The syNth Degree: Envelopes

In synthesizers, samplers and other virtual instruments, there are many different parameters (like pitch, volume, filter frequency, etc.) that can be dynamically controlled. One way to control these is through modulators. A modulator is something that affects a change in something else (think about modulating from one key to another). The first type of modulator that we'll look at are envelopes.

Envelopes

An envelope is something that opens and closes (just like a mailing envelope). Set an envelope to open or close a parameter over time, which allows for subtle (or not subtle) evolution of a sound. There are anywhere from 2 to 5 or more legs of the envelope, detailed below.

ADSR

ADSR stands for Attack, Decay, Sustain, Release, which are the four most common legs of an envelope. Some envelopes will feature others such as delay and hold. 

Delay: The amount of time that the envelope delays before kicking in (this is a less common envelope feature)

Attack: The amount of time it takes for an envelope to open

Hold: The amount of time the envelope stays open before beginning to decay (this is a less common envelope feature)

Decay: The amount of time it takes for the envelope to decay from the fully open position to the sustain level

Sustain: The amount that the envelope stays open for the duration of a note or phrase. NOTE: Sustain is the only parameter that does not have a specified length of time. A note can be held for one second or twenty minutes, and sustain determines the envelope level for the duration of a note after delay, attack, hold and decay have run their course.

Release: The amount of time the envelope takes to close again after a note or phrase is released

Amp Envelopes
Take for example an amp envelope - an envelope that controls the overall volume or amplitude of a device. Here is an audio clip from a synthesizer that employs a volume envelope. This envelope has a 2 second attack, one second decay, a sustain level of -12 dB (meaning that the decay will decrease the volume 12 dB from the peak volume of the attack) and a release of 2 seconds.
Using an amp envelope you can simulate how different acoustic instruments behave. Let's explore the naturally occurring volume envelopes for a piano and a clarinet:

Piano

A piano is a percussive instrument, and like all percussion instruments that are struck, they are at full volume immediately as the note is played, and can only decrease in volume thereafter.

Clarinet

While a clarinet can certainly play a note full volume, then decrease, this instrument has a fairly unique ability to begin a note from near silence, have a long slow attack, and a gentle decay and release.

Inverted Envelope

Whether using an envelope to affect volume or any other parameter, their function is to modulate a parameter change over time. Keep in mind that the parameter does not have to go from completely closed to completely full. An envelope can change a parameter between any two set locations. Perhaps you want a filter frequency to began mostly open, then close partway, then open back up again. This can be done with an inverted envelope, one that begins open, uses the attack to close, then uses the decay and release to open back up again. Here is an audio clip from a synthesizer that employs an inverted filter envelope. This envelope has a 2 second attack, one second decay, a sustain level of 50% (meaning that the filter frequency will return halfway after the decay) and a release of 1.5 seconds. Note that the amp envelope has a 2 second release. Without this, the filter envelope release would not be apparent.