[Anechoic]

First thing: unless that middle air gap is very wide (10+ inches), you've just created a resonant chamber that almost completely negates the sound reducing performance of the two double-pane layers. This is way party walls consisting of two separately framed walls (drywall-stud-drywall-gap-drywall-stud-drywall) tend to be very poor at blocking sounds between units.

There are other issues regarding the impedance mismatch between the outside and middle air gap openings.

As you said, it's a very hard problem.

[ww520]

Can the resonant chamber actually help in the sense that it tunes the sound into certain frequency range? Making the job of the ANC easier? Also can the glass material be tuned to reduce the resonance? Music instruments need to be make certain way to take advantage of the resonant chamber effect; any deviation would produce no sound.