Pretty much. Outer pipes open result in a pretty straight shot out of the muffler. When closed, all that junk gets dissipated Inside the muffler on it's way to the inner pipe outlets and consequently not so noticeable externally.
Theta pic:
View attachment 10194
There is one other thing at play which keeps the inner two pipes cleaner on NPP systems, if the outer NPP valves are open.
It turns out that Bernoulli's principle manifests itself, and, get this, the airflows on the center two pipes are INTO the mufflers instead of out of the mufflers.
I just saw a YouTube video of this tonight. The guy and his girlfriend kept trying to blow up a - er... membrane... yeah, let's call it that - placed over exhaust tip number 2. However, each time he started the car and/or drastically increased the RPMs, the, er, "cylindrical protrusion of the membrane" would keep getting sucked back into the muffler instead of blowing out like a balloon like they desired. They could not figure out why.
Yelp, it's the same thing at work there that causes an airplane wing to develop lift and fly.
How cool is that?
-Rodney a.k.a "Dr. Nerd" over and out