first, to be clear the topic of discussion is the "AR chamber", which IS a tube which exits inside a larger tube, as illustrated earlier. There is some variation on the size difference, or there might be a slight taper to the inner tube so the exit is a hair smaller, but that's the idea, general outline.
The use of a sharp edge tube which exists into a large volume, OR small details of the port/valve which promote flow in one direction more than another....are separate topics IMO.
The "device" is a concept that is fairly easy to understand, but how it actually functions is not as clear. The actual difference in flow forward/reverse in steady state testing is actually very small. The difference in reflecting pressure waves forward/reverse is also very small.
To "put in" an AR chamber....you MUST also increase the volume at that point. The added volume is what is making a difference just as much or MORE-so than any special flow characteristics of that shape.
If you are thinking about AR chambers you are thinking about stepped headers, and tapered pipes. Header pipes with steps or a header pipe that is tapered actually function in a very similar way. It's the overall volume/length/size that makes a difference. It is true that steps will give a return wave SLIGHTLY different than a gradual taper but it is NOT a large difference. If the step is a few inches one way or another....still operates largely the same.
As compared to a straight pipe, Tapered/stepped pipes tend to be far more PEAKY. You can even hear it. trumpet VS flute.
A "reverse stepped" or reverse tapered header is weaker but a lot less peaky than a straight one.
You may have never considered the possibility of a reverse stepped header....but they can exist, and they do exist. I bet a lot of folks who DO report large gains with AR chambers would have similar results with a "reverse stepped header".....but never tried one. People will not build or try a reverse stepped header because it's theoretically bass-acquards, and I'm not denying it, I agree!
BUT, if you try a stepped header, and it's WORSE, then the logical thing to try next is the opposite....right? sure. OR try different lengths, valve timing or change the port or valve, really. it's all a system. But what we all want is a free lunch. CAN the AR chamber be part of a balanced breakfast? Maybe.
We know the PORT ends up being smaller than the valve, say, 86% give or take, and the far end of the header pipe ends up being as big as the valve or sometimes larger, so WHERE and HOW exactly does the size step up from port size to header size? In some cases it works great to stay small (port size) 8-12" inches out of the head, because... you can. IF you do that....it's kind of like a stepped header, it's more peaky. If that's great, then great. If that's a bad thing, put an AR chamber in there where it steps up...then you add back the volume you took out, and it should resonate more like a straight pipe did.....and there might be a net gain. That's where there MAY be the free lunch.
So TO ME, the AR chamber does not work as imagined, BUT, it is still the best way to add volume to the engine side of the header pipe with the least downsides.. Think about it. If you can think of a better way.... please share.