Amplitude Envelopes

In his work, On the Sensations of Tone, Hermann von Helmholtz characterized tones by the way in which their amplitudes evolved over time, that is, by their amplitude envelope.

He described the envelope as having three parts:

1. the attack: the time it takes the sound to rise to its peak
2. the sustain: the steady state portion of the sound (where the amplitude has negligiable chane)
3. the decay: the time it take for the sound to decay or fade out.

Figure 3: A sinusoid with an amplitude envelope.