Koenig's manometric flame apparatus

Sound from the source to be measured was concentrated by means of a horn or tube into one half of the capsule chamber.

The sound caused the diaphragm to vibrate which modulated a flow of flammable illumination gas passing through the other half of the chamber.

The illumination gas was passed to a Bunsen burner, the flame of which would then increase or decrease in size at the same frequency as the sound source.

The frequency of the sound could then be calculated from the apparent distance between the flame images in the mirror and the known speed of its rotation.

[1] [2] Alexander Graham Bell used this type of equipment to study the performance of his microphones and demonstrated it in his display at the 1876 Philadelphia Centenarian Exhibition.