Simon, I'm curious about the internal porting - did you empirically or analytically tune that volume/neck as an absorber? Certain acoustic metamaterials are very similar in execution, but I didn't know if it was a controlled gas release thing, or an effort to dampen a particular frequency by throwing a resonator in (much like high end car exhausts do).
I am functionally an acoustic engineer at my day job, we can discuss the potential of your system offline if you like. Just thought of this due to a discussion with UV_HALO and was wondering if you had thoughts.