SIRENE, an instrument for the production of musical sounds in such a manner as to enable us to discover their ultimate nature. In the simplest form of sirene a vane con sisting of four equal plates, attached to a delicately supported axle, is so fixed in a metal tube as to close it almost completely (with the help of stops), when either pair of plates is perpendicular to the axis of the tube. When air is forced from a bellows through a pipe, it gives the vane a rotation, and thus produces a current which is inter rupted four times in each revolution. In other words, four times in each revolution the air escapes freely, giving rise to a sound. While the vane revolves slowly, the ear dis tinguishes these successive puffs; but when the revolutions are more numerous than about five per second, the successive puffs cannot be distinguished, and the recurrent sounds are merged into a uniform note, whose pitch rises it becomes more and more-shrill) the faster the vane revolves. Such an instrument works well when driven by water instead of air. What it shows is, that musical sounds consist of the repetition, at equal very small intervals of time, of some definite noise. By turning the vane by means of a train of wheels, so as to give it a definite rate of rotation, the number of repetitions per second, necessary for the production of a given musical note, may be measured.
But the sirene of Cagniard de la Tour is much more valuable for such a purpose, as it counts for itself the number of repetitions per second. In principle, it is identical with the simpler instrument just described; but the details of its construction are differ ent. It consists essentially of two circular disks, the upper of which is free to revolve
so as almost to touch the lower. In each a series of holes is cut, arranged at equal dis tances in a circle about its axis. Through the holes in the lower (fixed) plate, streams of air are admitted from a bellows, and pass through the corresponding holes in the upper (movable), plate, when the pairs of holes are superposed; but are checked when the upper plate is turned a little, readmitted when the plate turns a little further, 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 Sown), like those of the flute or tuning-fork. The axis of the upper plate carries an endless screw, which turns a light train of wheels (with dials) resembling that of a gas meter, so that when, by proper adjustment of the pressure in the bellows, the instru ment gives steadily some definite note, we may observelhe number of turns in any num ber of minutes by watch. The number of puffs is obviously to be found from this by multiplying by the number of holes in the plate, since during one turn any hole in the upper plate has been opposite each of those in the lower plate in succession. Thus we find the number of puffs per second necessary to the formation of any given musical note.
More complex forms, such as Helmholtz's double sirene, have been devised for more recondite branches of the science. See SOUND.