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post #11851 of 11862 Old 04-08-2017, 07:25 AM
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Yeah it will potentially reduce some of the midbass absorption and may even reflect some sound at a significantly reduced level. It may raise the low frequency absorption some. It's hard to say.

I know of a studio designer that likes to back OC703 with hardboard to reflect attenuated sound waves and he angles them so that the sound is redirected behind the listener. It can't be all bad of he's doing it as a professional.


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post #11852 of 11862 Old 04-12-2017, 12:05 PM
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Nice post. Boundary Interference is just one of the factors at play for most of us trying to get good sound in smaller rooms. Usually there are two ways to combat it: either move the speakers, or put a bass trap in the path the reflected low end takes to get back to the speaker. SBIR is often problematic with a big null between 100-200Hz, if the speakers are between 18-36" from a wall. These strategies can help, but should only be a part of the overall bass management strategy.

Speaking of which, I don't agree that bass trapping (such as superchunks) do very little. If they are implemented correctly in a good bass trapping strategy, the improvement is not subtle. That said, yes you do need a lot of broadbass trapping to solve very low frequency issues (under, 60Hz, say).



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Originally Posted by Mpoes12 View Post
SBIR is caused by low frequencies which are best treated by bass trapping of the boundaries near to the speaker. In reality, this is an issue that is best addressed by having a significant amount of absorption in the room, especially in small rooms. Baffle walls only treat one cause of the so called SBIR. I'd also note, not all acousticians like or use this term. I've mentioned it to a few who just looked at me funny or have noted its just a special case of interference and doesn't need or deserve a special name. Baffle walls treat the boundary effect of a speaker placed close to a back wall. However, it will not treat interference caused by reflections off the ceiling, floor, or sidewalls. This is why I think you need to treat ALL boundaries around the speaker for bass. How you treat the midrange or high's is a different issue and wouldn't fall into SBIR anyway. I can't speak for the other author, but for myself, I'd agree with the notion that you treat all boundaries for low frequency absorption that are near the main speakers. That would include sidewalls still.


There are sometimes notches in the response blamed on SBIR that are not SBIR in the way GIK defines it. At least I don't believe so. The reason I would make such an argument is that we see these notches blamed on SBIR and happen to roughly match something like the distance between the main speaker and rear wall. Then we note that in fact it also matches a width or length mode. Further we note that the main speakers have no significant output at that frequency in question and the subwoofers placement would not excite that mode due to its distance to a wall, but would due to the rooms length or width. I think this is what you are talking about when you say only treat above the crossover. That is kind of correct, but you still would want to fix that notch, right? The problem is that significant modal interference in a room is not easily treated by a few 6" deep bass traps on the wall or some corner traps. Even "superchunks" often do very little, especially when at very low frequencies. Fixing them ends up being a process of subwoofer quantity, location, tuning, room design, wall construction, and bass trapping in the traditional sense.

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post #11853 of 11862 Old 04-12-2017, 07:42 PM
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I think we are largely saying the same thing. Treating boundaries effects requires absorbing the reflection or moving the speaker to reduce its effect. My point was that absorbing it's reflection at even 200hz requires a lot of absorption across the entire boundary surface to be what I call highly effective. I also rarely find corner traps to be the solution to these problems. A super chunk is just a big corner trap.

My biggest problem with this is that if we have a boundary effect causing a 60db dip in the response, which isn't unheard of, then adding damping might be highly effective at reducing the dip from 60db down to just 40db down, but it's still 40db down. And in some cases it also lowers the Q of the dip, which can be audibly worse. That isn't to suggest that adding inadequate low frequency damping is worse than nothing, but I think sometimes the effect is blown out of proportion. If I have a 40db dip or a 60db dip, I don't consider either to be better, I want to fix that until it's minimal. If we are talking about say 120hz, then we are talking about probably 12" of insulation across the entire front and front/side wall before that dip goes away.


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post #11854 of 11862 Old 04-13-2017, 07:18 AM
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Okay guys, what would it do to the sound absorption if I put velvet over top of a sound absorber? I know I want to use an AT fabric, and I know that velvet is not AT. But what would it actually do to the sound?

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post #11855 of 11862 Old 04-13-2017, 08:09 AM
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You'll just get a little more reflectance above 2-3khz.

Edit: I'd wager it would teeter on not noticeable to barely noticeable in this instance.

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post #11856 of 11862 Old 04-13-2017, 12:44 PM
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Quote:
Originally Posted by 12B4A View Post
You'll just get a little more reflectance above 2-3khz.

Edit: I'd wager it would teeter on not noticeable to barely noticeable in this instance.
I'd bet the opposite is true. It will absorb more, not less.
http://www.rosebrand.com/product1210...Velour-FR.aspx
Rosebrand tested their velour's as acoustic curtains and found them to be highly absorptive when give a good airgap. There is nothing surprising about that.

Velvet or velour may or may not be acoustically transparent. It would depend on the material. At low frequencies most materials are acoustically transparent, the material lacks the necessary mass to be "seen" by the wave. At high frequencies it can do one of three things. It can be rigid and massive enough to reflect sound. It can be such that the particle's energy is dissipated within the woven fiber and thus the sound is absorbed. It be light and thin enough to essentially allow the particles to pass through unabated. Acoustic transparency means the latter. The question is what effect it will have on sound absorption, and in fact, it is absorptive itself to a point.

The problem will be the density and openess of the backing material. If the acoustical impedance of the material relative to the fiberglass provides a drastic shift (low to high typically) it will reflect. Sometimes the backing material of these fabrics have a high acoustical impedance and it becomes reflective when sound hits it (even though its surface is absorptive). This seems to be the case with a lot of synthetic suede materials and I think why we see a drop in the absorption of high frequencies for these panels.

What you really want is a gradual shift from lower to higher acoustical impedance as you approach the wall in order to avoid the wave from reflecting before it has traveled through the material.
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post #11857 of 11862 Old Today, 05:16 AM
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I'm looking at doing some DIY broadband panels, and would like to have them as efficient as possible in the lower frequencies. I am thinking of a sandwich construction with something like:

2" fiberglass -> Vinyl barrier -> 2" fiberglass.

If I understand the theory correctly, the middle layer should improve the low frequency absorption.
Now my question is, how is it determined what density to choose for the vinyl barrier? The site I am looking at sells 6kg/m2, 7.5kg/,2 and 15kg/m2. I guess the higher weight the lower frequency, but the weight should ensure a "linear" absorbtion coefficient for the whole system.

I am also making the same construction but with a vinyl barrier placed on the face of the panel as well to be used when only LF-absorption is wanted.

Inputs are appreciated!

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post #11858 of 11862 Old Today, 05:54 AM
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3" 2x4 panels or 1" linacoustic

Hi,

I'm finally building my first small dedicated room as part of a post-pipe-burst basement remodel. The room is 12 x 14 with 7.5' ceilings. I need to orient the room with the projector/screen along the 12' length owing to requirements to have a door to a mechanical area. I will have 4 seats and a 92 inch screen (accommodating for another door on the front wall). Not ideal given the limited space and access requirements but I will be moving from a mixed use space with windows to a dedicated space with no windows and openings.

I have wired the room for 7.2.9 with in-walls for SL, SR, RL, RR and 4 Atmos speakers in the ceiling. My L/C/R/SW will be cabinets placed around the screen; I don't have room for a false wall and so am not doing an AT screen with false wall. I have a regular Elite fixed screen from my prior multi-use space. I plan to run dual subs to balance room modes. The walls are up and painted dark blue with a black solid ceiling (i.e. sheetrock not drop). The projector is ceiling mounted with 10.5' throw.

My question is about acoustic treatments. I have tried to read as much as possible but this thread is really hard to get through. Here are my questions:

1. Since I don't have an AT screen, is there any point in treating the front wall? Presently I am not planning to treat the front wall. Most posters do 1" linacoustic but that seems to go with AT screens.

2. For side walls I am torn between 1" linacoustic to 48" height versus four 3" 2 x 4 acoustic panels on each side wall (for a total of 8 panels). Cost is close - with the raw materials from Fabricmate coming in at $200 more than eight completed GIK 3" panels. Other than the aesthetics, is there any reason to go one way over the other? The panels are thicker and would provide 70% of the surface area coverage of the Linacoustic so I'm not sure what would be "better." I need to determine is aesthetics is the only basis for making this decision or if there is another factor to consider.

3. Should I do anything with the rear wall? Many threads discuss diffusion, but my seats are 1" from rear wall which seems too close for diffusion. Some threads talk about not treating the rear wall to have reflections to minimize localization of surrounds. I haven't come to a conclusion yet and would appreciate some guidance.

4. Since I will be running dual subs, is it fair to assume that with DSP/Audyssey will minimize the need for bass trapping?

Thanks very much! Should be up and running in the next 90 days and I will post pics?

David
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post #11859 of 11862 Old Today, 05:58 AM
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Sorry just noticed typos after posting. Cant figure out how to edit my original post.

Wiring the room for 7.2.4.
SW will be in front right corner and rear left corner.
Seats will be 18" from rear wall.
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post #11860 of 11862 Old Today, 09:44 AM
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Quote:
Originally Posted by dgkula View Post
Hi,

I'm finally building my first small dedicated room as part of a post-pipe-burst basement remodel. The room is 12 x 14 with 7.5' ceilings. I need to orient the room with the projector/screen along the 12' length owing to requirements to have a door to a mechanical area. I will have 4 seats and a 92 inch screen (accommodating for another door on the front wall). Not ideal given the limited space and access requirements but I will be moving from a mixed use space with windows to a dedicated space with no windows and openings.

I have wired the room for 7.2.9 with in-walls for SL, SR, RL, RR and 4 Atmos speakers in the ceiling. My L/C/R/SW will be cabinets placed around the screen; I don't have room for a false wall and so am not doing an AT screen with false wall. I have a regular Elite fixed screen from my prior multi-use space. I plan to run dual subs to balance room modes. The walls are up and painted dark blue with a black solid ceiling (i.e. sheetrock not drop). The projector is ceiling mounted with 10.5' throw.

My question is about acoustic treatments. I have tried to read as much as possible but this thread is really hard to get through. Here are my questions:

1. Since I don't have an AT screen, is there any point in treating the front wall? Presently I am not planning to treat the front wall. Most posters do 1" linacoustic but that seems to go with AT screens.

2. For side walls I am torn between 1" linacoustic to 48" height versus four 3" 2 x 4 acoustic panels on each side wall (for a total of 8 panels). Cost is close - with the raw materials from Fabricmate coming in at $200 more than eight completed GIK 3" panels. Other than the aesthetics, is there any reason to go one way over the other? The panels are thicker and would provide 70% of the surface area coverage of the Linacoustic so I'm not sure what would be "better." I need to determine is aesthetics is the only basis for making this decision or if there is another factor to consider.

3. Should I do anything with the rear wall? Many threads discuss diffusion, but my seats are 1" from rear wall which seems too close for diffusion. Some threads talk about not treating the rear wall to have reflections to minimize localization of surrounds. I haven't come to a conclusion yet and would appreciate some guidance.

4. Since I will be running dual subs, is it fair to assume that with DSP/Audyssey will minimize the need for bass trapping?

Thanks very much! Should be up and running in the next 90 days and I will post pics?

David
You want to put bass traps in corners of your room if you can, if not then you need acoustic panels that are 4 inches or more to absorb low frequencies.

You want to find where your first reflection points are and put panels there.

Since you sit close to rear wall you will get alot of reflected sound bounced back to you from your L, C, and R speakers. Pu broadband absorption panels on rear wall.

Dual subs have nothing to do with helping absorbing bass....dual subs are good for 2 things....increase spl, and evens out bass response in the room which in turns to reduce localization issues.

But ALWAYS treat the first reflection points at least and it would also help if you pull out your couch 1 or 2 feet from your rear wall. When I pulled mine out 3 feet from rear wall the bass became tighter and my decay times improved.

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post #11861 of 11862 Old Today, 12:37 PM
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Quote:
Originally Posted by JonasHansen View Post
I'm looking at doing some DIY broadband panels, and would like to have them as efficient as possible in the lower frequencies. I am thinking of a sandwich construction with something like:

2" fiberglass -> Vinyl barrier -> 2" fiberglass.

If I understand the theory correctly, the middle layer should improve the low frequency absorption.
Now my question is, how is it determined what density to choose for the vinyl barrier? The site I am looking at sells 6kg/m2, 7.5kg/,2 and 15kg/m2. I guess the higher weight the lower frequency, but the weight should ensure a "linear" absorbtion coefficient for the whole system.

I am also making the same construction but with a vinyl barrier placed on the face of the panel as well to be used when only LF-absorption is wanted.

Inputs are appreciated!
Here is a graph giving a comparison of the different approaches. Use it as instructive rather than definitive, as the Q is certainly wrong for the limp mass. I believe the formula doesn't properly account for damping of the limp mass or of how a compound panel behaves. It's absorption coefficient estimates are also a bit low below 500hz for a velocity absorber as compared to what we seem to see with actual tested panels.


Low Dense is the panel with a 6kg/m2 vinly limp massed sandwiched between two OC703 panels (I used that panels low resistivity for the estimate). The green High Density is with the highest density vinyl, 15 kg.m2. Red is a straight 4" panel and Yellow is a straight tuned trap model (Limp mass membrane over 4" of OC703. Ignore the big peak and dip, that isn't what it would really look like. The center of the peak is probably close to right and the center of the dip is probably close to right, but the shape will be far flatter than what you see here. If someone from GIK or another acoustics company that does real testing of tuned traps is around they may be able to give better insight as to why the mathematical model gives these goofy results. I have testing gear to test things like the resonance of these traps, but I don't know how to measure absorption in situ to study this concept, so I've only been able to confirm that the resonance matches closely for the membrane.

If you are planning to put a heavy vinyl membrane in your trap, I'd take some measurements first and be certain the center frequency is going to be targeted in a good place. If you aren't sure or there isn't a real issue like that, you may want to go with a less dense membrane like 10 mil PVC. I'm having trouble confirming this but I believe the density of 10 mil PVC sheating is around 1-1.5 kg/m2. Doing that improves the absorption around the 150-200hz range rather than down at 50hz.

I hope this helps (and is right, like I said, I can't test this).
http://www.acousticmodelling.com/multi.php
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post #11862 of 11862 Unread Today, 12:52 PM
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Quote:
Originally Posted by dgkula View Post
Hi,

I'm finally building my first small dedicated room as part of a post-pipe-burst basement remodel. The room is 12 x 14 with 7.5' ceilings. I need to orient the room with the projector/screen along the 12' length owing to requirements to have a door to a mechanical area. I will have 4 seats and a 92 inch screen (accommodating for another door on the front wall). Not ideal given the limited space and access requirements but I will be moving from a mixed use space with windows to a dedicated space with no windows and openings.

I have wired the room for 7.2.9 with in-walls for SL, SR, RL, RR and 4 Atmos speakers in the ceiling. My L/C/R/SW will be cabinets placed around the screen; I don't have room for a false wall and so am not doing an AT screen with false wall. I have a regular Elite fixed screen from my prior multi-use space. I plan to run dual subs to balance room modes. The walls are up and painted dark blue with a black solid ceiling (i.e. sheetrock not drop). The projector is ceiling mounted with 10.5' throw.

My question is about acoustic treatments. I have tried to read as much as possible but this thread is really hard to get through. Here are my questions:

1. Since I don't have an AT screen, is there any point in treating the front wall? Presently I am not planning to treat the front wall. Most posters do 1" linacoustic but that seems to go with AT screens.

2. For side walls I am torn between 1" linacoustic to 48" height versus four 3" 2 x 4 acoustic panels on each side wall (for a total of 8 panels). Cost is close - with the raw materials from Fabricmate coming in at $200 more than eight completed GIK 3" panels. Other than the aesthetics, is there any reason to go one way over the other? The panels are thicker and would provide 70% of the surface area coverage of the Linacoustic so I'm not sure what would be "better." I need to determine is aesthetics is the only basis for making this decision or if there is another factor to consider.

3. Should I do anything with the rear wall? Many threads discuss diffusion, but my seats are 1" from rear wall which seems too close for diffusion. Some threads talk about not treating the rear wall to have reflections to minimize localization of surrounds. I haven't come to a conclusion yet and would appreciate some guidance.

4. Since I will be running dual subs, is it fair to assume that with DSP/Audyssey will minimize the need for bass trapping?

Thanks very much! Should be up and running in the next 90 days and I will post pics?

David
1) It is good to treat the front wall for SBIR. However 1" won't have any impact on that. The 1" is used because Dennis Erksine has been suggesting two layers of it with a plastic layer in-between. He does this because he finds 2" tends to over-absorb in the 200-500hz range (if I recall his old post on this). I've found he is right and have a measurement of my sparsely treated room that is consistent with his finding, too much absorption in that range. This recommendation is to absorb sound that radiates behind the speaker. as well as any sound that is reflecting off other surfaces onto that back wall and toward the listener. These are mostly later reflections but still bad.
2)1" is going to only absorb down to around 500hz or so. It won't provide a lot of absorption below that. In a later point you make a false assumption that leads to this. You want 4" panels here, not 1". You also don't need to treat the entire wall necessary and that decision should be dictated by your rooms RT60 response. Most small rooms that are nearly completely untreated still have RT60 values of like .3-.4ms, which is low enough that if you start covering all the surfaces with 1" absorption, would be too much. It would likely drop below .2. I would 2x4 panels, but you may consider two of these for a 4x4 area at 4" thick.
3) You should not use diffusion if you are sitting 1" from the rear wall. You want to make the wall disappear in this case, you want absorption and lots of it. you are so close to the rear wall you won't have room for thick, but I would still go with 2" on the entire area around your head, including the side walls there. It will make the room appear larger.
4) While it is true that multiple subs and Audyssey can reduce the problems, it is not true that you don't need bass traps. You can reduce the modal ringing somewhat by reducing the modal peaks, but you can't reduce the overall decay of the bass in the room. Most rooms have far too little LF absorption. You should think about bass traps on top of the rest of this. Also remember that 2 subs doesn't automatically equal improved LF response. You have to place them precisely. You can place them in the 1/4 points of the front wall, which is symmetric and possibly between your mains, but...while that might cancel the side to side modes, it might not and it very well may not do much good at your listening position. There is a power point floating around that talks about this, but my experience is the same, this isn't a great location. There really needs to be either two subs placed in very different parts of the room (front and back for example) or really, 3-4 at key points. I've also found that Geddes approach netted smoother bass than Welti's, that using non-symmetric locations seems to work better.

You don't mention this, but consider treating the ceiling. It helps make the ceiling appear to disappear too.
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