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Discussion Starter #1
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.

Bryan
 

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Discussion Starter #3
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:
Axial
28, 56, 84, 112Hz - length
48Hz - width
70Hz - height

Tangentail
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:

Edit:
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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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.

Bryan
 

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Discussion Starter #5
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.

Fred
 

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Discussion Starter #7
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.

Fred
 

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I did not experiment with the size of them. What I did was check absorption coefficients of everything, the carpet and plywood included.

It is rather interesting how I came about and designed my HT and first I have to explain a few details before how I made the Helmholtz Resonator.

The way I found my HT being built was I had left my apartment for a weekend vacation. When I arrived at the lake house at night, the usual automatic motion sencing light did not come on, so I stoped, walked back up the steps and turned on my headlights to see where I was going. There was a large hole in the ground dug up over 20ft deep with rebarb sticking out the ground right where I was about to walk into. I called my folks immediatly in distress and exclaimed that I was nearly impailed by rebarb, and demanded a good explanation. They explained they were having a storage area being built and a garage. By the following week I had plans for the 20' X 20' area to be a HT, and we made up the design as we went, also adding a bedroom above the garage.

I began design on the HT by first placing a subwoofer in the room near where I thought most appropriate. This was a DIY subwoofer I had constructed while the bedroom was being built. What I then did was place the subwoofer in the room with a radio connected via headphone jack, and I walked around the room. The room was entirely concrete and steel, with no framing. I played ace of bass on my radio at volume level 4, and walked around the room standing in locations that had dips or peaks, and moving my head in and out of these areas observing what they sounded like. When I was at the rear of the room there was an espicially large area on the rear wall from which the air pressure would actually breeze past me in a woosh kind of tone/feeling. The area next to it was a corner that had a light feeling gentle rising type of bass response. If you have read the REW thread, this is where two of the 12" woofers are now located. I designed my room size around this taking oblique room modes into high account for even modal distribution, and left areas suitalbe for room treatments within the given space.

To size the Helmholtz Resonator I took the axial and oblique modes of near 80Hz into careful consideration after doing the room mode calculators. I refered to an online caulcultor for caclulating the sabines for RT60 in several scenereos, changing room size and factoring that I did not want to live, or to dead a room. Using an online Helmholtz Resonator calculator, I tuned the size of the box for an effective 80Hz absorption, and used the vertical size based on the listening height of the rear row. I did not have excel to calculate the holes size, so I used the required size of a single hole, and spread this over an area I thought seemed appropriate. I wanted it to be effective at lower frequencies, so I used large holes. I attempted to do my best to make the coverage of these holes as best for broadband bass absorption, without overdoing it. Basically I took what knowledge of hearing I had in my previous HT, everything I had learned from people I consider experts such as Bryan or Ethan and applied it.

To do that I did not spend hours over the internet. I used what knowledge I had and contemplated each idea for many hours while simply sitting in my HT area, listening to my radio, and occassonally enjoying a few cold ones. A good environment for me to me design is a stress free one. I did spend a good week checking facts and doing calculations online.

I asked for some help designing the Helmholtz Resonator in an acoustics forum (not here), asking for a member with exel to do the calculations for me, but there was no reply. I'm happy it turned out so well.
 

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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.

Bryan
 

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Discussion Starter #10
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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Discussion Starter #11
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).

So...

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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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.

Bryan
 

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The bass is arrived at the listener the same time you feel and hear it in a concrete room, unless there is some kind of problem somewhere. :dontknow: Concrete rooms and subwoofers make it easier to open the door. :dontknow:
 

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Hi Fred,

Is the back corner near the left surround an option? You posted this graph on the REW Forum last week taken from that position, and it looks much less problematic as it has only a single peak from a room mode (which are easy to equalize) but no serious nulls (which can be a challenge).




You could use the BFD to tame that 38 Hz peak and have a decent curve. Of course, that won’t help any ringing issues you may have; the traps will do that.

Regards,
Wayne
 

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I notice I have more ringing at around 25Hz after equalization. Here is a peak I had similar to yours and then a more recent one at near the same location but with 3 more subwoofers and a new soundcard with newer computer. Still adding more room treatments but I just can't afford anything right now, and anything that looks like a room treatment I have to pay for. :hissyfit: Sure sounds pretty good so far though..

Before lots of bass trapping and while I was building some corner traps.



Intitial results:



same scale now



Correct scale

 

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Discussion Starter #16
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.

Fred

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


http://82.95.237.142/mhsoft/Helmholtzabsorber.asp#PanelAbsorber

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)

Bryan
 

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Discussion Starter #18
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.

Fred
 

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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.

Bryan
 

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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.
Could be that was a fluke. It's doubtful that moving a sub to another position will have an appreciable effect on ringing (other than perhaps a room mode measuring worse or better). It's best to take multiple readings at any prospective position if you're looking ar waterfalls. They can vary quite a bit from one to the next if you're using the Radio Shack meter.

Also, the peak being higher won't have any effect on whether or not it can be equalized.

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.
You won't know if it helps for other seats or not until you measure from those locations. If you're seeing the same peak at different seating positions, then EQing it will be a benefit for all those locations. I have a peak at ~42 Hz that shows up in 4 out of my 5 seating positions, and EQ made an improvement for all of them.

Regards,
Wayne
 
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