QUALITIES OF SOUND. 597 



becomes less loud, the vibrations are smaller, and the amount of 

 excursion made by the vibrating body is commonly spoken of as 

 the amplitude of the vibration, and upon it alone depends the 

 loudness of the sound. Thus the pitch of a tone bears no relation 

 to the amplitude of the waves of the vibration, but depends upon 

 their rate ; while its louduess is quite independent of the period 

 occupied by the vibrations, but is in proportion to the extent or 

 amplitude of the waves. 



So far only tones or musical notes have been mentioned. They 

 are produced by vibrations occurring at perfectly regular periods. 

 The simpler and more regular the vibrations, the purer the tone. 

 But the great majority of the sounds we are accustomed to hear 

 are not pure tones, but are the result of an association of vibra- 

 tions bearing more or less relation one to the other. When the 

 variety of vibrations is very great, their intervals irregular and 

 out of proportion, they give rise to a discordant sound devoid of 

 musical tone, which is commonly called a noise. But so long as. 

 such commensurability exists in the rate of the vibrations as to 

 produce a sound not disagreeable to the sense of hearing, it may 

 be called a note. 



By the use of a series of different resonators, each of which is 

 capable of magnifying a certain tone, it can be shown that the 

 clearest and purest notes of our musical instruments are far from 

 being simple tones, but are really compounds of one prominent 

 note or fundamental tone, modified by the addition of numerous 

 over-tones or harmonics. If one blows forcibly across an orifice 

 leading to a space in which a small amount of air is confined, 

 such as the barrel of a key or the mouth of a short-necked flask 

 or bottle, either a clear, shrill or dull, booming sound is heard, 

 which varies in pitch according to the proportions of the air- 

 containing cavity. This dull note is a simple tone. It is devoid 

 of character, and in this respect differs greatly from the notes 

 produced by a musical instrument. The notes of every instru- 

 ment have certain characters or qualities which enable even the 

 most unpracticed ear to distinguish them from one another. 



This peculiar quality, which is independent of the pitch (i.e., 

 rate of vibration) or the intensity (i.e., amplitude of wave), is 



