Originally Posted by volfan6415
As you can see from the graph I have a pretty good handle on the room resonances from 20 HZ and above via the FBQ1000. However, my calculations based on room size (19 x 40 x 12) show and the graph above confirms I have a big room resonance around 13 - 14 hz.
What can I do to help correct this resonance?
In my opinion, your modal calculations, and your observance of the FR dip in the measurement, aren't necessarily cause and effect.
Yes, room modal resonance behavior is absolutely tied to a room's dimensions between boundaries. Although there's three variations, we primarily are most interested in the axial, as they're often the most influential, thus the most important. The Q, or sharpness of said resonant behavior, depends on boundary impedance, ie., how lossy or rigid each boundary is. For example, a fully concrete bunker type room would possess sharply defined modal resonances. And if any boundary was relatively lossy, like a long stretch of typical wood stud/drywall that had no perpendicular walls built off of it, then the previously well defined modal resonance involving that boundary is now less so well defined.
I shared all that because your room appears quite complex, and therefore I'd be less confident in any type of modal calculations, and much more inclined to merely focus on observed measurements. In a best case scenario for accurate modeling, whereas there's a perfectly symmetrical rectangular box, with all boundaries identically built, and no furnishings, it's still difficult to model modal behavior in a perfectly accurate manner. So as easy as it may seem to link two components
, in this case your modeled expected results
, and the actual measurements
taken, in reality acoustics is very complex and often counter-intuitive.
That said, if your room possesses a longest dimension of 40', then the calculated
first axial mode is about 14hz. A room's first axial mode, 1, 0, 0, doesn't typically cause a dip in freq response. Modal resonances within the subwoofer range typically cause constructive, supportive acoustic observances. Conversely, dips in FR are typically destructive interference.
Examining your measurement, it shows your response peaking in the low 20s, and rolling off from there. In my experience, the dip in the low teens is likely a ceiling resonance, diaphragmatically subtracting energy from the playback. What's more concerning to me is the area above 50hz, as this is a vital range that's important for both music and movie playback. It's possible we're seeing a robust house curve of 15dB from 100hz downward, in that case the response is relatively smooth with the exception of the small excess of ~50hz energy. That small peak there may very well be a product of the floor to ceiling axial mode, and other modal influences a touch higher.
So bottom line, nothing really appears that unusual. It would appear that given the equipment used, the FR peaking in the low 20s, and rolling off from there is not unexpected at all. Continue to experiment, and remember you can't boost a destructive null. Peaks are mostly caused by modal influences, and nulls and most often destructive 1/4 wave acoustic interference. And most importantly in the context of this thread, the destructive dip in the low teens in your FR, is likely caused by ceiling motion.
That's my take, good luck and welcome to AVS
Originally Posted by sdurani
Unbolt your IB sub and re-install it at the midpoint of the 40' dimension (a mode cannot resonate if the source of sound pressure is at its null location).
For what I'm striving for in my playback system, I'd suggest most every room with a largest dimension eliciting reciprocal gains around 20hz and lower, is a good thing.