Apple Logic Express 7 - Manual (Page 80)

80 Chapter 12 Synthesizer Basics
When you strike a key, the envelope travels from zero to it’s maximum level in the
attack time, falls from this maximum level to the sustain level in the decay time, and
maintains the sustain level as long as you hold the key. When the key is released, the
envelope falls from its sustain level to zero over the release time. The brass or string-like
envelope of the following sound—the envelope itself is not shown in this graphic—has
longer attack and release times, and a higher sustain level.
The envelope generator can also control the rise and fall of the cutoff frequency. You
can also use envelope generators to modulate other parameters. In this context,
modulation can be thought of as a remote control for a given parameter. There are
more sources that can serve as a modulation source: the pitch (note number), velocity
sensitivity or the modulation wheel, for example.