That portion of the ENVELOPE of a sound or SOUND OBJECT when its AMPLITUDE is relatively unchanging, that is, the portion between the end of its ATTACK and the beginning of its DECAY, usually heard as stationary, i.e. more or less unchanging. Also called steady state.
However, unless the sound is produced electronically, for instance by an OSCILLATOR with a fixed WAVEFORM, the SPECTRUM of the sound will show subtle variations (see diagram under FOURIER ANALYSIS).
Compare: DRONE, HUM, INTERNAL DYNAMICS, STATIONARY SOUND.