ADSR stands for Attack, Decay, Sustain and Release and they are the 4 primary stages of an envelope. Attack, decay and release are all time measurements while sustain is a decibel level. The attack is how fast the instrument or sound reaches is loudest amplitude, the decay is how fast the sound reaches its sustained level. The sustain level is how loud the sound is while holding a particular key or note. Finally, the release is how long it takes for the sound to reach absolute silence after the key or note is released.
On a synthesizer or sampler, an envelope shapes a sound over time based on pre-determined values and can apply to volume, filters, LFOs and pitch. Many instruments offer customizable source and destination routing capabilities where you may use existing envelopes to trigger odd parameters.
Acoustic sounds have natural amplitude envelopes and can be manipulated by the performance of the instrumentalist.
Transients are described as spikes in amplitude. For instance, the moment a drummer hits a snare with a drumstick, the initial impact of the stick on the drum skin is considered the transient. The attack and the decay stages of an envelope dictate the transient information.
The faster the attack and decay times are the sharper the transient. Conversely, a slow attack and slow decay create a dull transient and if slowed down enough, the transient becomes undetectable and is perceived as a mere volume increase and decrease over time.
Let’s consider a violinist playing and holding the note C… naturally, a violin has a slow attack and decay time compared to that of a drum transient and therefore will be almost indistinguishable. As the violinist holds the note, a decibel level is maintained. This is considered the sustain level of an amplitude envelope. The level at which the note is being held. After a few seconds or so, the violinist stops holding the note and the final stage of the envelope is reached, the release stage. As the strings of a violin do not continue vibrating too long after the note is let go, this would create a fast release. An example of an instrument with a long release is an electric guitar as the strings continue to vibrate a considerable time after a note plucked.
With instruments that are struck, such as a snare drum, a guitar string or a bell, the sustain level is practically non-existent and the attack, decay and release stages predominantly shape the sound.
Let’s look at three instruments and their amplitude envelopes in terms of ADSR:
Here's a visual sample of each sounds amplitude envelope:
Envelopes are powerful tools to create movement and life, set the amplitude envelope before tweaking any others as the amplitude envelope is the only one that allows all others to exist!