The double siren is an instrument that produces sounds from two sources in a way that allows
the phase of one sound to be changed relative to that of the other. It is capable of producing everything in the range of a half-tone to an octave.
A double siren consists of two coaxial Dove sirens—disks rotating at high speed. As air flows through a series of holes in each disk, a distinct tone is produced, the frequency of which depends on the rotational speed of the disk, and the spacing of the holes in the disk. On top of the instrument is a crank for adjusting the phase of the top disk relative to the bottom. In addition, each disk has multiple concentric series of holes that can be opened or closed, for producing multiple tones. Each disk contains eight concentric circles of holes The frequency of the sound is determined by running the siren for a known period of time, and then reading the dials on the front of the device, which count the total number of revolutions of the siren.
This was the first of the instruments built by Rudolph Koenig based on the designs of Hermann von Helmholtz, who published a description of the instrument in Die Lehre von den Tonempfindungen als physiologische Grundlage für die Theorie der Musik (On the Sensations of Tone as a Physiological Basis for a Theory of Music).
The following video shows the double siren in action, at no particular frequency or phase. With a motorized belt and air source of constant pressure, the siren could emit a constant tone at a constant volume.