SM208     Siren     circa 1963     Sound

The siren is an apparatus that is used to measure the number of vibrations of a body in a given time.   The name was given by its inventor, Cagniard Latour, because it yields sounds under water.

The siren consists of a cylindrical box where air can be introduced under pressure.   This is closed by a plate with a series of equidistant circular holes placed at a constant radius.   The holes are inclined to the perpendicular.   A moveable disk with holes corresponding to the fixed disk but with holes inclined in the opposite direction is placed almost touching it.   The rate of rotation of the moveable disk in a given time period can be measured by means of a scale calibrated with the number of revolutions that it makes.

The pitch of the sound is directly proportional to the rate of rotation of the disk and the number of holes in it.

For the same velocity of rotation the siren emits the same pitch of sound in any fluid e.g. also in water.