Creating a window on a sound file

A "window" is a function (shape) used to control the amplitude of a sound, briefly turning it up then turning it back down. This example shows how to create a trapezoidal window on a sound file, which will quickly fade the file in and out for a specified duration, repeatedly at some specified triggering interval. The metro triggers a message to the line~ object. The message is formed by taking the specified window duration, subtracting 10 from that number of milliseconds to allow for a 5 ms fade-in and a 5 ms fade-out, then forming a list that instructs line~ to control the amplitude with that shape. The message says, in effect, go to 1 in 5 ms, stay at 1 for duration-10 ms, then go to 0 in 5 ms.