Effect of room construction on resonances?

[klettermann]

I'm trying to get my new media room set up with the help of REW. Unsurprisingly the room has several VERY prominent bass resonances. Before going down the DSP path, which I'm not set up to do at this point, I've been trying to get things best arranged to the extent possible. This has proved extremely difficult. Regardless of speaker positioning, subwoofer placement/phase/etc, I just seem to swapping one set of peaks/nulls for another, kind of a whack-a-mole scenario. In the end the outcome is the same - too much bass, especially under 100Hz. Too be clear, the low end is well defined, not muddy or boomy, just too much so the overall sound is unbalanced. The room is L22ft x W12.2ft x H7ft. The only treatments at this point are carpeting and 4 ASC Tube Trap towers (2 traps per tower). I've had the same (or very similar) equipment in several similar listening environments and never experienced this problem to this extent.

Now to the question: This leads me to wonder if there something specific to this room making it sound so different from its predecessors? And the answer is "yes." It's quite soundproofed and well sealed. Construction is double 5/8" sheetrock, green glue, isolation clips, etc etc etc. Is it possible that such constructions actually increase resonances in some fashion? This is a kind of academic issue, cause I need to solve it regardless of the cause, and will likely end up doing some kind of DSP. In the meantime I'm curious about this. Is it a known thing? If so are there known approaches? Thanks for any comments and cheers,

 

[DVDdoug]

It's quite soundproofed and well sealed. Construction is double 5/8" sheetrock, green glue, isolation clips, etc etc etc. Is it possible that such constructions actually increase resonances in some fashion?

Theoretically, yes. Stiff-hard walls prevent the sound from being absorbed by the walls or going-through. I don't think it makes a big difference.

But you've got bass traps, and it's probably more-related to room dimensions.

too much bass, especially under 100Hz.

How did you set-calibrate the subwoofer level? It should be easy to adjust the overall bass level.

EQ should help a LOT. Usually it is best to get the room as good as possible (or practical) first, but I wouldn't try to avoid EQ correction. Knocking-down the peaks (standing wave anti-nodes) usually works very well. And that seems to be your complaint. Dips (nodes where the direct & reflected waves cancel) are usually impossible to fix with EQ because it takes "infinite" power and "infinitely large" subwoofers, etc. Bass traps can help with both by absorbing the waves that would otherwise be reflected. Multiple subs in different locations can also smooth things out.

I just seem to swapping one set of peaks/nulls for another, kind of a whack-a-mole scenario.

Yeah... That's pretty normal. Usually you have to compromise for your main listening position.

 

[EERecordist]

With REW you might look at your unwelcome peaks and losses, moving your microphone around your listening position. Then look up the wavelengths. Your L is within the some wavelengths in air below 100Hz. Sometimes you will find standing waves. Bass traps tend to be large and weighty.

Your old room construction walls may have absorbed some bass, while your new well-constructed room does not.

 

[Flaesh]

it sound so different from its predecessors?

What was the construction of the previous rooms?
Do you have measurements?

 

[klettermann]

Thanks to all for the remarks. First, I concluded that my past experience actually wasn't so helpful. In retrospect my rooms really weren't very similar - open ended, L-shaped, etc. NOT a sealed shoebox! So there's that. And I spent a LOT of time playing tones, going around with SPL meter, moving stuff and moving MLP. Ditto for subwoofer placements.

At the end of the day I did manage to make a major improvement. The breakthrough turned out to be much, much finer subwoofer tuning. I've got 2 mid-market units (Heco Aurora Sub 30's). It turned out that the worst of the low end could be tamed by a very specific combination of SW volumes and phase. I was amazed that the subs could work in tandem to such effect. I missed it before cause my changes were too large. The result was a major smoothing of the low end, though very specific to the MLP. That's OK. There's still a significant hump from about 50-70Hz. For House/Trance this is borderline too much (though I'm sure some would love it), but on most other stuff it's fine.

In conclusion, the result is pretty good. Not quite as good as I'd like, but probably as good as I'm going to get without DSP &/or lots more bass traps. Thanks again and cheers,

 

[basscleaner]

Let me pay your attention at some circumstances, which not be taken into consideration before. First of all, in attempts to understand LF behavior one need to begin with initial room dimensions "at hard surfaces". What does it mean? It means concrete, brick, multilayered panels wall etc. Why? Because they determine LF base response for your room. The second - soundproofing constructions of them, which have your own resonant response and third - furniture, which can influence at the FR result. The result - mutual act of all the resonant features. You told about your room dimensions. Are they initial, or belong to finish construction? For correct comparing you need to get REW result for initial and finish cases and have a look for resonant sum of them.

 

[klettermann]

Let me pay your attention at some circumstances, which not be taken into consideration before. First of all, in attempts to understand LF behavior one need to begin with initial room dimensions "at hard surfaces". What does it mean? It means concrete, brick, multilayered panels wall etc. Why? Because they determine LF base response for your room. The second - soundproofing constructions of them, which have your own resonant response and third - furniture, which can influence at the FR result. The result - mutual act of all the resonant features. You told about your room dimensions. Are they initial, or belong to finish construction? For correct comparing you need to get REW result for initial and finish cases and have a look for resonant sum of them.

I'm not sure that I entirely follow. In any case, the room is new construction in a previously unfinished basement. There isn't any "before" to compare it to. To reiterate, walls and ceiling are double 5/8" sheetrock with green glue, sealed outlet boxes, solid sealed doors, etc. There's also a large soffit spanning the room (~10" high and 20" deep).

Since my OP I've gotten much better at REW and moved on to MMM and a far more systematic test methodology. I already know that respective subwoofer volume levels strongly interact, so that's my main focus for the moment. I started with a 3x3 factorial design (9 measurements) varying respective subwoofer levels to find the optimum combination of settings. I didn't include phase cause I already know the effect of that. This creates a nice map of the sub interaction. Then I took the best combination and repeated the measurements centered around that further define it. This brought me from the red response to to green. Scans obviously aren't level-matched, but you get the idea. Obviously still some improvements to be made, but not bad - I think!! -for just tweeking sub levels and no EQ at all. Next series will look at XO frequencies with green as starting point for levels. Thanks and cheers,