Page:Elementary Text-book of Physics (Anthony, 1897).djvu/163

 CHAPTER I.

ORIGIN AND TRANSMISSION OF SOUND.

127. Definitions.—Acoustics has for its object the study of those phenomena which may be perceived by the ear. The sensations produced through the ear, and the causes that give rise to them, are called sounds.

128. Origin of Sound.—Sound is produced by vibratory movements in elastic bodies. The vibratory motion of bodies when producing sound is often evident to the eye. In some cases the sound seems to result from a continuous movement, but even in these cases the vibratory motion can be shown by means of an apparatus known as a manometric capsule, devised by König. It consists of a block $$A$$ (Fig. 43) in which is a cavity covered by a membrane $$h$$. By means of a tube $$c$$ illuminating gas is led into the cavity, and, passing out through the tube $$d$$, burns in a jet at $$e$$. It is evident that, if the membrane $$b$$ be made to move suddenly inward or outward, it will compress or rarefy the gas in the capsule, and so cause the flow at the orifice and the height of the flame to increase or diminish. Any sound of sufficient intensity in the vicinity of the capsule causes an alternate lengthening and shortening of the flame, which, however, occurs too frequently to be directly observed. By moving the eyes while keeping the flame in view, or by observing the image of the