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Room Treatment Options?

7281 Views 43 Replies 6 Participants Last post by  bpape
I am in a situation where I have an irregular shaped room with some nasty bass peaks and nulls.

The Room:

The sub is located along the side wall (center of picture) and the surrounds are where the red Xs are.

Here is the bass frequency response graph:

I have a nasty peak at 52Hz and a nasty null at 76Hz.

From the reading I have done, traditional room treatements are good down to around 80Hz. If I understand correctly, my only option for that null is a tuned absorber like a Helmholtz/pannel resonator.

I am also extremely limited in placement options. I have the rear left corner from floor to ceiling, but can only put something in the front left corner in the top 1/3 of the room only. I do have the front and rear and some side ceiling/wall corners. I could also tuck something behind the main seating couch.

Where do I go from here?
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Not all treatments lose their effectiveness at 80hz. Also, they don't just 'stop', they roll off. What you need to do is identify WHY you have the null and peak and what's causing it to determine what is best to try to deal with it. If it's something like SBIR or a null cancellation occuring off the rear wall, then they can potentially be dealt with via conventional treatments.

Also, if the issues you're seeing are positional (seating) you may be able to impact them simply by moving the seating position. Others could be caused/fixed by small movements of the sub, adjustments to the phase, etc.

Soft treatments are indeed velocity absorbers - not pressure absorbers. Again though, it's not a black and white issue. You'll get OPTIMAL absorption at 1/4 wavelength of the frequency in question (distance from hard boundary to leading edge of absorption) but you'll still get some behind that. Thickness also does make a lot of difference. Again though, thinner can still do some good.

As an example, our Monster panel is 8" thick (6" absorption with an airgap built in). Theoretically, you'd get diddly down low. Testing shows that at 50hz and a flat wall mount, you'll get 0.18 coefficent. Corner mount will give you a 0.63 coefficient. Now, I do have to caveat that since there are NO standardized measurements below 125Hz and there are no standards for corner measurements. All we can do is show the sabines absorbed in the same lab room by the same samples on the same day. Still, it gives a decent relative idea.

Even with all of that, I've seen Monsters and even 244's and equivalent from other manufacturers placed behind speakers (SBIR) and on the rear wall (cancellations based on wall to head distance) have close to double digit impacts on frequency response aberrations.

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If you're doing a Helmholz resonator with holes, then in the corner also likely isn't the best place. Same would hold true for sealed panel absorbers. Corners work well for broadband treatment as all room modes terminate there - don't cut off what they can do.

Yes - most likely a large portion of the 70hz peak is height related. Treating it would require something over your head.

Just remember that there is more to treating a room that simple frequency response corrections. It's as much or more about getting a balanced and properly damped decay time across the spectrum.

I hate to burst your bubble, but I'm going to disagree with just about everything you said in that last post.

What happens below 80hz is the fundamental of the music. No, it doesn't happen tons in real life but MUSIC doesn't happen in real life. It does exist though. And believe me, it's a LOT more than just pipe organ. Now, if you wanted to make this same kind of statement for below say 35Hz, I'd agree with you. But, there's a ton of stuff in music and movies that is between 35 and 80Hz.

Lots of thin panels is better than fewer thick ones - couldn't be farther from the truth.

Lots of bass traps? Depends on the size of the room, the shape of the room, the usage of the room, and the construction of the room. You're better off to determine WHERE the issue is and do that rather than just randomly placing lots of bass control around the space.

Panel absorbers can be relatively effective. They work over about 2 octaves rather than a few notes like a Helmholz. That said, their efficiency per unit area compared to broadband absorbers is in general, less than 50% over much of their range. Also, when you try to use them for VERY low frequencies, they can cause issues of their own. This isn't even mentioning the cost to build them.

You can build panel absorbers till you're blue in the face but if they're not in the right places in the room, you're reducing their efficiency even further. What if it's a height issue? Putting them on the back wall will help a little, but not optimally, and you may cause other problems by doing so. Get out the scalpel, not the shotgun and figure out why you have that peak and null. As mas pointed out in another recent post, you can use a piece of broadband absorbant moved carefully around the mic position and watch what happens to the peak and nulls to get an idea of where things are happening.

I will agree with you on one thing - concrete rooms suck! :hissyfit: They provide absolutely zero, zip, nada, help in taming bottom end. They're bright and edgy in the upper mids and highs. They generally require a lot more overall acoustic treatment than a 'normal' drywall over stud room with carpeted floor.

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Not a reference book, but this link has good calculators for both rigid panel and slot/hole Helmholz resonators

The Master Handbook is an excellent book. Just be careful, depending on which edition you get, there may be an error in the Helmholz forumula. Reference to the error is pointed out in the link I posted. (The linked site had the error for a long time but has now been corrected)

There is NO reference for your room. YOUR room has specific issues that need to be dealt with with regard to YOUR context. The internet has a ton of info for generalities. The best idea is to analyze your space and what it needs and then proceed from there.

Probably so. Hard to communicate sometimes in forums.

As for the ringing, it's quite possible that the Q of the EQ filter is broader than the rolloff is which can cause this.

Well, you're consistently dealing with the 40Hz null. Above the aquarium shifts the null in the 70's.

Just for giggles, try re-running the test with no panels but shift the mic about 6-8" to either the right or left.

Without knowing a little more about the room or about what speaker(s) were running and their positions, it's hard to guess at the rest. Regardless, it's a good exercise to go through and see what you're room is doing and how you can impact it.

My suspicion is that they're more consistent as they're likely related to:

- Side to side seating/mic location
- Vertical ear location
- Sub location.

Those 3 things always stayed the same - as did the nulls. Probably there is a little reinforcement from some other non-axial modes that is accounting for a bit of the shift in intensity.

What you need to do is to address sub, seating, and speaker positions first, prior to any treatment and get it the best you can. After that, we can see what's left to fix. You may find that what's a problem now can be minimized via placement.

They really don't tell you a lot given the fact that pretty much all placement options is variable. Spend the time to do that right off the bat then come back and we'll see what needs to happen. Also, remember that you're only looking at frequency response. There's a ton more than that to consider before we look at the proper solution.

While it seems variable, that's not a bad thing. IF you can adjust them so that you can treat where the problem is occurring vs leaving something that you can't treat, you'll be better off.

As I said, even though you may not be able to leave something there, play with it anyway so you KNOW more specifically what's causing it.

Yup - that can be a real test.

If you can't do any side to side movement of the seating and setup, then you may end up having these anomolies. In that case, and assuming it is width related, you might need to experiment with treating walls directly beside the listening position.

Well, that would certainly lean toward explaining it. If the REAL space is basically square, then they'll be very very pronounced. In the calculator, which tangential is it?

What I meant with the treatments was panels directly beside the seating on the one wall that you can treat.

If it's length and width, combine that with a thick set of panels on the rear wall and you've dealt with both.

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