Helmholtz resonator

From Ccpi

In addition to the double siren, Helmholtz also described in his 1862 book, On the Sensations of Tone, an apparatus able to pick out specific frequencies from a sound, now called the Helmholtz resonator. It consists of a rigid container of a known volume, usually round in shape, with a small neck and hole in one end.

Air is allowed to flow freely in and out of the container; air pressure changes outside the container cause the air in the neck to oscillate in and out. By placing the resonator's 'nipple' inside one's ear, specific frequencies of the complex sound can be clearly heard.

Among other things, the audible frequency depends on the volume of air in the container, so numerous resonators were necessary for complex experiments.