by [Theo Verelst]

Musical instruments, and in fact all sound sources, can be analized by plotting the amplitude of the sound against time.

That plot of evolution of that amplitude is in synthesizer terminology called the 'envelope' of a sound.

To create a synthetic sound, the making of such an envelope is one of the ingredients for a succesfull sound 
imitation, which since the first analog electronic synthesizers is done in ways which can also be simulated 
by computers.

 amplitude
 ^
 |        /--
 |       /   \
 |      /     \
 |     /       -------------//----
 |    /                           \
 |   /                             \
 |  /                               \
 | /                                 \
 +--------------------------//------------> time

The above is a schematic representation of how the loudness of a sound can vary from the start to the end of the sound, 
using what is called a ADSR curve, which we will simulate in tcl.

ADSR stands for Attack, Decay, Sustain, and release, corresponding with the phases of a sound coming up, decaying, being sustained (for instance like the key on an organ bein held down), and finally dies away.