224 ACOUSTICS APPLIED TO PUBLIC BUILDINGS. 



a musical note is the result. Each of these sounds is produced by the 

 human voice, though they apparently run into each other. Usually, 

 however, in speaking, a series of irregular sounds of short duration 

 are emitted, — each syllable of a v7ord constitutes a separate sound of 

 appreciable duration, and each compound word and sentence an as- 

 semblage of such sounds. It is astonishing ths.t, in listening to a 

 discourse, the ear can receive so many impressions in the space of a 

 second, and that the mind can take cognizance of and compare them. 



That a certain force of impulse, and a certain time for its continu- 

 ance, are necessary to produce an audible impression on the ear, is 

 evident ; but it may be doubted whether the impression of a sound on 

 this organ is retained appreciably longer than the continuance of the 

 impulse itself; except in cases of loud sounds. If this were the 

 case, it is difficult to conceive why articulated discourse, which so 

 pre-eminently distinguishes man from the lower animals^ should 

 not fill the ear with a monotonous hum ; but whether the ear con- 

 tinues to vibrate, or whether the impression remains a certain time 

 on the sensorium, it is certain that no sound is ever entirely instanta- 

 neous, or the result of a single impression, particularly in enclosed 

 spaces. The impulse is not only communicated to the ear, but to all 

 bodies around, v/hich, in turn, themselves become centres of reflected 

 impulses. Every impulse must give rise to a forward, and afterwards 

 to a return, or backward, motion of the atom. 



Sound from a single explosion in air, equally elastic on all sides, 

 tends to expand equally in every direction ; but when the impulse 

 is given to the air in a single direction, though an expansion takes 

 place on all sides, yet it is much more intense in the line of the im- 

 pulse. For example, the impulse of a single explosion, like that of 

 the detonation of a bubble of oxygen and hydrogen, is propagated 

 equally in all directions, while the discharge of a cannon, though 

 heard on every side, is much louder in the direction of the axis ; so also 

 a person speaking is heard much more distinctly directly in front 

 than at an equal distance behind. Many experiments have been made 

 on this point, and I may mention those repeated in the open space in 

 front of the Smithsonian Institution. In a circle, 100 feet in diame- 

 ter, the speaker in the centre, and the hearer in succession at different 

 points of the circumference, the voice was heard most distinctly di- 

 rectly in front, gradually less so on either side, until, in the rear, it 

 was scarcely audible. The ratio of distance for distinct hearing 

 directly in front, on the sides, and in the rear, was about as 100, 75, 

 and 30. These numbers may serve to determine the form in which an 

 audience should be arranged in an open field, in order that those on 

 the periphery of the space may all have a like favorable opportunity 

 of hearing, though it should not be recomended as the interior form of 

 an apartment, in which a reflecting wall would be behind the speaker. 



The impulse producing sound requires time for its propagation, and 

 this depends upon the intensity of repulsion between the atoms, and, 

 secondly, on the specific gravity of the matter itself. ^ If the medium 

 •were entirely rigid, sound would be propagated instantaneously ; 

 the weaker the repulsion between the atoms, the greater will be the 

 time required to transmit the motion from one to the other ; and the 



