SOUND 



5444 



SOUND 



The to-and-fro movement of the gong is the 

 cause of what you hear. As the sides move 

 outward they 'compress the air in all directions, 

 and as they swing back they allow the air to 

 expand ; each particle of air presses against the 

 particles beyond it, then draws back, and so the 

 atmosphere about the gong is formed into a 

 series of globular shells alternately of com- 

 pressing and of expanding air. When these 

 reach the air in your ear the changes of pres- 

 sure on your eardrum send the sensation of 

 sound to your brain. 



How Sounds Differ. Since the surges of air 

 against the eardrum are the cause of sound, 

 it is plain that whether a noise is loud or soft, 

 high pitched or low, pleasant or unpleasant, 

 must depend upon the characters of the sound 

 waves and of the individual ear. 



Loudness. If you put your feet down gently 

 when you walk, you move quietly; if you 

 stamp, a loud noise results. The greater the 

 force you exert, the stronger the compression 

 which the resulting vibrations produce, and the 

 more intense the effect upon your ear. The 

 tramp of soldiers marching is louder than the 

 noise from one person's feet because the simul- 

 taneous waves from sources close together com- 

 bine their force. But sometimes two sounds 

 result in no sound, for if the compression from 

 one meets an equal expansion from another the 

 waves at that point cease. Thus, if you turn a 

 tuning fork slowly around near your ear you 

 will find four positions in which you cannot 

 hear it unless you hold something between 

 one of the prongs and your ear. 



If you hear four trumpets blown one hundred 

 yards away, the sound seems to you about as 



RIPPLES ON WAVES 



As waves enlarge in circles from the point 

 where disturbance occurs, so do sound waves 

 travel from the exciting cause. (See illustration 

 on next page.) 



loud as that from one trumpet fifty yards 

 away, for when the one source of waves is twice 

 as far from you as is the other, the spherical 

 surface of a wave from the first is four times 

 as great as the surface of one from the second, 

 and it contains four times as many particles 

 to share the energy of the sound. The scien- 



tific statement of this law is that the intensity 

 of a sound varies inversely as the square of the 

 distance from its source. 



Most of us have tried the experiment of 

 holding a watch between the teeth or pressing 

 it against the forehead. At each vibration 

 caused by the ticking, a wave of compression 

 and expansion travels through the bones of 

 the head just as it would through the air. In 

 fact, the denser the object, the better, as a rule, 

 will it transmit sounds. The reason will be 

 plain to anyone who has played with pool 

 balls; if half a dozen balls are placed in a 

 row, all touching, a slight tap at one end will 

 ' move the ball at the other end, but if the row 

 is formed with space between the balls the first 

 one must be hit quite hard in order to affect 

 the last. The denser a substance the closer 

 together are its particles of matter, and a sound 

 will therefore travel better through steel or 

 wood or even through water than through air, 

 but will not progress at all in a vacuum. 



Why is it, then, that the walls of a house 

 soften the noises of the street? The reason is 

 that when sound waves traveling in one me- 

 dium meet the surface of another they are 

 partly reflected, just as light is mirrored by a 

 transparent pane of glass. Nature has taken 

 advantage of this law in giving rabbits and 

 other animals ears which they can hold erect 

 to gather in a greater volume of sound; each 

 wave impulse that strikes the wall of the ear 

 at an angle is reflected inward at an equal 

 angle, and so reaches the eardrum. Sound 

 reflection is also responsible for echoes. 



High Tones and Low. The rapidity with 

 which an object vibrates is what determines 

 the pitch of the sound given out. Thus a 

 soprano, to sing high C, must send out four 

 w.aves from her vocal organs in the time that a 

 contralto singing middle C gives out one. The 

 actual number of vibrations for middle C, 

 whether sounded by human vocal chords, piano 

 strings or wind instruments, is 256 per second. 

 The ear takes note of sounds containing as 

 many as 30,000 or even 40,000 vibrations a 

 second, but is unaffected by fewer than 30. 



The conditions which regulate the rate of 

 vibration of any object are nearly all known, 

 and our knowledge of them is utilized in mak- 

 ing and in tuning musical instruments. Thus 

 the low strings on a mandolin are thicker than 

 the high strings, for to double the weight of a 

 string halves its number of vibrations. To 

 raise the pitch of a string you tighten it, 

 doubling the number of vibrations if you quad- 



