Chapter One: An Acoustics Primer
6. What is amplitude? | page 5
Dynamic envelope refers to the amplitude change over time of a sound event (usually a short one, such as an instrumental or synthesized note). As a very simple example (because there is usually much more going on in acoustic sounds), a note can have an initial attack characterized by the amount of time it takes to change from no sound to a maximum level, a decay phase, whereby the amplitude decreases to a steady-state sustain level, followed by a decay phase, characterized by the time it take the amplitude to change from the sustain level to 0.
Not only do real world (and complexly synthesized) sounds have more complex overall envelopes, but they often exhibit different envelopes for all their individual frequency components.
For further study, see Hyperphysics->Sound Level Measurement