Subwoofer causing cancellation at 120hz?

[beren777]

I've been poking around with REW trying to figure out some issues I've had with understanding dialogue through various center speakers. It's probably my room or my ears, but I've noticed that my subwoofer also appears to cause cancellation at ~120Hz.

Green and blue is with subwoofer and center active, center set to small. The others are variations of the subwoofer either being off, or on and center set to large.

If all the speakers are off and I only test with the subwoofer, I see normal expected roll off through 120Hz.

Center is a Genelec 8030C, sub is a Monolith 12 THX Ultra.

My room is definitely not optimal and no clear improvements are really doable there. Should I try reversing the subwoofer phase or could that just cause nulls elsewhere?

(I had the wrong picture initially posted. Corrected.)

 

[raindance]

Reverse the subwoofer phase and measure. We can't tell you the result. Once you do that, we can ask more questions if necessary.

 

[syzygetic]

This is super interesting to me. I haven't had time to post yet, but I am having the same issue at around the same frequency in an otherwise really solid system. I will be following closely!

 

[audio2design]

Reverse your phase and that becomes a peak.

Move your sub .... and keep moving it till you get the best response. Even better, get a second sub (or 4).

 

[beren777]

So I checked, sub polarity was set to 0 degrees. I changed it to 90, and results differ.

Green is 90 degrees. You can see there's a new null around 80 Hz, and the 120 Hz null is effectively gone.

 

[raindance]

What about 180 degrees? That would be interesting to see.

 

[beren777]

Reverse your phase and that becomes a peak.

Move your sub .... and keep moving it till you get the best response. Even better, get a second sub (or 4).

In my setup a second sub is going to be easier than moving this one around and this sub model is on sale, so it could happen. However with polarity set to 90, I think it's balanced enough that room EQ might handle the rest satisfactorily.

 

[beren777]

What about 180 degrees? That would be interesting to see.

180 caused a significant null at ~250 Hz.

 

[beren777]

Just a note, as you might expect, the interaction is different with a different speaker. So I'll have to make sure I spot check it whenever I change speakers out, unless a second sub evens it all out on its own.

 

[raindance]

What are your crossover frequencies? I don't understand how a sub can cause anything to happen that high up.

 

[raindance]

Also what you're getting at the 90 degree setting is pretty darn decent.

 

[audio2design]

What are your crossover frequencies? I don't understand how a sub can cause anything to happen that high up.

Yes, this is also what I was thinking. The crossover may be set too high, and/or the filter order is too low.

 

[beren777]

The center speakers I've been testing with are a Genelec 8030C, a Revel Voice2, and a Monolith THX-365C. AVR is a Denon 3700H. Crossover is set to 80 Hz.

 

[GalZohar]

I think you should just find the setting that gives you the smoothest response overall. None will be perfect, but some will be way better than others.
It might be easier to do with changing the subwoofer distance rather than the phase knob (should achieve same results, but may be easier to understand and handle).

In any case, to my understanding, it's not at all uncommon to have cancellation between speakers and subwoofer, mostly within the 1/2 octave above and below the crossover.