Pure tone

A pure tone is a tone with a sinusoidal waveshape.

A sine wave is characterized by its frequency — the number of cycles per second, or its wavelength — the distance the waveform travels through its medium within a period, and the amplitude — the size of each cycle. A pure tone has the unique property that its waveshape and sound are changed only in amplitude and phase by linear acoustic systems.

Fourier theorem

The Fourier theorem states that any periodic waveform can be approximated as closely as desired as the sum of a series of sine waves with frequencies in a harmonic series and at specific phase relationships to each other.

The lowest of these frequencies (the fundamental frequency), which is also the inverse of the period of the waveform, determines the pitch of the tone, which is perceived by the human hearing. In music, notes are assigned to tones with different fundamental frequencies, in order to describe the pitch of played tones.