Neil,
this is interesting information, might just make another exhaust,
mine is real simple, short & robust. (the milk tin is just a heat shield). but I also used bigger & more hole area in the inner tube, reports I get are that it is no louder than any other aircraft around here just "different". above about 4000'agl had people say they never even notice me fly over, but I guess this is due to placement of tailpipe not pointing at the ground.
But If I can get quieter & "different" all the better.