Hey Zak, don't know where I read 60-something dB on the Welrod. It could have been wrong. I do know it's listed as being a 73dB setup on Wikipedia (
http://www.timelapse.dk/thesilencer.php --this is their reference, says it used what look like wipes along with a ported barrel, .32 Mk2 didn't have ports as it was factory sub). I may have the design of the Welrod somewhere, I have the old army "silencer evaluations" manual from the 60's or 70's and I think it's in there. The most advanced one in there is one for a Walther SMG IIRC. You've probably either read if not own this book I'm sure.
So I got to looking up some math for another problem I was dealing with and came upon the FFT, or "Fast Fourier Transform" and thought of you and what it does is to take a portion of a broadband signal and isolate and analyze the constituent waveforms that makeup a broadband signal. Usually there are spikes and those broadband signals would be, IMO, the ones to target as they're responsible for most of the bang. The primer for sure, then any other peak. If a way to neutralize those waves could be found, that'd be the first step in making very, very quiet next generation silencers.
Maybe the answer is whistle ports in the brake and not the suppressor, and/or force gas through a "whistle chamber" that cancels the waves. Another more organic idea is that the entire construction of the can itself is designed with that in mind and every inch of interior surface plays a part.
I have no idea how to go about engineering something like that, I don't have engineering software or anything that I can use to play with changing physics or harmonics with respect to changes in the 3d construction of the suppressor and then witness the results virtually without building it. I wish I did, but wouldn't know which one to choose that can do what I need, or if it even exists. But hey, it's an idea. If it worked though, it'd be a gamechanger.