Acoustic Envelope   

This is another important cue in addition to spectrum that allows us to discriminate one sound from another. 
From the moment a sound begins until it ceases there is usually a characteristic variation in the sounds overall intensity or amplitude. 
In its most simplified form three parts of this acoustic envelope can be distinguished: 
The initial attack portion or transient is very important in providing cues as to the nature of the sound being started.  The transient is followed by a period of more or less stable internal dynamics often described as a sustain.  The sound finishes with some kind of decay to silence. 
Sound Examples:  
  clarinet    attack only    sustain only    decay only 
An envelope more commonly used in electronic music instruments is the ADSR which provides for an attack, a decay to some sustain level and a release to silence: 
Amplitude fluctuations and the speed at which these fluctuations occur in each of these parts of the envelope account for a great variety in the perception of a timbre