Porting the room - flat to 6hz

[Aijan]

We recently moved to a new house. I setup the subs (three 12" dual opposed cabs) in one of the two rooms in the basement (22'x12.5'x7.5'), and took some measurements, but something wasn't right: I had flat response down to 6hz! The subs start to roll off at around 40hz. I know it's not unusual to get flat response down to 12hz or so in a room like this with the help of some room gain, but getting another octave below that, now that's very unusual. Then it hit me -- we had quite a large internal window between the rooms in the basement for some reason, and I had filled it up with low density mineral wool to minimize sound leakage between the rooms. The window measures roughly 45"x32"x9", and it appears to work like a port tuned to 7hz or so if I'm correct. So, the question is why build large ported subs when you can use compact sealed subs and simply add a window to one of the walls instead?

[MemX]

Could the mineral wool be acting as the 'dead' wall in a SBA, reducing destructive cancellation?

 

Or are the wavelengths too long at that depth for that to happen?  (IIRC constructive-only interference starts to occur once you get down past 30Hz??)

[Aijan]

I think the window's dimensions are too small for it to act like an entire wall in a SBA. My guess is that the mineral wool in the window slows down the speed of the sound and decreases the "port tune" a bit.

[SME]

Even if the filled window is behaving like an SBA dead wall, that is nowhere near sufficient to explain flat response to 6 Hz.  I think the hypothesis described in the OP is reasonable.

The mineral wool probably has the effect of making the port lossy, but I doubt it lowers the tune much if at all.  I wonder if even more output at around 6 Hz would be seen if the mineral wool were removed?