SIREN. An instrument for the production of musical sounds in such a manner as to enable us to determine the number of vibrations which produce a given sound. or. in other words, the pitch. In the simplest form of siren there is a revolving disk which is pierced with a series of holes arranged in a circle whose centre is the centre of the disk. If air forced through a tube from a bellows or other source of pressure strikes the disk at a point which is passed by the holes in their revolution, a series of sounds will be produced by the successive puff of air escaping through these openings. While the disk revolves slowly. the ear distinguishes these successive puffs; but when the revolutions are more nu merous than about ten per second, the successive puffs cannot be distinguished, and the recurrent sounds are merged into a uniform note, whose pitch rises (i.e. it. becomes more and more shrill) the faster the disk revolves. Such an instru ment works well when driven by water instead of air. What it shows is that musical sounds eon sist of the repetition, at equal and very small intervals of time, of some definite noise. By turning the disk by means of a train of wheels, so as to give it a definite rate of rotation, the number of such repetitions per second necessary for the production of a given musical note may be measured.
The siren invented by Cagniard de in Tour in 1819 is better adapted for such a purpose. as it registers the number of revolutions per second. In principle it is identical with the simpler in strument just described ; but the details of its construction are different. It consists essen tially of two circular disks, the upper of which is free to revolve upon the lower, being pivoted at A and b. In each a series of holes is cut, ar
ranged at equal distances in a circle about its axis. Through the holes in the lower (fixed) plate, streams of air are admitted from a reser voir. B, connected with a bellows, and pass through the corresponding holes in the upper (movable) plate, when the pair of holes are superposed; but are checked when the upper plate is turned a little, readmitted when the plate turns a little farther, and so on. The holes are pierced obliquely through the upper plate, so that the issuing stream makes it turn about its axis. The sounds given by this instrument are exceedingly pure (see ACOUSTICS), like those of the flute or tuning fork. The axis of the upper plate carries an endless screw. S, which turns a light train of wheels, G and H, with hands and dials resembling those of a gas meter, so that when, by proper adjustment of the pressure of the bellows, the instrument gives steadily sonic definite note, corresponding with that of an organ pipe or tuning fork whose pitch we desire to ascertain, we may observe the number of turns in any number of seconds by a stop-watch. The number of puffs is obviously to be found from this by multiplying the number of holes in the plate.
A large instrument operated by steam is used as a fog signal, while more complex forms, such as Helmholtz's double siren, have been devised for investigations in the more advanced fields of acoustics. Consult: Tyndall. Sound: Lehrbuch der Physik (Brunswick, 1S97 ) ; Helmholtz, Tonempfindungcn. English translation by Ellis (London and New 189.5). See ACOUSTICS ; FOG-SIGNALS.