RELATION OF AIR TO SOUND 



17 



Experiment 12. How high a tone can the ear dis- 

 tinguish ? 



Test the upper limit of hearing by use of a Galton's 

 whistle which you can borrow from the physics department 

 of your school or by cutting short lengths of iron rod and 

 suspending them with string in such a manner that they 

 may be sounded with a small hammer. The shorter the length, 

 the higher the pitch. Who has the highest limit of hearing in 

 your class? 



In your notebook 1 record the results of this experiment. 



OTHER INVESTIGATIONS WHICH YOU CAN MAKE 



1. Secure an old phonograph and take the sound repro- 

 ducing mechanism apart. 



2. Study a phonograph record under the microscope. 



3. Make a collection of bottles and pipes of varying 

 pitch. 



4. Study several musical instruments, some string and 

 some wind, and see how various tones are produced. 



5. Measure the speed of sound in air. Have one person 

 go away a measured distance of at least a half 'mile and 

 shoot a blank cartridge from a gun. Time the difference 

 between the flash and the instant the sound reaches you. 

 A stop watch will be found useful for this. 



& READINGS WHICH- WILL HELP ANSWER THE 

 , PROBLEM QUESTIONS 



How does sound affect our daily lives? We live in 

 a world of shakes and quivers. Nearly everything 

 that we do starts something moving or quivering. 

 If we walk across the floor, it moves ; if we speak, 

 the air about us is disturbed and begins to move. It 

 is very easy to set the air moving because it is a gas 

 and touches almost everything. When the ocean 

 moves, it stirs the air above it; an animal stealing 

 noiselessly through the forest moves the air about 

 him. 



So noisy have some of our great cities become with 

 the clang of metal on metal, the auto horns, and the 

 fire sirens, that in many places steps are being taken 

 to measure the noise and to make laws that at certain 

 points in the city near hospitals and schools, it must 

 be kept below a certain point. Perhaps our policemen 

 will soon be carrying sound-measuring devices in 

 their pockets. 



In spite of all the noise there is also a beauty in 

 sound. What a drab and uninteresting world this 

 would be if it were not for music. Even though not 

 educated to it, all have a love for the enjoyment that 

 only music can give. 



We live in this world of horrid and beautiful 

 sounds where each air disturbance is carried along 

 hither and yon as if it were the only one there, some 

 to die out unheard when they have spent their 

 energy, others to fall upon an ear of some lower ani- 

 mal or man and there, through that marvelous 

 mechanism, to be changed into a nerve current, car- 

 ried to a brain, and recognized as noise or musics 



What is sound and how does it travel? If a rubber 

 band, string, or metal rod is vibrated, if a tight skin 

 is hit as in a drum, or if one blows across the mouth 

 of a bottle, setting the air in motion, sound waves are 

 set up in the air around and travel away from the 

 center of the disturbance in every direction as shown 

 by the illustration in Figure 31. Figure 32 shows some 

 common sources of sound waves. 



FIG. 31 



$) 

 1 See accompanying workbook, p. 8. [_j 



FIG. 32 



If you have ever thrown a stone into a still pond 

 and noticed the waves as they spread over the water, 

 you have a picture of how sound tr-avels ; we must re- 

 member, however, that it will go upward and down- 

 ward as well. To a certain extent it will travel 

 through almost anything. Have you ever put your 

 ear to a railroad rail to see whether a train was com- 

 ing or heard some one cracking stones under water 

 when you were swimming? These common expe- 

 riences show that sound travels through steel and 

 water; in fact, many other liquids, solids, and gases 

 carry sound as well as or better than air. Sound travels 

 about llQCUfeet per second in air at a temperature of 

 _32 degTCeTF. For every added degree of temperature 

 its speed is increased two feet a second. To get the 

 soun-d produced by a vibrating object from one place 

 to another then, it is necessary only to have some 

 solid, liquid, or gas in contact with the sounding body. 

 A simple experiment may be done to prove this. If 

 an alarm clock be set on a piece of felt or heavy cloth 

 and placed under the bell jar on an air pump, its bell 

 can be heard. If now the air be pumped out of the jar 



