Point of Attack

[drpaulng]

So far, we are attacking the problem room modes with every effort dealing with EQ. Did anyone try to attack the problem by removing the room modes? In my opinion, a parametric EQ can be set to the 3 basic room modes with "peaks" rather than "dips". We filter out the 3 room modes and send them to a subwoofer to be placed in a tri-corner for maximal effect. OK, this subwoofer excites the room modes accordingly, BUT to be set in 180 degree out of phase to the normal music playing through the Hi-Fi system. In theory, the Hi-Fi plays the original music while the anti-phase subwoofer simultaneously does it's job to cancel out the room modes being excited by the Hi-Fi system.

To be exact, there should be 2 anti-phase subwoofers placing at the 2 tri-corners corresponding the ther 2 front stereo speakers, each being fed by the corresponding stereo channel. The left anti-phase sub only excites the 3 room modes with 180 degree phase-out against the left front speaker while the right sub goes against the right front speaker.

Just give it a try.

[brucek]

You're assuming the "hi-fi" system is producing the frequency range to excite those modes that you propose to remove with phasing. Most systems mains are crossed at 80Hz for obvious reasons and don't really create first order axial resonant frequencies in a typical room.

Consider a room of 20' x 15' x 8' where the first order axial resonant requencies are 28.2Hz, 37.63Hz and 70.56Hz. These frequencies would be mostly exclusive to the subwoofer. It's fairly easy to EQ reduce the peaks that may be produced by that corner loaded subwoofer and the only phase adjustment that would be neccessary would be around the shared frequencies at the crossover.

It is the very nature of using a crossover that tends to eliminate the problem you site where multiple speakers are producing the same frequencies at the room modes.

I've been down the multiple subwoofer road before and for me it wasn't as simple as dialing in 180 degrees of shift to get them eqaualized. It was a nightmare. I'm in the camp of a single monster sub with a bit of EQ......

brucek

[drpaulng]

In theory, it should work. In practice, it's up to someone with endurance through lots of experiments.

http://www.bagend.com/bagend/downloads/E-Trap.pdf

E-Trap might be one of the solution for fighting room modes. However, I doubt it for the mic-picking method of detecting room modes (live-feeding?). Filtering with EQ upon the source line is of course OK, especially for damping the positive feedback such as micking inside of an auditorium. It is the cheapest method also when applied to fighting room modes at home. The most costly method might be recruiting bass traps that work with very large amount of space and acoustic materials.

My suggestion is simple. However, anyone has the idea why it won't work at all? Or, why the E-Trap not known to the world? There must be some weak points here.

[Wayne A. Pflughaupt]

My suggestion is simple. However, anyone has the idea why it won't work at all?

Well for one, not everyone has “2 tri-corners corresponding to the front stereo speakers.” I never have in any place I've ever lived.

Regards,
Wayne

[drpaulng]

No tri-corners?
Good point.

Tri-corner is the best place for exciting all room modes of a "shoe-box" listening room. A rectangular room is supposed to be well-predicted for room modes calculation (in fact it is). In order to eliminate the need of tri-corner, we might place an anti-phase sub very close to one of the stereo full-range speakers and feed to it simultaneously with music. The anti-phase effect should work out well to fight against the room modes (The sub is pre-filtered with parametric EQ. and plays only the anti-phase room mode peaks at selected frequencies, not acting normally as ordinary sub).

I propose some would-be reasons of failure here:
(1)The anti-phase adjustment might not be too easily adjusted to fully 180deg against the excited room modes however closely sided with the main speaker.

(2)The phase adjustment circuit cannot adjust the frequencies simultaneously in one phase (some of them lag behind the others).

(3)The room interaction is not predicted well. The anti-phase sub should be indivdually re-adjusted to copy the room modes of the main speaker since they are placed side by side, not overlapped in exact position.

(4)The main speakers should be full-range in order to play good stereo, not the 2.1 system. However, if it's a 2.1 system, put the anti-phase sub side by side with the 0.1 subwoofer, not the small stereo speakers.

I got this idea when I accidentally reverse the speaker cable connection to one of the front speakers. The low frequencies were attenuated very much while the upper range still seems to be OK. Obviously, the "lows" are much more "in phase" when coupled to the air while the "highs" are relatively immuned to the reversed phase. So, it is up to people who can carry out experiments with an extra sub. Sorry, I don't have an FBD at the moment.

Edit: Point 4, and some typos.