Brad, your post is comforting if not enabling...lol  I have yet to run any of my 3 project engines I've acquired in the last 3 years. First and 3rd ones are all apart. I was feeling quite guilty until I read your 15 year number....then I was just scared. I need and will get back to my project this summer! Thanks for the inspiration!

38ac, BruceM

Thanks kindly for the complements!

A lot of trials (and failures) before this system.
One muffler
Two mufflers
Two mufflers and a long pipe
Two mufflers and a drum
All of them gave a deep WUMP...WUMP...WUMP low frequency sound that could be heard 100 ft away.

The dual expansion chamber seems to be a good way to cut the pulses

Outstanding!

All good points,

Thanks BruceM

My thoughts not based on specific experience for you application:

Plan A- put a duct to the generator head air inlet, and make a deeply folded and fiberglass covered plywood box muffler for it. A similar muffler path for the air outlet.  Insulated flex duct, with a deeply bent path might be a way to do this also.

Plan B-  Put the generator head outside the sound isolating box, cover most of the belt slot with a removeable plate for better sound isolation.  This might not work for your frame/enclosure design but it is quite simple.

Plan C- In a book I have, one author used fully enclosed squirrel cage type blowers as a way of cooling his insulated for sound isolation diesel generator shed, which he claimed blocked the mechanical noise quite well.  A pair of very small, low power ones might be worth a try.  One on inlet and one on outlet; inlet might be directed to the generator head inlet.