Section 105.—SOUND.
Instruments for the production of sound are scarcely within the province of the mechanical engineer, but of late years several of them have been employed in connection with mechanical means for producing sound—for fog signals, whistling, and other forms of sound signalling. Musical sounds are produced by vibration of air from wind, string, or reed instruments. In wind instruments the vibration is produced by the lips and modified by the shape and length of the tube. Strings are either bowed, as in the fiddle, struck, as in the piano, or fingered, as in the harp. Reeds are springs vibrated by a current of air. In the harmonium and concertina class of instruments there are no tubes or pipes added to the reeds to modify the sounds produced; but in the organ pipe the reeds have pipes added which greatly augment and qualify the sounds. Other special sound-producing instruments are illustrated here.
[1917]. The Siren, or steam turbine whistle, the loudest instrument known, consists of a slotted cylindrical drum revolving inside a fixed drum; the slots are angular (see plan), so that the rush of steam revolves the inner loose drum rapidly and the sound is directed by the trumpet-shaped hood. A pair of slotted discs is also sometimes used for the same purpose instead of the slotted drums.
[1918]. Mechanical fog-horn; ordinary bellows are often used to supply the blast.
[1919]. Iron gong, struck with a muffled hammer.
[1920]. Harmonium reeds, or free reeds; the tongue covers a slot of same size and shape, and can vibrate into and out of it, but without touching its edges; the gravity of tone or pitch depends on the size and thickness of the tongue.
[1921]. Organ reed pipe; the tongue A in this case beats, with a rolling contact, upon the reed B, which is tubular, closed at bottom and opening at top into the pipe C, which extends upwards from the block D; E is the tuning wire which regulates the vibrating or free length of the tongue.
[1922] & [1923]. Wood and metal organ pipes, which are practically large whistles, the vibration of the column of air in the pipe being produced by the wind striking the edge of the lip A.
Steam whistles are bells with a ring-shaped slit below, from which the issuing steam strikes the lower edge of the bell.
Other forms are now made giving a more musical sound, and in some cases a double note, usually an interval of a major third, as C-E, by a modified form of pipe with two lips.
Striking bells of various shapes are extensively used.
Gongs are cheese-shaped metallic hollow suspended vessels, and are struck by a muffled hammer.
Musical sounds are also produced from slips of glass, or chilled iron, glass bells and tumblers, and also from resonant magnetic iron blocks.
[1924], [1925], [1926]. See [Sec. 98].
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