For many people, especially those fond of presets, the envelope controls are never explored to their full potential. Almost everyone will know the words Attack, Decay, Sustain, and Release (or ADSR for short) but mastering these four controls will give you more power over your synth than any other feature.
The amp envelope on Free VST
There are many possibilities for using these envelope controls and they can control pretty much any parameter, but you will be most familiar with Amp and Filter envelopes, hence I'll stick with these for examples. So let's look at easy ways to understand each function, I'll try and include examples where possible to help you imagine the effects.
Attack: This is the one control in particular you want to play with at first. It controls the onset of the sound. The higher the attack, the longer it takes for the sound to reach full volume. A drum for example has a very fast (low value) attack as it goes pretty much from silence to full volume instantly. Compare that to a soft cello, which takes much longer.
(Top - a drum hit; Bottom - a cello note)
I'd like to draw extra attention to this image which includes
"key pressed" and "key released".
If you want your synths to hit a little harder, it may be worth turning down your amp envelope attack and making them hit full volume faster. If you wish to soften them up a little bit, turn the attack up so it takes longer.
Decay: When you strike a note on say, a guitar or piano, it has an initial loudness (which hits at the attack phase). The decay is the time the note lasts at that initial peak volume (or cut-off if you are using a filter).
Imagine you hit your guitar string very hard, it will sound a lot brighter initially than it will after holding the note for a second, likewise, it will be a lot louder for a brief moment shortly after the string is struck.
We can use the envelope decay to mimic this. If we want a guitar sound, use a fast attack (not instant) and a moderately fast release on both the amp envelope and the filter. Adjust the filter decay to be slightly longer if you want a brighter sound that lasts longer into the note.
Sustain: is the level (volume or cut-off frequency) that the note remains at while it is being played. After the initial note strike, when holding a piano key down it will play at a sustained volume.
This is the confusing one, Sustain is the only control which is (almost always) not a time value. Instead it is the volume the note remains at, after the decay, until the key is released. A sustain at full will be pretty much full volume, while a sustain with a low value will sustain quietly.
If you are designing a synth sound where you want the note to stop, even if you are still pressing down the key, the best thing to do is set the sustain to zero and make the decay last for the duration you want the note to last. Otherwise the note will sustain for as long as the key is held down.
Release: is the time taken for the note to disappear after you release the key. It is useful for longer percussive sounds e.g. mimicking a glockenspiel or a bell. A long release will last long after you let go of the key and can sound especially nice for dreamy pads. It is not ideal for complex or fast melodies as the note tails last well into the next notes - not a problem if you want a wash of sounds in harmony, but it can be hard to compose something that sounds nice with those tails.