ADSR Envelope Explained in Simple Terms
An ADSR Envelope is one of the key elements of shaping a sound via samplers and synthesizers. ADSR is the abbreviation for Attack, Decay, Sustain and Release. All 4 of these terms refer to different types of modulation on the sound, often in the form of modifying it’s amplitude (volume) and the set durations (time) of these changes.
As a whole an ADSR envelope will determine (attack) how fast a sound reaches it’s maximum volume, (decay) how long it takes to drop down to the sustained level, (sustain) what level of volume the sound will remain at while the sample or synth remains triggered (key is held down), and (release) how long it takes the sound to fade out from the sustain level down to silence after the key is released (no longer holding down the note, or triggering the sample).
Why Do Samplers And Synths Need To Implement ADSR Envelopes?
Although ADSR envelopes can be used for various different applications, one of the most important things it allows in a synth or sampler is the emulation of “timbre”. Defined as “the character or quality of a musical sound or voice as distinct from its pitch and intensity”, timbre is essentially what allows us to distinguish the sound of one instrument from another, even if they’re playing the same note simultaneously at the same amplitude or volume level.
ADSR does not entirely replicate the timbre of a physical instrument, but it is requirement to achieve something even remotely similar to the sound of a real instrument. If you were to trigger a sample to play a tone that abruptly started at the maximum amplitude, stayed at that volume level as you held the key/note/pad down, and then abruptly stopped when you released the key, it would sound extremely unnatural and foreign in relation to your typical woodwind, brass, strings, and percussion instruments.
Below I’ll go into detail of each aspect individually to help explain the modulation caused by an ADSR volume envelope, and also touch on when a “hold” parameter is added to the mix resulting in an AHDSR envelope.
Attack is the duration or time that it takes for a signal to reach the highest point of amplitude after being triggered/sounded. Often the attack of a sound is so quick that it is perceived as happening instantly, and then quickly followed up by the decay phase of the ADSR envelope, although it’s common to have a longer attack phase on certain synthesized sounds (pads come to mind). It’s important to note that ADSR is not always applied to just volume envelopes, as can be seen in the attack or release parameters found on a compressor.
Decay, like attack, is a parameter that sets a duration – But rather than the time it takes to increase the volume level, decay represents the time it takes to drop down to the sustain level after reaching the initial peak of the attack phase.
Sustain is the only factor in ADSR that doesn’t represent time. The sustain is a level of amplitude that the signal remains on for as long as the sample/synth/sound is being triggered. In regards to a keyboard, the duration of the sustain is determined by how long you hold down a key.
Release, as with attack and decay, represents a change over time. The release phase begins as soon as the sample stops being triggered (Back to our keyboard example, this would be when you stop holding the key down). The release parameter determines how long it takes for the sound to fade out completely from the sustain level. You can think of it as the opposite of the attack in a sense.
While an ADSR envelope utilizes the attack, decay, sustain and release, an AHDSR envelope also implements what is called “hold”. Rather than decaying to the sustain level immediately after the initial peak as in ADSR, an AHDSR envelope allows you to set a duration that the signal remains at the peak before initializing the decay phase. The addition of a hold parameter gives you more control over the instrument or sound, and can be useful for a variety of different applications – for instance on an 808 bass drum.