ADSR Envelopes are very commonly used in synthesiser designs – in fact you’d be hard pressed to find a synthesiser that didn’t feature an ADSR Envelope generator.
But what are they and how are they used?
What is an ADSR Envelope?
ADSR is an acronym which means Attack, Decay, Sustain and Release.
In an ADSR envelope these four elements are given values. Generally these will be time-based values for Attack, Decay and Release (such as seconds). Sustain will generally be a percentage (of maximum volume).
An ADSR envelope is generally used to control the intensity, or volume, of a particular signal but may also be used for other things such as managing the frequency cut-off of filters. For demonstration purposes we will focus on using ADSR envelopes for controlling volume as that is probably the easiest to visualise.
The diagram below is a pictorial description of the ADSR envelope over time.
Imagine that the envelope is applied to the volume on a sound at the moment the sound starts. Using a synthesiser example, that would be at the moment a note was triggered.
Starting in the Attack phase, the volume would start at 0 and then move up towards peak amplitude. The time taken for the note to reach the peak amplitude from the zero point is the Attack setting – i.e. 2 seconds. As a result the note will fade in over this time.
Once the note reached peak amplitude the envelope would then transition to the Decay phase. In this phase the note volume reduces from the peak amplitude to the Sustain level. The time taken for the note to reach the sustain level from the peak is the Decay setting – i.e. 2 seconds. The note will therefore fade to the sustain level during this phase.
Once the note has reached the sustain level then the Sustain phase is entered and the amplitude will remain constant at the defined level. The envelope will remain in the sustain phase at a constant level until the note was released.
At the point the note is released the Release phase will be immediately entered. The release phase will then transition from the sustain level to zero, taking the length of time defined for the release, i.e. 2 seconds. The note will therefore fade to zero during this time.
If the note were released at any point before the sustain phase then the Release phase would immediately be entered and the note would fade away to zero amplitude in line with the release settings.
Why would you use an ADSR Envelope?
As described above, ADSR envelopes can serve a multitude of purposes.
Let us consider here the application of an ADSR envelope settings operating on the volume of a note.
Here are some example settings and the effect they would have.
1. 0s Attack, 0s Decay, 100% Sustain, 0% Release
In this case the attack, decay and release settings are set to 0s so these phases will execute immediately. This would result in a note that reached maximum volume immediately, sustained during the Sustain phase at this volume level and returned to zero volume as soon as the note was release.
Using these settings the ADSR envelop would open as a simple gate, triggered by note on and off.
2. 1s Attack, 0s Decay, 100% Sustain, 1s Release
Here a note would have a 1 second fade in and out time, making the note sound softer than the above gate. There would also be a slight fade out, giving a little extra depth on note off.
3. 5s Attack, 0s Decay, 100% Sustain, 5s Release
This would have the effect of a very long fade in and fade out, giving a distinctive ethereal sounds. Very atmospheric and ‘floaty’.
4. 3s Attack, 5s Decay, 0% Sustain, 0s Release
In this case, due to the sustain being set at 0%, the note will be reduced to a zero volume once the decay step was complete, regardless of whether the note was still active or not. As a result, these settings would result in a note that faded in and out to zero while the note was still being held.
This type of envelope would be useful for controlling filters etc..
Hopefully that has given you a quick overview of what ADSR envelopes are and how they can be used to create some basic effects.
To get a full appreciation download the RedLion VST/AU FM Synth and experiment!
There are also several video tutorials available in our blogs that could assist your understanding: