This video demonstrates the principle of a disk siren.

The device consists of a rotating machine accompanied by a series of cardboard perforated discs and bearing indications on their use. The siren of the Opelt disk has 24 series of concentric holes. The top 15 produce simple sounds; others generate different arrangements and musical intervals. The disk is placed on a rotating axis. One of the drive belts connects to a large pulley fitted with a handle. The disk is rotated rapidly. A flow of compressed air coming out from a glass nozzle is directed against the disk. The rapid periodic interruptions of the air flow merge into a note whose frequency depends on the speed of rotation and number of holes. The different notes and chords are clearly audible. Using two or more nozzles, it is possible to generate more complex sounds.