The audible realm depends on a condition of dynamic equilibrium, an overall stability of the air or other medium - within this realm occurs a kind of sound - very noticeable, very carrying- the sound of singing or a flute or organ pipe or of a stretched string.

This kind of sound, harmonic sound, consists for example in a pipe of air an oscillation of the whole, of halves, of thirds, quarters and so on with generally decreasing loudness. Within each note there is a complex of many notes, a vibration, and these all bear simple whole number divisional relationships to each other, a harmonic vibration.

• Acoustics