I started out with this
then I made up a small spiral / auger insert to go inside the 2" perforated pipe inside the 3.5" pipe.
I found that it was a bit restrictive so I thought what if I removed the centre pipe. I made up a spiral and slipped that in
It made a difference so I machined up some thicker washers and turned them into a spiral
inserted that into the 2" perf tube and wrapped it with s.s brillo pads and got this
I'm still after a few less decibels. I'm wondering if the 'cones' would help. The whole baffle insert is only about 9" long
I tried adding an extra one but it didn't make any difference.