When musicians talk about the sustain of an instrument, The sustain, listen to it. They don’t mean exactly the same thing synthesis means. You would though, if you’re playing.
And that’s true of decay as well. To a traditional instrumentalist, both of these are measures of time. And while sometimes synthesizers will conflate the terminology, ADSR is all about the intersection of time and level.
So while you wouldn’t be wrong to think of attack and release as being durations, typically you’re plotting a coordinate with an assumed value of 100 and 0%. With sustain and decay, that’s not typically the case. So while decay does plot a duration, sustain plots its level.
And combined they form an axis that describes a single coordinate. Which stands in contrast to release and attack, which typically are fixed at 0% and 100%, unless of chorus. These count to 11.
