A siren is a device basically used to make a warning or to give a signal. It produces a loud, piercing sound of a definite pitch. It is powered by electric motors, steam or hand cranks.
Sirens are used for various purposes, for example, in factories warning signal for any danger or accident; for cautioning people in case of enemy attacks; for traffic clearance by the vehicles of emergency services etc.
Siren was invented in the late 18th century by a Scottish natural philosopher, John Robinson. It was named thus by a French engineer, Charles Cognaird de La Tour, who devised an acoustical instrument of the type in 1819 which is not generally used nowadays.
The Cognaird siren consists of a cylindrical hollow box ‘C’, known as the wind chest. It has a fixed top lid ‘B’ that consists of a number of equidistant slanting holes arranged in circles. The chest is fixed at its lower end with a tube ‘T’, through which air can be blown in it by means of bellows. Another disc ‘D’ of the same size, with equal number of equidistant holes slanting in the opposite direction covers the first disc. This disc can be rotated by a mechanical arrangement. The air under pressure is forced into the wind chest of the siren due to which the disc begins to rotate. The air passes through the holes in the lower disc, and comes out through the holes in the upper disc. Every time the holes in the upper disc are above the holes in the lower disc, a puff of air escapes. This creates a sound. The pitch of the sound depends upon the speed at which the disc rotates.
Now improved versions of this siren are available. One of such types of siren uses two small cylinders one inside the other, which have holes in them. An electric motor rotates the outer cylinder. Another device forces air or steam through the cylinders. The holes in the cylinders are actually small slots which are cut on a slant in lines encircling the cylinders. The air enters the slots in the first cylinder, and then passes through the slots in the second cylinder. Every time the slots are exactly opposite to one another, a sound is created by this movement of air through the slots. The faster the rotation of the outer cylinder, the louder the sound.
