it is r4 vs r8 that is the heat insulating factor but I suspect it will be a sound factor also.
I am going to have left over material I could build some and ship them out I need to see how much it will pack down.
is it wise to install another fan at the other end of the duct/muffler to get as much heat out as possible?
if you make the duct really long yes it may need a helping fan. my ducts are short and I get very good exit pressure.
Phil, did you consider cutting out a vent hole low in the wall between garage doors and ducting the exhaust outside? You could put a nice vent cover over the outside and it wouldn't look like anything out of the ordinary, and then all the heat and much of the exhaust noise would be directed outside.
the wall between the garage doors is a support beam.
No can do it is 2400 watts more or less once we get below 50f the heat will be a non-issue.
as for sound it is under 55db at exit point with the fans at 4100 rpm
In the summer I would need to vent, but my power jumps from 10 cents to 17 cents so I can't summer mine.