CHAPTER XI. 



ACOUSTICS. 



402. Acoustics is that department of ohvs- 



WTifit is tiiG X ■ %f 



science of ical scieiice which treats of the nature, phe- 

 nomena, and laws of sound. It also includes 

 the theory of musical concord or harmony. 



403. Sound is the sensation produced on the 



What is Sound? /• i • i 



organs oi hearmg, when any sudden shock or 

 impulse, causing vibrations, is given to the air, or any 

 other body, which is in contact, directly or indirectly'-, with 

 the ear. 



underwhatcir- 404. "Whcu au clastic body is disturbed at 

 ^bratorymove- ^^J point, its particles execute a series of vi- 

 mcnts anse f bratory movcments, and gradually return to a 

 position of rest. 



Thus when a glass tumbler is struck by a hard body, a tremulous agitation 

 is transmitted to its entire mass, whicli movement gradually diminishes in 

 force until it finally ceases. Such movements in matter are termed vibra- 

 tions, and when communicated to the ear produce a sensation of sound. 



The nature of these vibratory movements may be illustrated by noticing 

 the visible motions whicli occur on striking or twitching a tightly extended 



Fio. 184. 



cord, or wire. Suppose such a cord, repre- 

 sented by the central line in Fig. 18-4 to be 

 forcibly drawn out to A, and let go ; it 

 would immediately recover its original posi- 

 tion by virtue of its elasticity ; but when it 

 reached the central point, it would have ac- 

 quired so much momentum as would cause 

 it to pass onward to a ; thence it would vi- 

 brate back in the same manner to B, and back again to b, the extent of its 

 vibration being gradually diminished by the resistance of the air, so that it 

 would at length return to a state of rest. 



In vibratory movements of this kind all the separate par- 

 ticles come into motion at the same time, simultaneously pasa 

 tion, the point of equilibrium, or rest, simultaneously reach the 



maximum of their vibration, and simultaneously begin their retrograde mo- 

 tion. Such vibrations are therefore called stationary, or fixed vibrationa 



Describe the 

 nature of a sta- 

 tionary vibra- 



