Thanks, Jens
One more curiosity question-- I was playing with the models at acousticmodelling.com and get puzzling results--
In multi-layer absorber calculator, if I specify a slotted panel, air gap, absorber depth. And then specify a perf panel, with identical percentage of open area, identical panel depths, identical air gap and absorber depth. The only difference is that one model is a slotted panel and the other is a perf panel. I get different curves, different center frequency and different Q.
To further confuse the issue, as best I can tell, I get another slightly different curve if entering the exact same parms into their Helmholtz calculator. Three different answers for the same problem.
For Instance--
The helmholtz calculator (calculating a perf panel on certain parms) gives a center frequency about 115 Hz.
The multi-layer absorber calculator perf panel model, with the same parms, gives a center frequency of about 90 Hz.
The multi-layer absorber calculator slotted panel model with the same parms, gives a center frequency of about 71 Hz.
I'm not blaming you or anybody because the answers differ. Just reporting the results for opinion if you have any.
One thing of interest-- If two otherwise identical helmholtz absorbers were constructed, one for instance with a 12 mm thick slotted panel with 3% open space, and another with a 12 mm thick perf panel with 3% open space. Same air gap, same depth, same amount of damping in each trap-- Is there a real-world reason that slots might have a different resonant frequency and Q, compared to an identical box with holes instead of slots?
Can the bass really tell a difference between the slots and the holes?

Or would such disagreement be most likely artifacts in the different models, but yielding about the same performance in the real world?