Felix Savart (1791-1841) is probably best known for his 1820 investigation (with Biot) of the strength of a magnetic field as a function of the local geometry and the current through a wire. Most of his research dealt with acoustics, including Savart's wheel, which he used for research on the lower frequency limit of hearing. .
Demonstrations with Savart's wheel ought to convince anyone of the source of sound. The wheel is set spinning, and a playing card or a file card is held against the serrations on the rim of the wheel. At very slow rotation rates, the eye can see the edge of the card moving, but at higher rates it blurs, and a tone is produced. The frequency is directly proportional to the rotation rate. If some sort of counter were provided, Savart's wheel can be used, along with a stopwatch, to find the frequency of the sound. Thus, like the siren, it can be used for the absolute determination of pitch.