http://www.autoevolution.com/news/2015-dodge-challenger-srt-hellcat-brutal-exhaust-sounds-video-81680.html

After lots of sucking, squeezing and banging from all 8 cylinders, the supercharged 6.2-liter HEMI V8 sends gases through a low-restriction exhaust system. It's an active one of that, featuring butterfly valves that stay closed at low rpms to limit noise levels. However, if you step on the gas pedal more firmly, those valves open to reduce back pressure and the staccato exhaust note's volume is turned all the way up to 11.


Altough, these days it is likely to be closer to Yamaha's driven EXUP valve rather than being vacuum operated.