3. On Beats of Imperfect Harmonies
William Thomson · Proceedings of the Royal Society of Edinburgh · 1878
According to a usage which has been adopted from the German of Helmholtz by the best English scientific writers on sound, a sound is called a “simple tone,” or without qualification a “tone,” when the variation of pressure of the air in the neighbourhood of the ear which is the immediate excitant of the sense is according to a simple harmonic function of the time; that is to say, when the whole pressure of the air varies in simple proportion to the distance, from a fixed plane, of a point moving uniformly in a circle.