Sound differs from light, heat, and electricity in that it can be propagated through matter only. Sound-waves are matter-waves, not ether-waves. This can be proved by placing an electric bell under the bell-glass of an air-pump and exhausting all the air. Ether still remains inside the glass, but if the bell be set in motion no sound is audible. Admit air, and the clang of the gong is heard quite plainly.
Sound resembles light and heat, however, thus far, that it can be concentrated by means of suitable lenses and curved surfaces. An echo is a proof of its reflection from a surface.
Before dealing with the various appliances used for producing sound-waves of a definite character, let us examine that wonderful natural apparatus
THE EAR,
through which we receive those sensations which we call sound.
Fig. 133.—Diagrammatic sketch of the parts of the ear.
Fig. 133 is a purely diagrammatic section of the ear, showing the various parts distorted and out of proportion. Beginning at the left, we have the outer ear, the lobe, to gather in the sound-waves on to the membrane of the tympanum, or drum, to which is attached the first of a series of ossicles, or small bones. The last of these presses against an opening in the inner ear, a cavity surrounded by the bones of the head. Inside the inner ear is a watery fluid, P, called perilymph ("surrounding water"), immersed in which is a membranic envelope, M, containing endolymph ("inside water"), also full of fluid. Into this fluid project E E E, the terminations of the auditory nerve, leading to the brain.
When sound-waves strike the tympanum, they cause it to move inwards and outwards in a series of rapid movements. The ossicles operated by the tympanum press on the little opening O, covered by a membrane, and every time they push it in they slightly squeeze the perilymph, which in turn compresses the endolymph, which affects the nerve-ends, and telegraphs a sensation of sound to the brain.
In Fig. 134 we have a more developed sketch, giving in fuller detail, though still not in their actual proportions, the components of the ear. The ossicles M, I, and S are respectively the malleus (hammer), incus (anvil), and stapes (stirrup). Each is attached by ligaments to the walls of the middle ear. The tympanum moves the malleus, the malleus the incus, and the incus the stapes, the last pressing into the opening O of Fig. 133, which is scientifically known as the fenestra ovalis, or oval window. As liquids are practically incompressible, nature has made allowance for the squeezing in of the oval window membrane, by providing a second opening, the round window, also covered with a membrane. When the stapes pushes the oval membrane in, the round membrane bulges out, its elasticity sufficing to put a certain pressure on the perilymph (indicated by the dotted portion of the inner ear).