I once watched a lycoming powered RV6 undergoing a dynamic propeller balancing.
cowlings were off the engine and it didn't appear to have any muffler, so as it started up I put my fingers in my ears.
then I took them out again.
the engine had a tuned length exhaust and during the full power run ups it was so quiet you could hear the prop tip noise.
by far and away the quietest lycoming I have ever heard.
exhaust note was almost silent.
I tried to find out what they had done but I am still to see the details.
sorry no idea which system it was but it was incredibly silent.
just looking at the photos if it wasn't a power flow system it was something similar.
the curved pipes are all a tuned length hence the curves to tuck away the length.