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1. The word sound is commonly used in two different senses: (1) to denote the sensation perceived by means of the ear when the auditory nerves are excited; and (2)to denote the external physical disturbance which, under ordinary conditions, suitably excites the auditory nerves.
This usage will generally be followed here. It rarely leads to any ambiguity, the context generally showing in which of the two senses the word is employed. When necessary, for clearness' sake, the first or subjective sense will be represented by " the sensation of sound," and the second, or objective sense, by "sound waves," or other similar expressions.
It is a matter of common knowledge that the source of sound is always a body in a state of vibration, or rapid toand-fro motion. This body may be a solid, as the string of a harp; or fluid, as the column of air in a wind instrument.
We may, therefore, define Acoustics, or the study of sound, as that branch of physics which deals with vibratory motion as perceived by the sense of hearing. It is usual, however, to include with this a few other closely allied phenomena.
2. Production of Sound.—But in order to produce sound it is not sufficient to have some body in a state of vibration as its source. We need also (1) some medium to receive and transmit this vibratory motion, otherwise neither the sensation of sound nor the external disturbance would be present. (2) It is imperative that the parts of the body in vibratory motion should have such shape, size, and motion as to cause a disturbance to advance through the air, and not such as to produce a local flow and reflow of the air simply. (3) Our ears enable us to perceive the sensation of sound only when affected by to-aud-fro movements whose number per second lies between certain limits. Therefore, to produce sound sensations, it is necessary that our vibrating body should conform to this requirement also. These points are respectively illustrated by the following experiments:—
Expt. 1. A Medium Essential.—To illustrate the necessity of a medium to convey sound from its source to the ear, hang a bell by india-rubber cords within a glass bulb fitted with a tap. Sound the bell in the bulb full of air with the tap open, and then with the tap closed, so as to indicate its loudness in each case. Next, exhaust the bulb as completely as possible by an air-pump. Detach the bulb when exhausted and shake so as to attempt to make the bell sound. No sound, or only an extremely feeble one is heard. While the bell is inaudibly shaking, open the tap so as to admit the air; the sound is very quickly restored. Contrast with this the case of an ordinary electric glow-lamp, whose incandescent filament sends light to us across the space within its bulb although it is practically devoid of air. Light, like sound, needs a medium for its propagation. But the medium essential to the propagation of light is the ether which we are at present unable to remove from any space.
Expt. 2. Importance of Sound Board.—Fit up two steel wires each about 1 mm. diameter—one on a monochord, the other on the bars of two 56 lb. weights. The latter should have the same length between the bars that the former has between the bridges, and they should be tuned to the same pitch. It is desirable that the 56 Ib. weights should not rest upon wood. They may both be on a level tiled floor, or slab of stone or slate. Or, the wire might be