SOUND 



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SOUND 



ruple the tension, and to play a high note you 

 shorten the string with your finger, doubling 

 the number of vibrations if you halve the 

 length. The same laws govern other stringed 

 instruments and the drum. The pitch of wind 

 instruments depends on their length, and on 



HOW SOUND WAVES SPREAD 

 If sound waves could be seen they would appear- 

 as an ever-widening circle, moving from the ex- 

 citing 1 cause. Though unseen, they are as real as 

 a widening circle of ripples on water. If the ex- 

 citing cause be violent, as when a cannon is fired, 

 the force of the waves of sound may destroy the 

 eardrum. 



whether or not they are open or closed at the 

 ends ; the reasons for this will be made clear by 

 reference to the diagram in the article ORGAN, 

 after a study of the explanation below of the 

 length of sound waves. 



Noise or Music. If we leave out of con- 

 sideration the differences in personal likes and 

 dislikes, the only distinction between musical 

 and unmusical sounds is that the former have 

 regular, the latter irregular, vibrations. A con- 

 fusion of sounds makes noise, for the various 

 sound waves mingle and cause the ear to re- 

 ceive a jumble of sensations. If you stand on 

 a hill above a city, or high up in a skyscraper, 

 you find many of the sounds of the city, which 

 below are so disagreeable, to be distinct musical 

 tones; but, as in music, notes which separately 

 are pleasant may produce discord if sounded 

 together. 



Discord in music seems to be due to what are 

 known as beats. If you strike any two adja- 

 cent piano keys simultaneously, you can dis- 

 tinctly hear throbs in the sound. The waves 

 from the two notes start at the same instant, 

 but because the upper string vibrates faster 

 than the lower its first push against the air is 

 completed sooner and it starts to draw back 

 while the other is still moving outward. It 

 starts its second push still farther ahead and 

 continues to gain until after a time an instant 

 comes when the expansive force of one exactly 

 balances the contractive force of the other and 

 there is silence. The next instant the one wave 

 has gained on the other again, and with the 

 two once more moving in the same direction, 



the sound surges out with doubled intensity. 

 Since a beat occurs only as often as one wire 

 has made a complete vibration more than the 

 other, there will be as many beats per second 

 as the difference between the number of vibra- 

 tions each makes. Notes which sound about 

 thirty beats per second make the most disagree- 

 able combination; above and below that num- 

 ber the unpleasantness gradually decreases, and 

 seventy or more beats per second are not 

 noticeable. When notes differing more than 

 seventy vibrations produce discord, as do C 

 and the B above it, there are beats with or 

 between the overtones. Piano tuners always 

 listen for beats. 



Quality of Sound. Overtones are the notes 

 produced when an object vibrates not only as 

 a whole but also in parts. If you rest a small 

 piece of paper on the center of a violin string 

 it will not be thrown off while the string is 

 vibrating, for while the left half moves up the 

 right half moves down and the center is mo- 

 tionless. Thus, while the whole string is pro- 

 ducing the fundamental note, each half is 

 producing a note an octave higher, called the 



SCALE OF C ON THE STAFF AND 

 KEYBOARD 



first overtone. By vibrating in thirds also, the 

 string sounds another overtone, in fourths an- 

 other, and so on. Overtones are very important 

 in music, for, as proved by Helmholtz, their 

 number and strength determine quality of tone. 

 Sympathetic Vibration. If you hold down 

 the loud pedal of a piano, so that all of the 

 wires are free to vibrate, and sing a note into 

 the wires, they will send back the same tone 

 with so much realism that they seem to be 

 mocking you. This is because the waves from 

 the fundamental and overtones of your voice 

 set in motion the wires which are of the proper 

 length, tension and thickness to sound them. 

 Sometimes a certain note on a piano will cause 

 window panes to rattle, and sometimes glass is 

 broken by vibrations set up by music. On the 

 other hand it is because of sympathetic vibra- 

 tion that an old house gives to music such a 

 mellow sound. 



