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Room Treatment Options?

7271 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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Bryan, I think we are saying the same thing. I will look into the Master Handbook...

Wayne. You are the first person to mention equalization having a broader effect, but thinking on it, it makes sense that if it is all the same frequency and caused by the same mode. Hmmm.

Thanks again gents.
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.

I finally got a chance to dig into the Master Handbook of Acoustics today. By any measure this seems to be a weighty book. I gave it a National Geographic read (charts and pictures first :bigsmile: ) and it is going to be good for giving me speficic guidance should I decide to go with a panel resnoator as well as filling in the significant gaps in my knowledge.

I gave it a National Geographic read
:rofl2: I have that book also. It's very good. Don't forget to check out all recommended books that it refers to go into greater detail with.:reading::reading::scratchhead::explode::hsd:
Missed one. :dizzy: I think I 'm going to wait until I have at least 1/2 of the book read before I tempt myself with more details.
After much thinking, a number of re-reads of this thread and some procrastination, I finally built a couple of panels and did some testing.

Here is a pic of the two panels stacked in a 4' x 4' configuration at the back of the room behind the center seat:

The front of the panels is 2'8" from the back of the room in this picture the idea was to get them out where they would be more efficient.

They hold 6" of 3lb fiberglass board.

The positions tested were at the back as shown above, at the front of the room above the aquariums, in the rear right corner and in the dining area on the table. Wherever possible, I tried to get them out from the wall to increase efficiency.

Below is a pic of them at the front position:
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The measurements:

I have included a non-treated base line (purple) in each plot for reference. I took this measurement with the treatments safely out of the way in the bathroom so they would not influence the baseline. I did a quick calibration to make sure everything was correct and then did all the measurements without any changes.

Rear room position, Centered behind the meter:

Front position above the aquarium:

In the Corner:

On the diningroom table in a 4x4 configuration:
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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.



1. All positions did pretty much the same thing with only minor variation.
2. The 40Hz null is gone and the 25 to 30 Hz region is improved. :eek:
3. Nothing really touches the 52Hz mountain. :explode:


The front position did narow and reduce the 52Hz peak slightly, though I do not know that it would make much difference to the sound of the room. The other positiona actually increased the peak a little. What the does this tell me about the related room mode(s)??

The front and corner positions, the only two that I can realistically use, also narrowed and reduced the 75Hz null.

In addition to eliminating the 40Hz. null there is also a reduction in the 80Hz and 120Hz. Are they all part of the same room mode?

The real test comes next. I want to build another two panels to see if there is further effect or if I suffer the same fate as the last person who tried this.

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Hi Bryan. I guess I should have reposted the room info. The measurements are of the sub only and is in the position shown in the diagram at the beginning of this thread: on the side wall facing forward.

Just for giggles, try re-running the test with no panels but shift the mic about 6-8" to either the right or left.
I will certainly do that in the near future, What I was after with this set of measurements was to see the effect of treatments in different positions so I went with a consistant mic placement.

I do, however have multiple position measurements I have done in previously. Out of interest, I went back to look for comon elements across measurements from diferent mic positions.

The positions are: center left and right seats and 1.5' forward of these three positions (38% distance point in the room). The sub was always at the side position either facing forward or into the room. Measurements were taken across days, so mic positions would have varied by inches as well.

What I found was peaks and nulls at almost the same positionsacross all measurements. What changed was db level.

34-35Hz -peak varying by mayby 10db
37-40Hz null varying from nothing to 6db down.
52-53Hz peak varying by maybe 3db
72-77Hz null varying from nothing to -15db
80.7-83Hz peak varying by maybe 3db
93-97Hz null down 15-20hz varying a bit
103-104Hz peak missing in some measurements
114-124Hz peak sometimes a shelf

The 40Hz null is the most variable and the 52Hz and 80Hz peaks are the most consistant.

I am wondering if those that are the most consistant are the most reinforced by multiple modes?? That would make them the most difficult to treat, and the ones likely to respond the least to a single treatment placed in the room. Am I on the right track here?
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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.

OK, I see your point, they are very much related to the sub and mic position. But the fact that various peaks and nulls react differently to different treatment positions should tell me something more about the nature of those peaks and nulls. Those differences are accounted for by one variable, treatment location. What does that tell me?

For example, the 40Hz null is affected pretty much the same way no matter where in the room I put the treatment (in a horizontal position). That tells me that I have a 40Hz reflection that is bouncing around in the room in all sorts of (horizontal?) directions. I can trap it once, and it is largely gone; multiple directions, single source (??).

Contrast that with the 52Hz null. When I put the traps centered in the room behind the mic, it actually increases the peak by around 1.5db. That tells me that there is some cancellation going on in the room length direction that is getting absorbed by the trap more than whatever is adding to that frequency.

Putting the trap in the dining area, centered on the wall does nothing, telling me, that in this area (side to side), either there is no reflection at 52Hz, or the trap is absorbing both in and out of phase reflections equally.

I guess what I am getting at is that I would like to move away from the "throw it at the wall and see what sticks" approach, which could end up quite costly, to a more analytical or deductive reasoning approach.
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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.

I thought I had already addressed this, but I guess only in my head...

My seating position is limited to the current couch position or as much as 1.5' forward (at the 38% position).

I have played with sub position within my limits (the current side position or the one back corner) as well as orientation at those positions. All that has done is shift problems around. Given the variability in measurements at the same position it may be worth while playing with fine positioning at the current location.

I also want to broaden the 'sweet spot' and moving the sub around hasn't helped any with that so far. Measurements show a lot of variability between seating positions.

Also, given that I live in a concrete box, adding broadband treatement will help me limit nasty reflections above 100Hz where comb filtering shows up.

I am still curious to know what my most recent measurements tell me about the room. Perhaps that is beyond comprehension with the limited measurements I am taking??
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.

...I am still curious to know what my most recent measurements tell me about the room...
After playing a bit with this stuff, here's my take on it:

1) Your measurement tell you that your room has several low frequency modes.

2) They also tell you that no matter where you put the absorbers they have an insignificant impact on those modes.

This makes sense since it would be a very good resistive absorber indeed to achieve 20% absorption at 40Hz in a real room. If, for example, you consider a 40Hz wave that gets 100% of its energy reflected back in phase then you will see a 6dB peak. Absorbing 20% of the reflected wave still leaves a 5+dB peak. The difference is not significant.

So you are asking broadband resistive absorbers to do something they cannot do. It's not a worthwhile endeavor. Better to work on speaker and listener location adjustments.
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.

Thanks gents. I hope to get back at it in a couple of days.

Even if the broadband absorption doesn't end up doing much low down, given that I live in a concrete box I still need all the help I can get higher up.
Fred, what are your impressions after adding the traps in your room ?

Well, its only two 2' x 4' traps so I'm not expecting a dramatic difference. I have not done and critical listening tests to see what the audible differences might be.

The plan is to re-visit sub positioning once more to see how minor changes in positioning affect the response of the sub at multiple positions.

I plan to be able to put a few more broadband absorbers in before I do much in the way of critical listening. I also want to do some full range (well 2hHz - 3KHz anyway) measurements to see how the broadband absorbers affect the higher frequencies.

The big test will be to see how pipe organ music responds to the room. I have found this the most demanding both on the low and high end because the organ just puts out so much sound energy.
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