exhaust sound

[joshf2]

what characterises the way the exhaust sounds in a motor?

when i say this, there are different tones when the engine is under load, when its not under load, and when its under compression braking etc.

understandably, its "loud" when under load, ie WOT, but what about when its nearly silent in a small half-throttle range. im really hoping this makes sense.

and then theres a different tone when engine braking. and what causes the burble and pop on over-run...?

just curious