 of eight tones beginning with middle C on the piano 

 and the number of vibrations given by each one. 



The human ear can recognize sounds which vary 



259 290 326 345 388 435 488 517 

 FIG. 35 



in pitch from about 16 vibrations per second to 20,000 

 vibrations per second. Not all ears, however, are the 

 same in this respect, some being more able to recog- 

 nize high tones while others can better recognize the 

 low tones. 



A third important thing about musical tones is that 

 even though a piano and violin are playing a tone of 



