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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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Bryan. I guess I should have mentioned that I have played with both phase (0 & 90 are my options) and seating position. The above measurement was taken forward of my current main seating position at the 38% spot in the room. Mostly I get very severe drops in that 70 Hz range and quite often I get the same thing in the 36-38Hz range.

The only small movement that has made any difference so far is rotating the direction of the sub at its current position at the side of the room from facing in to facing forward. Even then, it only seems to improve one seating position.

Mybe I have read just enough to be dangerous, but I was under the impression that for absorbers to be effective they need to be away from walls/corners enough to be in a spot where the sound wave to be absorbed is at high velocity/low preasure. For longer waves this seems to be fairly far into the room.

OK, plugging more accurate room dimentions into a room mode calculator gives some interesting results:
28, 56, 84, 112Hz - length
48Hz - width
70Hz - height

56Hz - length/width
76Hz - length/height
85Hz - width/height

Some of those numbers line up very nicely with the peaks in my graph ie: 56, 84, 112Hz for length and the 70Hz (sort of??) for height.

Now, what does all this mean? Or to put it another way... Is there good reference material out there that relates these measurments to treatments and how/where to apply them?

So many questions... :dizzy:

1. phase made no noticable difference in any of the measured positions.
2. I remember reading somewhere that the rolloff for traditional absobtion materials was steep and that they needed to be quite thick to be effective at lower frequencies.
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Hmm. Not quite as straight forward as it seems. I did check out the monster pannels last night.

I also did further reading last night on room modes. It would seem that since the 70Hz mode is in the vertical direction, putting something in the corner would do very little for that frequency.

Does this also account for its consistancy in the measurements that I am doing since I am always measuring in the same vertical plane?

As I understand it, a pannel resonator, being a pressure absorber, would also be much more effective in the corners.

Is the book Master Handbook of Acoustics a good place to start if I want to understand both the principles and their application a little better?

I still feel a little :dizzy: , but its starting to come together now.

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Thanks for the link Steven. I found that page a couple of days ago, but couldn't make sense of it at the time. It makes a little more sense now, but I still have lots to learn.

It even recommends absorption
I can't say that I could find anything like an obvious absorption recommendation. :scratchhead: I tried.

Lines like this: '(sabins - front wall - carpet) / Left+Right+Rear wall: 30 %' are tantalizing, but make absolutely no sense to me (yet)

By the way, I read through your REW thread and found it very helpful. Out of curiosity, how did you figure out the dimensions of your helmholz/pannel resonator? Did you have to do any tuning that involved changing the dimensions to get to where you were satisfied?

I suspect that my best bet for taming some of those low bass modes is to use a pannel resonator, but I really wonder how much tuning is involved to get it right.

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Steven. Thanks for the info.

Thinking further, it seems the most important thing to reduce in the bass region is the 52Hz. peak, so that it does not dominate.

That peak has caused me to turn up my sub to the point where it measures quite hot when it is set at a 'satisfying' level. If I leave it at its current 'calibrated' level, I lose all the low frequency rumble in movies and significant bass in the 60-80Hz region.

Next would be to reduce ringing where possible, and then to do something to deal with the 76 Hz null.

Does this make sense??

Bryan. I understand your point on using the corner for broadband absorbtion. One thought was to put a panel resonator against the wall and then put broadband absobtion in front of it across the corner. I'm not sure I have enough room to do that though.

I had also wondered if putting something above the seating position to deal with that 76Hz null.

OK, more quasi-informed speculation:

Ideally, I want a trap that is tuned to 52Hz with enough internal absorption to broaden the absorbtion roll off to cover as much of the low end below say 80Hz as possible oriented along the length of the room and into a corner. This would deal with the broad peak at 52 Hz and help out with other low frequencies.

One space I happen to have is at the front of the room above the aquarium. I have the top 3 feet of that wall to work with for the entire width of the room. Would treating this area have a significant impact on the 52Hz peak in my listening position?

Waterfall at the current sub position:

52Hz is an issue here as well.
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OK, while waiting for an answer I spent the last two days searching reading and re-reading stuff.

Some of this is starting to sink in.

It seems that stuff below 80Hz is not seen as that important in the music world because not that much exists in this realm (unless...).

Anything that cleans up the mid and upper bass is going to have a noticable effect on the clarity of bass in music, regardless of what is going on below 80Hz.

It takes a LOT of bass traps to have a significant effect below 80Hz and even then, they don't seem to reach below 55Hz(ish). I have seen this in post after post including two recent ones here by bjs and some real world testing by Real Traps.

Tuned traps are much more efficient at reducing frequencies below 80Hz, yet, with the exception of slat Helmholtz traps (not so tuned) nobody uses them (except thewire). Tuned Helmholtz resonators are finicky, panel resonators are not so finicky. Why don't people use panel resonators (on purpose)?? Panel resonators of sorts exist in any drywall and stud construction room

Last, but not least, concrete rooms are the pits! :crying:

Oh yeah, lots of thinner traps is better than less thicker traps: Concrete rooms with limited trap placement options are even worse. :rolleyes:

Oh yea, that unless... Unless you really like pipe organ and synth music (which I do) or you use your rig for movies as well (which I also do).


After all that, the way forward is not really that different except one thing. I should go ahead with broadband treatments while I figure out the finer points of panel traps as I am going to need them anyway and they will have an immediate positive impact on my music listening experience.

If anyone has anything to add, feel free...
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Steven. I think with that big helmholtz resonator full of fiber, you have built an effective low frequency absorber. I wish I had that kind of room at the back of my room. Unfortunately I have a balcony door behind the seating position.

I think your room measures exceptionally well below 80Hz.

Wayne. That position is possible. The thing I didn't like is that the peak is higher and shifted down to 38 Hz. There was also more ringing between 20 and 40Hz.

The concern I have with equalization, is that it (probably) solves the problem for only one seat. If there is a way to considerably reduce the peak over a wider area, I would like to pursue that, even if it means considerably more effort and learning on my part.

If it turns out that something like panel absorbers isn't a viable option, I can fall back on this.

Brian. I have no bubbles left to burst. They have been stomped on squashed burst and flattened too many times. :bigsmile:

The only way for me to learn is to put stuff out there as [incompletely as] I understand it.

My below 80Hz comment was a poor choice of words. The exact quote, taken from a studio forum, was "Forget about 55Hz. There is just not much music that low. Concentrate on broadand...". Perhaps a more accurate statement is that there is a lot of bass between 70/80 Hz. and 400Hz (??) that you can do a lot more to control.

Again, the lots of thin... statement could be intrepreted a lot of ways. This was from a test done at Real Traps comparing relatively few positions 6" thick in corners with the same panels 2" thick (I think) placed across more corners, including floor wall interfaces. This was in a small, reasonably proportioned room.

The results were that the more spread out absorbtion showed noticably better results from about 80Hz on up. This is working from memory as I do not have the links here at home.

On panel absorbers:
Wood, which I understand is the construction material, is relatively cheap and I am very handy. I would be looking to treat the 56Hz frequency and its immediate companions. Any positive effects outside of that peak are just an added bonus.

From what I have read, panel resonators are the ideal choice for this sort of single peak issue, and are not nearly as troublesome as Helmholtz resonators. Close is supposed to be reasonably effective.

As to placement, so far what the calculator tells me seems to be a close match with a room length mode. I did notice mas' post when I re-read those threads so I will try the movable broadband absorber trick to see if it verifies the above.

Thank you gentlemen for your patience and answers. I am on days for the next two weeks and have my son over most nights, so I won't be able to do much for a while.

I will post here when I have some additional results/info.


P.S. Is there a good reference book that help on both the practical side (building/tuning panel traps??) and the theory behind room acoustics? The Master Handbook of Acoustics seems like a good candidate but I would like a second opinion.
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Thanks Bryan. I think I have visited every site on the internet relating to panel/Helmholtz resonators twice. Thats a great site for a higher level view and for calculators.

I am now looking to fill in the gaps that inevitably come from using the internet as a reference.

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.
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.

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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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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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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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??
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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.
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.
If you can't do any side to side movement of the seating and setup
With 11' of width to work with and two couches I loose all seating flexibility. It also dosn't give me much to work with for the mains because, if I understand correctly, I should maintain a minimum 8' distance between speakers and keep them 1.5' from the side walls.

However, I thought one of the advantages of using absorbtion/diffusion was to broaden the sonic 'sweet spot' beyond the primary seating position? Unfortunately, I have such a difficult room to work with, I may never be able to achieve that.

In that case, and assuming it is width related, you might need to experiment with treating walls directly beside the listening position
You lost me with this. Which anomalies do you suspect are width related? The 52Hz peak should be length related. Actually, it shows up as both a 56Hz axial and tangential mode on the room calculator.

Hmm... I just measured the room width including the dining area and it is... 19'. Looking at my room diagram, another way to view this space is as a 20' square with a few extra walls in it. :crying: :hissyfit: Maybe that explains why I have such a pronounced peak in the 52/56Hz area? Sigh...

On treating width related next to the seating position:

How do I do this in an asymetrical space? The right side of the couch is 10' from the diningroom wall, the left side, 2' from the livingroom sidewall.

The rear wall:

I remember reading several times in the past that you don't want rear wall reflections at all(low, mid or high frequency). I have been looking at my setup, and I can actually treat most of this wall. I do not have anything in the lower shelf of the aquarium stand so I could actually put treatments in that space as well. I can easily put a panel 3' high the width of the stand and 2" deep in front of the stand and then put additional treatment behind and under the stand as needed.
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