A surprising amount of character can be added to a sound by changing its amplitude over time.
A very common means of defining the shape of amplitude is to use an ADSR envelope which stands for
Attack, Sustain, Decay, Release.
* Attack is the amount of time it takes a sound to reach its maximum volume. An example of a fast attack is a
piano, where as a cello can have a longer attack time.
* Decay is the amount of time after which the peak amplitude is reached for a lower amplitude to arrive.
* Sustain is not a time, but a percentage of the peak amplitude that will be the the sustained amplitude.
* Release is the amount of time after a note is let go for the sound to die away to zero.