A pure audible tone’s only correlate is a single sinusoid

Evidence

The tuning fork in its lowest, most easily excited mode of vibration emits just one sinusoidal tone at a single frequency. This is a “pure” or “simple” tone—the atom of sound. A sinusoid is perfectly periodic, with the period given by the inverse of the frequency. Most sound sources generate complex tones consisting of many simultaneous sinusoids, even if they are periodic. It was by no means obvious in the nineteenth century that the only correlate to a pure tone is the sinusoid. This is the essence of Ohm’s law, which will be taken up in earnest in the chapters on pitch (chapter 23) and timbre (chapter 24). — Eric J. Heller, Why You Hear What You Hear- An…, 287

Categorization

• DEFINE: Defines the ‘atom of sound’ — the pure tone as a single sinusoid — and flags Ohm’s law as the governing principle.
• QUESTION: Why was it not obvious in the nineteenth century that the sinusoid is the only correlate to a pure tone? The non-obviousness itself is curious — what competing candidates existed?