Exhaust note has a lot of inputs besides mufflers. A long time ago, I had a ’67 stang. Iron manifolds, glasspacks, built motor. You could hear that thing blocks away. Much nicer after the turbo mufflers were installed.
Later I did headers and turbo mufflers on my first bronco, much quieter. It should have been louder, but the engine was much more tame.
The latest issue of hotrod ran a few mufflers and pipes. Same exhaust, same engine, same mufflers, different pipe diameter: 10dB louder at idle with the larger pipes.
So here are the things that tend to make a louder exhaust:
Headers, short runs (exit in front of the rear tires), large pipes, built engines.
For a little more quiet:
Manifolds, full length exhaust, small diameter pipes, stock engines.
The only thing I haven’t figured out is if it is quieter to have the muffler at the front of the exhaust (old cars) or near the rear of the run (new car style).
With that in mind, what I run is a mildly built engine with manifolds into a single 70 series flowmaster under the passenger floor. It can set off alarms if they are on the right side of me. It still sounds good, but is quiet enough that I have many other noise issues to deal with (tires, wind, fan, intake) before I worry that my exhaust is too loud.