Variable Slope Curves
The solutions to this are either, use the Multi-BP envelope Macro which provides a very flexible graphic breakpoint tool a bit like those in Absynth and FM7. (Under no circumstances attempt to understand how the internals of this module work without a packet of asprin).
Alternative Solutions
Or take a regular envelope and raise the output to the power of something (numbers between 0 and 1 will make the envelope start more dramatically, numbers greater than 1 will produce a curve that starts gently then accelerates).
If you're working on a Mac, there's a very handy Grapher tool in the Utilities folder that you can plug equations into to experiment with.
