What we mean with this word is the amplitude-flow of a sound from the moment it is generated to when it fades out. To introduce this concept we need to use a practical example. The most eloquent example is a string-instrument, for example a guitar.
When a guitarist plays a note, we perceive the note's first sound impact, and then gradually the note fades away. The flow of the amplitude of the note is called the ADSR Envelope (an acronym of the words Attack, Decay, Sustain, Release), and has a behavioural pattern that can be applied to any sound and instrument.
Let's describe the 4 phases it undergoes in detail
Attack: the amplitude rapidly reaches its highest amplitude-value
Decay: After the attack, part of the initial energy is lost and amplitude diminishes
Sustain: Amplitude maintains an almost constant level for a certain amount of time
Release: L'ampiezza ricomincia a diminuire fino ad annullarsi.
The following is an example of ADSR enevelope. The wave form of a sound is delimited by a curve that descrives the amplitude-flow and which in mathematics is called envelope. A waveform is symmetrical so, in order to evaluate the envelope, we take into account the positive part only.
In the figure we have also highlighted the fact that the initial part of the sound has more high frequencies, which are the ones that effectively fade out first. Generally in the Sustain phase the presence of high frequencies has diminished whilst low frequencies continue to be present.
This is the sound produced by the 5th string of an acoustic guitar (A) and its visualization in time.
A note played by a guitar [Track 10]
![Sound theory - A note played by a guitar [Track 10]](/images/en/figures/layout/icona_suono.jpg "Sound theory - A note played by a guitar [Track 10]")
We can quite easily identify ADSR envelope.
ADSR Envelope
As we have already mentioned, this behavioural pattern can be applied to most musical instruments and sounds in general. What varies is the duration of the various phases. For example a snare drum will have very brief attack and decay phases. A violin on the other hand shall have far longer Attack and Decay phases.
Generally the Attack-Decay phase is the most important in terms of what characterizes a sound the most, and infact in modern synthesis tecniques the Attack-Decay part is preferably a real sample of the instrument played and the Sustain-Release part is often synthetically produced. There are two reasons for this: the first is to make the synthetic instrument get as close as possible to the real sound of the instrument, the second is in order to have greater control on the instrument itself. For example the simulation of a vibrato is very easily obtainable with a synthetic sound whereas it is a rather difficult procedure with a real sound sample.
Related topics on Wikipedia
ADSR Envelope