Need some help for living room acoustic treatment - Page 3 - AVS Forum
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post #61 of 73 Old 01-02-2013, 11:12 AM - Thread Starter
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Originally Posted by localhost127 View Post

and that's basically all that needs to be said. all of these attempts at generalizations regarding which density is "better" is utterly meaningless without the context of the other variables that are part of the system - the most important variable being the specific acoustical problem the 'treatment' is being procured to solve.
eg, why are some here discussing "panel" absorbers with respect to LF absorption? panel absorbers are for indirect specular reflection attenuation. the design, placement, approach, etc is different. one user here just stated "You would need to use a lot of these panels to make a dent in low frequency absorption.". of which i would reply, why are you attempting to achieve LF absorption with panel absorbers???
So based on the different constraints mentioned in my initial post, plus the various notes about my perceived issues, what would you recommend me to do ?

For information, I just ordered a mic, so I will be able to provide some measurement in the nearer future.
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post #62 of 73 Old 01-03-2013, 09:21 AM
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For Localhost327, or whoever else wants to respond.


It looks like I will be ordering a bunch of OC703 to construct my absorbsion panels. The proper method is to stack two of the 2" inch think OC703 panels, for a total of 4" inches of thickness, with a 4" inch air gap, correct?

I do realize, of course, that I can not make accurate predictions as where to best place the panels, with out measuring, but, with that being said, I am under the impression that it is almost always beneficial to treat the rear wall, and first reflection points on the side walls, (using the mirror method to determine first reflection points), correct? So with that being said, is it a safe bet to say that placing these panels on the first reflection points of the side walls and the rear wall will improve things, and definitely not cause any problems? Is that a safe assumption to make until I can afford to pick up the correct measurement gear?

With regards to treating the ceiling, I will use some more of the two stacked OC703 panels at 4" of thickness with a 4" inch air gap. Where are the best locations on the ceiling to place these? (note that I will be building these panels as 4 foot by 4 foot and also some smaller 4 foot by 2 foot).

As far as basstraps go, being that I am free to do anything that I want to my theater room, and not having to worry about uglyness at this point....How would it work to use some of the regular fluffy insulation cut into triangles and stacked in the corners? I will likely also use some type of frame to support them so that they do not compress under their own weight. As far as coving them with some type of plastic in order to help with not attenuating the high frequencys, could I use a decently strong fabric from the likes of Hobby Loby, in leu of plastic? I would imagine that some of the nice and thick fabric found at that place would work somewhat ok to keep the high frequenys out. Does this sound like a good idea?

Any other suggestions as to this entire project of mine?
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post #63 of 73 Old 01-03-2013, 10:48 AM
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Any other suggestions as to this entire project of mine?

I think that you may be at the point where simply getting started actually doing something is in order.

As far as obtaining 703 goes, your local or regional yellow pages may have a insulation distributor who has 703 in stock and is within easy driving distance. Roxul rock wool is a different material with similar acoustic properties. Just FYI here is the web page of the guy who sells me this stuff in my area:

http://www.service-partners.com/companies/2750_ISM/index.aspx

You may find a similar relatively low-profile but very professional enterprise in your region.

I agree with Local's point that absorption is not always the best way to go, particularly at low frequencies. After you've experimeted with corner traps, a next logical step might be trying multiple subwoofers, along the lines suggested in this paper:

http://www.harman.com/EN-US/OurCompany/Innovation/Documents/White%20Papers/multsubs.pdf

I don't know if you have looked at this page, but it shows a fairly credible attempt at determining the differences you get when using similar materials in slightly different ways:

http://www.ethanwiner.com/density.html
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post #64 of 73 Old 01-03-2013, 10:11 PM
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Originally Posted by Gistum View Post

Hello,
It's my 1st time posting here, though I've been an avid reader for quite some time now. Today, I would really need some help regarding my living room, as I'm not very happy with the sound result I have today.
First, to avoid any unecessary reply, I would like to speak about the constraints I have:
- I can't move my sofa (because of kids, it has to stay against the rear wall)
- I can't move my TV desk (all my plugs are there, none are where the sofa is currently located)
- If anything must be added, as it's a living room, it must remains in the "good looking" category

Additional information:
- The floor is made of tiles
- The wall behind the TV is made of concrete
- I'm currently using port bungs in both rear ports of the front speakers
- There is nothing against the front / rear walls
- There is a play mats (about 160cm x 80cm x 4cm) in front of the TV desk, where kids can play on
It's hard to explain how it sounds, but all I can say is the bass are really boomy, even though I'm using the port bungs. As I'm a novice regarding acoustic issues, I didn't buy anything, waiting for some expert advices (you smile.gif).
If you need more information/have additional questions, I will gladly answer to everything I can.

Without measurements you are trying to fix a problem that you can't clearly define. Having said that, IMO putting a 4" absorber (with an air gap) directly behind your head will help with smoothing the bass response, but will become decreasing effective below the transition frequency. Corner traps will help as well, but I think the room will start getting ugly (for a living room) and you may inadvertently overly absorb mid and high frequencies especially with a big curtain in the room. If you knew what the offending frequency is, you could buy (http://www.rpginc.com/product_Modex_Module.cfm) or build a tuned Helmholtz resonator or pressure based absorber to fix at least the main part of the boom you are hearing. The boom is likely because your low frequency sources are against the wall exciting the axial mode(s) and you are sitting against the wall putting you in the highest energy area for those modes. I realize you said that subs aren't an option, but multiple subs around the room can fix this (someone mentioned and IB and if you have an attic or basement, you could do that; in-wall subs will work too). Audyssey will likely help, but once again, if you can't measure what it is doing, it may very well be doing as much damage to the sound as good.
There are ways to fix the boom you are hearing, but due to the asymmetry of the room or rather your constraints and the room, you are going to have a hard time making your seating position sound very good.
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post #65 of 73 Old 01-04-2013, 01:31 AM - Thread Starter
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Without measurements are you trying to fix a problem that you can't clearly define. Having said that, IMO putting a 4" absorber (with an air gap) directly behind your head will help with smoothing the bass response, but will become decreasing effective below the transition frequency. Corner traps will help as well, but I think the room will start getting ugly (for a living room) and you may inadvertently overly absorb mid and high frequencies especially with a big curtain in the room. If you knew what the offending frequency is, you could buy (http://www.rpginc.com/product_Modex_Module.cfm) or build a tuned Helmholtz resonator or pressure based absorber to fix at least the main part of the boom you are hearing. The boom is likely because your low frequency sources are against the wall exciting the axial mode(s) and you are sitting against the wall putting you in the highest energy area for those modes. I realize you said that subs aren't an option, but multiple subs around the room can fix this (someone mentioned and IB and if you have an attic or basement, you could do that; in-wall subs will work too). Audyssey will likely help, but once again, if you can't measure what it is doing, it may very well be doing as much damage to the sound as good.
There are ways to fix the boom you are hearing, but due to the asymmetry of the room or rather your constraints and the room, you are going to have a hard time making your seating position sound very good.
What I can do is moving my couch off the rear wall when we watch movie, but what minimum distance from the wall would you recommend/be enough ? As you can see on the drawings, I don't have much space to move the couch forward. Based on your feedback, I will also rerun Audyssey in this particular position.

To come back on the sub topic, would a single sub be night and day compared to my current set up ? I can't afford to buy 2, and you know my constraints for one, but if it's that good, I will try to find a way to add one to the room. Keep also in mind that I don't need deeper bass, and I'm pretty sure the bass from my speakers, in good conditions, would be enough for us. In wall subwoofer is really not an option though.

Btw, I ordered a mic, so I should be able to provide some measurements soon.
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post #66 of 73 Old 01-04-2013, 04:36 AM
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Without measurements are you trying to fix a problem that you can't clearly define.

Man, you said a mouthful there, and I mean that in an excellent way!
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Having said that, IMO putting a 4" absorber (with an air gap) directly behind your head will help with smoothing the bass response, but will become decreasing effective below the transition frequency.

All true, but what does this mean in the real world?

I managed to get a ca. 4" thick absorber maybe 8 foot high and running the 45 foot width of a church sanctuary constructed just above my head behind the mixing booth at my church. It covers the front of the balcony.

The goal was to reduce a very pronounced standing wave that caused a massive bass build up below 150 Hz that was strongest at the mix position. It's hard to mix well when what you hear is vastly different from what the audience hears.

If you run an absorber with 2" of 705 and a 2" air space behind it through Chris Whealy's calculator, it doesn't look like it would do a lot at say 100 Hz. However in actual use it was highly effective down to about 60 Hz, and had significant but reduced benefits below that. Measurements showed that a pronounced standing wave at the back wall was pretty much killed @ 100 Hz, and even somewhat below.

So what was happening? I think that one important point is that it doesn't take 0.90 absorption for an absorber to be useful. 0.30 can be a big help. Another point is that an absorber that covers a lot of area picks up effectiveness at lower frequencies simply because it becomes so pervasive in the room. To some degree you can trade off area for thickness.

Like the man says, build and measure and listen. Unless you are working with cookie-cutter listening rooms, every one is different.

Also, a good sounding room comes from the balance between reflection, diffusion, and absorption. I don't know of anybody whose preferred listening room is an anechoic chamber or a bare basketball court. Good is someplace in-between.
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post #67 of 73 Old 01-04-2013, 05:46 AM
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Another point is that an absorber that covers a lot of area picks up effectiveness at lower frequencies simply because it becomes so pervasive in the room. To some degree you can trade off area for thickness.

"8 foot high and running the 45 foot width "

it's more than likely the simple fact that like all things in acoustics, objects must be LARGE with respect to wavelength to be effectively 'seen'. a physically SMALL absorber regardless if constructed from ideal material and thickness is still not going to be effective to attenuate a much larger wavelength.

the primary reason is if it is due to a standing wave, then by nature there is going to be many passes of that wave through the absorber (since it is persisting/resonating), vs another acoustical problem such as SBIR, where you only really get one shot at attenuating the signal (only a single reflection takes place) before it reflects back into the space destructively --- if that makes sense.
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post #68 of 73 Old 01-04-2013, 05:56 AM
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Another point is that an absorber that covers a lot of area picks up effectiveness at lower frequencies simply because it becomes so pervasive in the room. To some degree you can trade off area for thickness.

"8 foot high and running the 45 foot width "

it's more than likely the simple fact that like all things in acoustics, objects must be LARGE with respect to wavelength to be effectively 'seen'. a physically SMALL absorber regardless if constructed from ideal material and thickness is still not going to be effective to attenuate a much larger wavelength.

Well you seem to be the acoustics expert, and I'm just a general practitioner. So, what do you think?

I'm not aware of any horizontal wavelength effects when the sound has perpendicular incidence. What I am aware of is the fact that absorption's effectiveness seems to be highly dependent on the percentage of the reflective surface that is covered.

If an absorber covers say 10% of a reflective wall, is that or is that not less overall effective than one that covers say 30% of the same wall?
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post #69 of 73 Old 01-04-2013, 06:46 AM
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What I am aware of is the fact that absorption's effectiveness seems to be highly dependent on the percentage of the reflective surface that is covered.

what i am aware of is that the absorber's effectiveness seems to be highly dependent on the absorber being large with respect to wavelength of the relevant signal.

the same physics/logic for acoustics is applied to any sort of 'treatment'. take reflection phase grating diffusers for example. regardless of how deep the wells are, the physical size of the array must still be LARGE with respect to the design frequency wavelength! you would not employ a 24" x 24" QRD with 20" deep wells for a design frequency of 300hz and expect the unit to truly diffuse a 300hz wavelength (3.7ft).

what i am saying is, sound has size and your particular absorber from the information you've provided is quite massive - so more of the signal (low frequency large wavelength) 'sees' the absorber (vs diffracting around) - and adding to the fact that if it is applied to attack that particular axial mode, then the signal (by nature of resonance) is passing through the absorber many times. so even if the absorption coefficient is relatively low, you are getting a large number of passes through the absorber vs another acoustical problem or application where the signal may only pass through the absorber twice (incident into the absorber --> reflecting from rigid boundary --> and back through the absorber into the acoustical space).

for example: if one applies a front wall porous absorber to attack the SBIR front wall phasor (1/4wave null), you only get "one shot" to attenuate that signal ... vs a modal resonance where the wave is passing many times through the absorber as the signal persists due to resonance. therefore, the SBIR absorber will need a higher absorption coefficient to be more effective.

and all of this still ignores other complex behavior such as speed of sound changes within the porous absorber / refraction / etc...

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Originally Posted by arnyk View Post

If an absorber covers say 10% of a reflective wall, is that or is that not less overall effective than one that covers say 30% of the same wall?

"overall effective" ??? overall effective at what, specifically?
i think that's too simplistic and vague, as your example is ignoring whether the signal being attenuating is a sparse indirect specular reflection in a small room or if one is removing energy from a statistically reverberant sound-field of a large acoustical space.

in small spaces such as residential rooms, we deal with local areas of variable pressure with respect to the ambient noise floor. we do not have a reverberant sound-field at any appreciable frequency to us humans, therefore we do not "treat" or apply absorption in that manner. the particular indirect specular energies and how they impede the listening position (receiver position) can be easily measured. from that information, we can identify the incident boundary and apply 'treatment' there (specifically) based on the design requirements (absorption, diffusion, etc). in large rooms, the absorption is applied statistically to reduce the reverberant sound-field.

this is why we simply do not "randomly apply" absorption on the boundaries in small rooms - as we are not looking to reduce the statistical energy flows but instead to attenuate or treat specific, focused indirect specular reflections based on source/receiver positions and the geometry layout of the bounded acoustical space. and as such, the absorber must be large with respect to wavelength.

a larger absorber (covering more sq area than needs to to attenuate that indirect signal in the small room) will still remove more energy from the room - but whether this is your definition of "effective" is not for me to decide. an overly large absorber for specular energy attenuation may quickly lead to a highly damped space, where-as a properly sized absorber could be used to still attenuate the destructive indirect signal, while removing as little of acoustical energy from the room as possible. it depends on the design requirements.


the energy flows are fundamentally different and thus it is not an apples to apples comparison (which is why we don't 'treat' small rooms (residential rooms) like one would 'treat' large rooms (auditoriums, concert halls, churches, etc)).

as for LF absorption in small rooms, we obviously have boundary limitations with respect to the modal wavelength sizes - which is why LF porous absorbers still need to be as LARGE as physically possible based on user's real-estate constraints to be effectively 'seen' by the longer (larger) wavelengths - and this is in conjunction with appropriate THICKNESS and FLOW-RESISTIVITY of the absorber to be effective.

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Originally Posted by arnyk View Post

What I am aware of is the fact that absorption's effectiveness seems to be highly dependent on the percentage of the reflective surface that is covered.

in large spaces where absorption is applied statistically, yes.

and no, im not a self-proclaimed "acoustics expert", sorry.
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post #70 of 73 Old 01-04-2013, 06:12 PM
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What I can do is moving my couch off the rear wall when we watch movie, but what minimum distance from the wall would you recommend/be enough?

Move it as far forward as gives you the best sounding bass. I can't guess at this, but listening as you pull it forward should give you an indication. Better, take measurements as you pull it away and you can easily see when the boom is removed sufficiently.
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To come back on the sub topic, would a single sub be night and day compared to my current set up ?

If by night and day, you mean to remove the boom, it depends on your room and without some kind of very expensive fluid dynamics simulator, no one can tell you. Empirical trumps simulation anyways, so best to borrow or buy one and try it out. I am pretty confident that it won't help you one bit if you place the sub near the main speakers though (against the wall with the screen).
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I can't afford to buy 2, and you know my constraints for one, but if it's that good, I will try to find a way to add one to the room. Keep also in mind that I don't need deeper bass, and I'm pretty sure the bass from my speakers, in good conditions, would be enough for us. In wall subwoofer is really not an option though.
Btw, I ordered a mic, so I should be able to provide some measurements soon.
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post #71 of 73 Old 01-06-2013, 05:53 AM
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So with regards to a typical small room, ie 14 x 18, is it beneficial to have several larger porous absorbers, say around 4ft x 6ft, versus a 2ft x 4ft, for treating indirect specular reflections? I would imagine that one would need to be pretty carefull with using an absorber this large as it could possibly over-dampen the room, right?

Localhost, are you saying that it is not a good idea to use too large of an absorber in a typical small room?

How would a person go about figuring out how to effectively treating the rooms problem areas, with regards to peaks and nulls, without killing the rooms acoustical energy?

What is SBIR? What is the front wall phasor and why do you only get one chance to treat it? What is the most effective way of treating this, SBIR or front wall phasor?
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post #72 of 73 Old 01-06-2013, 09:35 AM
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is it beneficial to have several larger porous absorbers, say around 4ft x 6ft, versus a 2ft x 4ft, for treating indirect specular reflections?

It might be useful to cover more than just a few 2x4 foot areas. It depends on the size of the room, where the speakers are located, and where you (and others) sit.
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I would imagine that one would need to be pretty carefull with using an absorber this large as it could possibly over-dampen the room, right?

Over damping a room is not usually a problem unless you line half or more of the surfaces with absorption. The more common mistake people make is using thin foam, because that absorbs only the mids and highs leaving the room unbalanced.
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How would a person go about figuring out how to effectively treating the rooms problem areas, with regards to peaks and nulls, without killing the rooms acoustical energy?

A room doesn't have energy of its own. All acoustic problems are caused by reflections from the walls, floor, and ceiling. The key is adding absorption at the right places, and not where it isn't needed. This should help:

Acoustic Basics
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What is SBIR?

Speaker Boundary Interference Response. It causes peaks and nulls that are not related to the room's dimensions. Versus modal peaks and nulls that are related to the dimensions. More here:

Frequency-Distance Calculator

--Ethan

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Ethan's Audio Expert book

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post #73 of 73 Old 01-06-2013, 10:01 AM
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So with regards to a typical small room, ie 14 x 18, is it beneficial to have several larger porous absorbers, say around 4ft x 6ft, versus a 2ft x 4ft, for treating indirect specular reflections?

One might take that question to be one of two questions.

One would be what is more beneficial, more absorber (4 ' x 6') or less absorber (2' x 4').

The other question would be whether it is more beneficial to implement 32 square feet of absorber as one 4'x8' absorber, or two 4'x4' absorber.

Only the second question can be answered in general. The answer to the first question depends on information that has not been given.

The second question can be generally answered by saying the same amount of square foot of absorption is usually more effective and pleasing when broken up into smaller separated areas.
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I would imagine that one would need to be pretty carefull with using an absorber this large as it could possibly over-dampen the room, right?

Depends on what is in the rest of the room. A bare 14' x 18' room would probably not be over damped by two or even four 4'x 6' panels.

The same room with a lot of overstuffed furniture, a heavy rug and an acoustical tile ceiling could be overdamped.
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