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Acoustical Treatments Master Thread - Page 310

post #9271 of 10188
Diffusion behind is a good idea yes, sorry I didn't pick it up, guess I mostly looked at the plan. cool.gif
post #9272 of 10188
Well I'm glad you commented because I really like the gills idea. smile.gif
post #9273 of 10188
Quote:
Originally Posted by HaroldKumar View Post

Well I'm glad you commented because I really like the gills idea. smile.gif

smile.gif it's being used more and more frequently here in Sweden. I thought I would have to build them too, but according to my acoustics consultant, my room is wide enough to warrant diffusion on the sides instead ( which diminished the cost of having him too, in a way ).
post #9274 of 10188
Quote:
Originally Posted by Nightlord View Post

You may bounce other sound too early towards listener as well.


A down firing subwoofer pointing to a riser filled with sand is a option?
post #9275 of 10188
Quote:
Originally Posted by HaroldKumar View Post

Well I'm glad you commented because I really like the gills idea. smile.gif

Yes, these are typically called "sawtooth" walls (at least from what I've seen). They're really great when coupled with diffusion as instead of absorbing the early reflections, you can send them to the rear of the room to be diffused and returned at a later time, which will keep the liveliness of the room while providing a much more balanced, neutral sound.
post #9276 of 10188
Quote:
Originally Posted by GIK Acoustics View Post

Yes, these are typically called "sawtooth" walls (at least from what I've seen). They're really great when coupled with diffusion as instead of absorbing the early reflections, you can send them to the rear of the room to be diffused and returned at a later time, which will keep the liveliness of the room while providing a much more balanced, neutral sound.

I'm not sure the two are completely the same. These aren't meant just to bounce the sound off, they are open on the short end towards the speakers and are to be filled with insulation (or other absorbing material). Is that common practice with your "sawtooth" walls?

(Gill-absorber is what this has been labeled as in Sweden )
post #9277 of 10188
My room with speakers so close to sidewalls it makes sense to try a custom gill assembly at the first reflection point. Since it's so close to wall and I have monitors the gill trap will be pretty small and easy to build with some black foam board, oc703 and hot glue.

But getting back to the real project is the diffusors. Did anyone have any comments on the three N11 panels vs the one N31 panel? The carpenter has slotted time for this now and I have to decide.
post #9278 of 10188
I'd go for the N31. If nothing else for no better reasons than that it's much better looking. biggrin.gif
post #9279 of 10188
Quote:
Originally Posted by Nightlord View Post

I'm not sure the two are completely the same. These aren't meant just to bounce the sound off, they are open on the short end towards the speakers and are to be filled with insulation (or other absorbing material). Is that common practice with your "sawtooth" walls?
(Gill-absorber is what this has been labeled as in Sweden )

Yes, that is the common design of what I've seen called "sawtooth" walls. Otherwise, they would work against their purpose and bounce sound back towards the listener!

They are very neat, and a great design concept for smaller rooms that don't have the room for fully angling the sidewalls.
post #9280 of 10188
What could have cause a big valley and a serious null for my stereo bookshelf speakers? Each speaker is about 3.3 feet from sidewall, 2.6 feet from the wall behind speaker. Speaker is on 2 feet stand. Floor to ceiling height is about 10 feet. Sidewall to sidewall is about 16 feet.
post #9281 of 10188
Quote:
Originally Posted by Skylinestar View Post

What could have cause a big valley and a serious null for my stereo bookshelf speakers? Each speaker is about 3.3 feet from sidewall, 2.6 feet from the wall behind speaker. Speaker is on 2 feet stand. Floor to ceiling height is about 10 feet. Sidewall to sidewall is about 16 feet.

I would suspect SBIR (speaker boundary interference response) to be the culprit in this case (I would suspect this due to the extremely narrow dips). The easiest way to tell is through testing. I would recommend taking these steps to get to the bottom of it.

Do a standard test of the room (considering response changes day to day, I always recommend starting with this, even if you already have a measurement)
Move the speakers back or forwards one foot from their original position. Test.
Move the speakers towards or away from the sidewalls from their original position by a foot. Test.
Move the speakers up or down in height from their original position (you can use books to raise their height). Test.
Move the speakers back to their original position, and move the microphone back or forwards a foot. Test.

Whichever one of these variables changes will show which boundary interaction is giving you these nulls. More than one may be present. We do have an article you can read about SBIR on our website, here: http://gikacoustics.com/speaker-boundary-interference-response-sbir/

The solution would be to place bass traps between the speaker and whichever boundary is causing the problems.
post #9282 of 10188
Quote:
Originally Posted by GIK Acoustics View Post

The solution would be to place bass traps between the speaker and whichever boundary is causing the problems.
How thick should the bass traps be to tackle the problem? 4" ? 6" ?

My local hardware shop has this Roxul RHT 80 : http://www.roxul.com/industrial/products/roxul+rht%C2%AE+30,+40,+60,+80,+100,+-c12-+150
post #9283 of 10188
RHT 80 would work fine. Thicker is almost always better, so I would recommend 6" but 4" would likely do good as well. The farther the front surface of the absorber is to the wall, the lower in frequency it will absorb (typically). So 6" would go lower in frequency, or a 4" absorber with an air gap behind it would also go lower in frequency. So 6", if not, 4" with an air gap, and still if not, then a 4" absorber.

If the nulls are caused by both the front wall and the side walls as I suspect, some corner trapping (either broadband traps straddled on the corner, or superchunks) would help combat both sides of the problem.

Usually the packs of Roxul come in packages of 6 (48 sq feet). I would do a test of the room without any Roxul installed. Then, lean a batt of RHT against the sidewalls inbetween the speakers and sidewalls, inbetween the speakers and front walls, and one in each front corner, and do a test. If the nulls are reduced, but not enough, then try different combinations (for example, two high in each corner and one on each sidewall, etc). Or try doubling them up on one side (so 8" thick) and just testing the speaker individually. If the nulls aren't reduced, try different positions with the Roxul (first reflection points, or back wall corners, etc)

Edit: Are your speakers decoupled from the stands? Or are the stands decoupled from the floor? These can also cause nulls, but I've never seen nulls that deep due to coupling, I could see it as possible.
post #9284 of 10188
Quote:
Originally Posted by Nightlord View Post

I'm not sure the two are completely the same. These aren't meant just to bounce the sound off, they are open on the short end towards the speakers and are to be filled with insulation (or other absorbing material). Is that common practice with your "sawtooth" walls?
(Gill-absorber is what this has been labeled as in Sweden )

Never realized the mthod had a name.

I've posted about such an approach many times. Grazing angles and effectiveness of sidewall absorbers is important, and often over-looked. I've found a variety of methods for "catching" that incident energy, one such very much akin to what you posted.

A side benefit is it absorbs in one direction, and reflects in the other. That energy is nice to keep bouncing around, but the early sidewall stuff,...especially within the first couple dozen millisec, is detrimental in clarity and it's beneficial to lessen it's level (with absorption or diffusion), or re-direct and delay it's arrival.

I've found simply exposing the sides of the absorbent panels, staggering them like lap-board siding, with each new edge facing the speakers works great too.
Rigid 703, wrapped in fabric, no frame. This is how my ceiling is config'd too. The incident energy sees multiple panel edges, in addition to the remainder of the panel.


Thanks
post #9285 of 10188
I didn't know that the front "edges" were absorption, and I thought the angles were such that the listening area received no first reflections.

Jeff
post #9286 of 10188
Quote:
Originally Posted by pepar View Post

I didn't know that the front "edges" were absorption, and I thought the angles were such that the listening area received no first reflections.
Jeff

yep;

Quote:
"Gills" aimed towards speakers stuffed with insulation

The walls are directional, energy one way gets absorbed, stuff from the other direction gets reflected. As I stated, ceiling tretments work this way too.
post #9287 of 10188
Quote:
Originally Posted by GIK Acoustics View Post

RHT 80 would work fine. Thicker is almost always better, so I would recommend 6" but 4" would likely do good as well. The farther the front surface of the absorber is to the wall, the lower in frequency it will absorb (typically). So 6" would go lower in frequency, or a 4" absorber with an air gap behind it would also go lower in frequency. So 6", if not, 4" with an air gap, and still if not, then a 4" absorber.
If the nulls are caused by both the front wall and the side walls as I suspect, some corner trapping (either broadband traps straddled on the corner, or superchunks) would help combat both sides of the problem.
Usually the packs of Roxul come in packages of 6 (48 sq feet). I would do a test of the room without any Roxul installed. Then, lean a batt of RHT against the sidewalls inbetween the speakers and sidewalls, inbetween the speakers and front walls, and one in each front corner, and do a test. If the nulls are reduced, but not enough, then try different combinations (for example, two high in each corner and one on each sidewall, etc). Or try doubling them up on one side (so 8" thick) and just testing the speaker individually. If the nulls aren't reduced, try different positions with the Roxul (first reflection points, or back wall corners, etc)
Edit: Are your speakers decoupled from the stands? Or are the stands decoupled from the floor? These can also cause nulls, but I've never seen nulls that deep due to coupling, I could see it as possible.
For your info, my room is fully brick-concrete, which is a reflection nightmare for many audiophile. I know many people have good result after adding bass traps (6" of OC703) but their rooms are NOT made of bricks, just plain wood frame and board instead. Therefore, I would like to know the effectiveness of bass trap in a room like mine...maybe I need 12" of bass trap to be effective?

Will denser material work better? Will RHT100 or RHT150 be more effective?

Is it OK to leave the Roxul batt plastic wrapping on for bass trap?

Speakers have BluTack stuck at the bottom, to prevent them from falling off the stand. I doubt the stand is causing the null.
Edited by Skylinestar - 12/22/12 at 7:46pm
post #9288 of 10188
I'm pretty sure that the recommendation for placing the batts above was just to test where placement would be most effective. For the actual installation, you'd want to unwrap the batts, not so much to remove the plastic, but to uncompress the batts from the packaging, and get the most volume for your money.
post #9289 of 10188
Quote:
Originally Posted by Skylinestar View Post

For your info, my room is fully brick-concrete, which is a reflection nightmare for many audiophile. I know many people have good result after adding bass traps (6" of OC703) but their rooms are NOT made of bricks, just plain wood frame and board instead. Therefore, I would like to know the effectiveness of bass trap in a room like mine...maybe I need 12" of bass trap to be effective?
Will denser material work better? Will RHT100 or RHT150 be more effective?
Is it OK to leave the Roxul batt plastic wrapping on for bass trap?
Speakers have BluTack stuck at the bottom, to prevent them from falling off the stand. I doubt the stand is causing the null.

Regardless of the room construction, an absorber will be equally effective in a room with any boundary. The difference is you will have longer modal resonances, which would take a larger quantity of absorbers, but not necessarily thickness. A 6" absorber goes as low as a 6" absorber regardless of the room conditions, if that makes sense. With that said, depending on where the problem frequency is, you may need them thicker than 6". For example if you're trying to absorb 40 Hz, many 6" absorbers will get you some small gains, but likely not what you need. If you need to absorb 70 Hz though, they would work great straddling a corner.

Denser material works well for thin traps, but I wouldn't consider anything greater than Roxul 80 (even 60 would be fine) for thicker traps. Once you get to a foot or so thick, standard fluffy insulation works best. So, the thicker it is, the lighter you want it to be. But to be specific, "density" isn't the important factor here, but "gas flow resistance". Though for most of these products, lighter density means less gas flow resistance.

Don't leave the Roxul in the plastic bags as they are extremely compressed in the bags. You can wrap them afterwards with thin plastic though if you are concerned. They will not lose bass efficiency.

You would be surprised at the amount of damage speakers being coupled to the floor can make - specifically, nulls. Proper decoupling is important! I'm not suggesting that IS what is happening, but it is certainly possible.
post #9290 of 10188
Quote:
Originally Posted by GIK Acoustics View Post

Regardless of the room construction, an absorber will be equally effective in a room with any boundary. The difference is you will have longer modal resonances, which would take a larger quantity of absorbers, but not necessarily thickness. A 6" absorber goes as low as a 6" absorber regardless of the room conditions, if that makes sense. With that said, depending on where the problem frequency is, you may need them thicker than 6". For example if you're trying to absorb 40 Hz, many 6" absorbers will get you some small gains, but likely not what you need. If you need to absorb 70 Hz though, they would work great straddling a corner.
Denser material works well for thin traps, but I wouldn't consider anything greater than Roxul 80 (even 60 would be fine) for thicker traps. Once you get to a foot or so thick, standard fluffy insulation works best. So, the thicker it is, the lighter you want it to be. But to be specific, "density" isn't the important factor here, but "gas flow resistance". Though for most of these products, lighter density means less gas flow resistance.
Don't leave the Roxul in the plastic bags as they are extremely compressed in the bags. You can wrap them afterwards with thin plastic though if you are concerned. They will not lose bass efficiency.
You would be surprised at the amount of damage speakers being coupled to the floor can make - specifically, nulls. Proper decoupling is important! I'm not suggesting that IS what is happening, but it is certainly possible.
Thanks for the info.

I'm thinking of placing 2 stacks of RHT80 bass traps (4' x 2' x 1' each) horizontally behind my entertainment center...wondering if it's too tick?
post #9291 of 10188
I would think that making an extra 1/3 the traps by making them 8" thick would be more beneficial for most rooms. Hitting as much corner area as possible will help again to absorb the most bass to balance out the room most. Don't forget that there are 12 possible corners - not only 4!
post #9292 of 10188
This is my first attempt at a HT and realized I have some acoustic problems. The bass is full/boomy around the room then drops off to nothing in the middle of the room. I’m sure I have other problems but the bass is the first to jump out at me. Any help with placement for bass traps and panels (plus size/thickness/quantity) in my room would much appreciated.

Here is some information that might help about my room size and setup.

Room size 18x13x10 notes: The vaulted ceiling is different in the front of the room vs. the back. The white doors are butted up to the back corner. Carpet is mid shag with thick DuPont pad. Two rows of seating and minimal placement options for the Sub (corner placement).





Audio setup: Denon AVR-3313CI, Klipsch RF-62II front, RC-52II center, RS-42II surround, SW-112 powered subwoofer. Changes I made after running the Audyssey setup (8 positions): all speakers changed to small, sub – yes LFE 80Hz raised 5db, crossover 80Hz.

Again thanks in advance for your help!!!
post #9293 of 10188
Mr Rogers,

Bass will always drop off / cancel in the center of the room, no matter the acoustic treatment. The only way to change that is to alter your room dimensions (and it will still do it in the center of the new ones, etc) so you should always be sitting in the back half of the room, at least a foot or two from the center of the room.

The boominess you described is typical of an untreated room. The basics to cover would be your first reflection points from your mains/LCRs along with thick trapping in the corners of the room. A set up like this would be preferred: http://gikacoustics.com/acoustic-advice/ (this is a studio and not a home theater, but the same principals apply)
post #9294 of 10188
Quote:
Originally Posted by Mr Rogers AK View Post

The bass is full/boomy around the room then drops off to nothing in the middle of the room. I’m sure I have other problems but the bass is the first to jump out at me. Any help with placement for bass traps and panels (plus size/thickness/quantity) in my room would much appreciated.
I had that same problem. The most effective remedy is to add another sub, with the two of them placed on opposing mid-walls (as a start). This will help cross-cancel the nodes that are killing the bass in the seating area. This technique is based on research from Harman (Welti) if you care to get into more of the physics.
post #9295 of 10188
Quote:
Originally Posted by Roger Dressler View Post

I had that same problem. The most effective remedy is to add another sub, with the two of them placed on opposing mid-walls (as a start). This will help cross-cancel the nodes that are killing the bass in the seating area. This technique is based on research from Harman (Welti) if you care to get into more of the physics.

This is most effective in rooms that are closed and rectangular.

Jeff
post #9296 of 10188
I solved a nagging null problem by hoisting one of my subs up to the ceiling. Since I have 10 ft ceilings there's lots of room up there.
post #9297 of 10188
Here s a good tutorial for multiple sub placement, http://mehlau.net/audio/multisub_geddes/
post #9298 of 10188
Quote:
Originally Posted by HaroldKumar View Post

I solved a nagging null problem by hoisting one of my subs up to the ceiling. Since I have 10 ft ceilings there's lots of room up there.

We tend to think two-dimensionally when placing subs. But YES! I thing Geddes or Welti advocates raising subs ...
post #9299 of 10188
Thanks for the info everyone! So it looks like adding another sub and some acoustic treatment will help. Does anyone have any suggestions on how I can treat (bass trap) the corner by the white doors. I don’t think my wife will go for not being able to open both doors.
post #9300 of 10188
Quote:
Originally Posted by Mr Rogers AK View Post

Does anyone have any suggestions on how I can treat (bass trap) the corner by the white doors. I don’t think my wife will go for not being able to open both doors.

sure, here you go:
I'd suggest you make movable non-wall attaching corner bass traps.
Eric took my concept and applied it with pink fluffy, so this is my recommendation to you, a win-win with your wife.
He gives step-by-step instructions for you to follow.
http://www.avsforum.com/t/1312693/diy-construction-methods-of-hang-able-acoustic-panels-not-fixed-frames/120#post_22131618
Quote:
Originally Posted by Eric2000 View Post

I followed a similar construction technique that Mike posted for his corner bass traps, but made mine from pink fluffy R-19 instead and wrapped the fabric around the outside of the threaded rod. Here is the photo journey:

My traps are 24x24x34 inch triangles, are about 3.5 feet tall, and probably weigh less than 10lbs each. Since I need access to one of the corners for a doorway, I made them stackable and moveable. I started by cutting triangles, drilling holes for the threaded rod, and inserting T-Nuts for the sides that stack on one another:
287

Then I cut 23" squares of insulation, cut them diagonally for triangles, and clipped off the corners so they fit snugly between the threaded rods:
390

A wire mesh made from separated Cat5 wire goes between each layer to keep the insulation from sagging over time. The numbered arrows indicate the direction of winding the wire to support the insulation. A bead of solder keeps the wire from unwinding. Each layer of insulation is about 5" thick.
400

Here is the first one all stacked up. You can see the supportive wire mesh wrapped around the threaded rod on each side:
516

And then with the Kraft paper glued to the front with spray adhesive:
529

Then, turn the trap upside down to affix the cloth wrapping - a two-pack of curtains from the giant W for $15. Each pack is enough for two traps.
400

The cloth is stapled to the underside of the top plate to prevent sagging over time:
291

When the fabric is fully secured to the top panel, turn the trap right side up again and pull the fabric around to the back. Trim off the excess and then just pull it tight and use a desk stapler to hold the fabric together in the back:
550

The staples produce a few ripples in the sides, but you won't see those once you put it in the corner. The front looks nice and clean:
528

Here is a closer shot of two of them stacked together:
600

And finally, the entire back half of the theater. With a few bean bag chairs up front, we can comfortably seat 10-12 people. The colors are a little off from a combination of CFL and flash lighting:
326

Below are the before and after REW plots. The purple trace is the original measurement with no traps and no EQ. The yellow trace is the difference made by the bass traps alone. The traps took 5dB off of the room-induced peak at 45Hz without sacrificing anything else in the audible range:
329

This is the original waterfall plot made by REW - no traps, no other corrections - just a mess with room modes at 45Hz and 90Hz:
443

And here is the waterfall plot after ONLY the traps are put in place. I was surprised by how much of a difference the traps made in the decay:
447

After several days of tweaking the parametric equalizer with the traps in place and a first order high-pass filter in place, here is my "final" room response curve - flat from 7Hz to 100Hz, plus/minus 3dB :'( The peak at 105Hz won't ever really happen because the preamp crosses the LFE channel at 60Hz.
449

And the "final" waterfall plot - nice and smooth. If I adjust the waterfall graphing limits in REW, the entire response curve is down by 20dB within the first 100-120ms with the exception of a 2-3dB narrow bump at about 23Hz. I am really pleased with this result:
443

It sounds great! The EQ and high-pass filter reduce (but don't eliminate) the incidence of amp clipping, but still provide enough punch to cause visitors to literally jump up from the couch :P

All of my other projects are on my web page, which I think is linked in my signature.

Eric
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