“Envelopes” in general are an important concept in synthesised music, because they are used to shape sounds in many ways, but the one most often encountered is the ADSR envelope. Let’s have a look at what this is.
Above, you can see a diagram of an ADSR envelope. I have actually drawn this with realistic proportions. If you imagine a key being pressed on a physical or virtual MIDI keyboard, with a pad patch selected, it might typically produce a sound matching this envelope, and the envelope represents the amplitude (or volume) of the sound generated over time. Let’s have a look at its phases:
The attack phase begins when the MIDI “note on” message is received – when the key is pressed or the note is encountered in a sequencer. You can see in the diagram that this pad begins inaudible then rises in a period of only half a second to the maximum digital volume of 0dB. So this pad starts quite quickly.
The decay phase defines how long the note will take to reach a settled volume after hitting the attack peak. That settled volume is usually lower than the attack peak, since notes in natural instruments tend to “stab” in then fade in intensity a little, but it could indeed be higher. In the example above the pressed note fades again over half a second to a slightly lower amplitude.
The sustain phase is an important one to understand because the envelope does not determine its duration. Thus the length of the horizontal bar in the diagram (indicating 2 seconds) is actually quite arbitrary. The sustain level is the volume at which the decay will finish and the note will be held at this volume until the MIDI key is released and a “note off” message is received. So this could be after one second or perhaps six seconds.
Once the note has been released, the envelope enters its release phase. The line of the envelope here defines how long the note’s volume will take to fade to zero. So in our pad here, it takes a full five seconds, so this is quite a long fade out (with reverb applied it would take even longer!).
Envelope lines do not need to be straight: many synthesisers will allow you to make them curved, making sharper attack sounds, for example. Some even allow additional phases. This, in any case, is the essential outline of the ADSR envelope. I hope it helps you!