Double siren, after Helmholtz

In a siren air is pressed through the holes in a fast rotating disc. The air vibrations thus formed cause the characteristic, penetrating tone. This double siren, after the design of Helmholtz, consists of two cylindrical chambers with a constant air flow producing the tone. Each siren has two discs with a different number of holes, which can be closed with copper sticks. With these sticks various pitches can be set. The frequency can be read from the dials. By means of the ivory knob at the top small variations in frequency can be set. Thus the device produces a tremulant note, which can be further explored. The Helmholtz-siren is one of the finest acoustic devices from a 19th century laboratory. It was made by the Berlin instrument maker Sauerwald.