DUDE! OEM electric cutouts, basically? How do they achieve 4 levels of adjustment? Dual mufflers, with options routing to both, one, or none, plus a route that bypasses a resonator? Or maybe something with exhaust valve timing is part of it? Maybe the mufflers themselves have multiple routes inside them? Or maybe it's not simple binary routing at each cutout, but mixture routing, where it goes halfway so some exhaust bypasses noise control and some doesn't?
I went to look it up, and found Motor Trend saying "Lots of cars have buttons or menus from which to change the exhaust setting"...really? I haven't been paying much attention to all these cars I can't remotely afford I guess. Neato!
C&D says "As in other cars equipped with active exhaust systems, the 2018 Mustang GT’s optional pipes are able to adjust the amount of noise released from its exhaust pipes courtesy of valves that open and close as operating modes and throttle input dictates." ...ok, so that confirms my cutouts guess, but still leaves me with my original questions.
C&D also says "In its loudest setting, the Mustang GT’s active exhaust lets out more than 80 decibels of unadulterated V-8 fury at startup, about the same as your average garbage disposal. Quiet mode holds back the raucousness and limits the 460-hp 5.0-liter V-8’s roar to just 72 decibels at startup. That’s equal to the amount of noise that enters an automatic-transmission-equipped Honda Fit’s interior at 70 mph."
Oh. That's not so huge of a difference (and odd to select a garbage disposal as their example). Decibels are logarithmic so it's not merely 10% difference, but still, a garbage disposal isn't much louder than a quiet car starting - especially when you consider that there's more than just exhaust noise involved, but that 72 decibel rating was just for the exhaust itself.