Chris, it hurts my brain to be unable to figure this out. I've been reading at gearslutz, since those guys - being in the business of making money based on the performance of their rooms - take this seriously. I've seen threads discussing this type of thing, but no one seems to do the math in public. They all must rely on spreadsheets and software, I suppose. The other challenge here is the access to appropriate materials. I think we can conclude that I allowed myself to believe in an oversimplified version and should have known better.
This thread: http://www.gearslutz.com/board/studio-building-acoustics/667929-my-experiment-metal-panel-absorber.html
illustrates my points pretty nicely. However, I bet it's the place to find answers if you are a careful and patient reader. I'm having trouble devoting the time and concentration that would be required - especially when sifting through the jargon and abbreviations - not to mention that a lot of the posters there are European and have access to different materials than we can find readily here.
I suspect that something like RPG's Modex products are what you need, like the modex plate http://www.rpginc.com/ProductDocs/MDXP_Modex%20Plate/Modex%20Plate_2%20Page%20Brochure.pdf
which is advertised to be effective to 50Hz. I have no idea what it costs - and they only make one size. I also hate their website - couldn't they make it easier to get an idea what these products cost or how a person can get them? (No, that would open them up to a mass market, and undermine their exclusive image and relationships with pro designers)
To be honest, these are the reasons I decided to build my room in a way that would allow me to try to use Helmoltz resonators. While not as sexy and compact as these more complicated engineered solutions, I have more confidence that I can engineer a Helmoltz device based on a single cavity resonance. The key (not explained at this link
) is the port are compensation - which means you have to add something like 60-80% of the port diameter to the length of the port to get the effective port length. You can basically build a ported subwoofer box with no driver, and use the port dimensions and box dimensions to tune the thing to absorb at its fundamental (and odd multiples, I think). I'm hoping to build on in the adjacent room, and extend the port through the wall into the theater. Just put the box under a bench or cabinet, creatively cover the port with some fabric (which will change the port compensation), and measure - adjust port, repeat. I've previously found people building these out of large tubes, but they're not "tube traps" and I can't remember the name that google can find. But the more I read about that
, the more I'm reminded how much trouble people have with them.