Acoustical Treatments Master Thread - Page 55 - AVS Forum | Home Theater Discussions And Reviews
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post #1621 of 11856 Old 09-11-2006, 05:22 AM
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Quote:
Originally Posted by bpape View Post

... 17x27x24" corner with the farthest part being out 12" from the corner at 45 degrees.

You meant 17x17x24 I presume.
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In addition, for the same amount of upper bass, mid and HF absorbtion, you'll get slightly better really deep bass control.

to be sure I am clear, you meant slightly better when using the solid design.[/quote]Let me know if I have that incorrect. Thanks for the clarification.
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post #1622 of 11856 Old 09-11-2006, 05:46 AM
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Originally Posted by GetGray View Post

You meant 17x17x24 I presume. to be sure I am clear, you meant slightly better when using the solid design. Let me know if I have that incorrect. Thanks for the clarification.

Corner Absorption Comparision test

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post #1623 of 11856 Old 09-11-2006, 06:21 AM
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That appears to show foam outperforming the fiberglass. Thought the foam wasn't so good realtive to the FG?
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post #1624 of 11856 Old 09-11-2006, 06:28 AM
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Originally Posted by GetGray View Post

That appears to show foam outperforming the fiberglass. Thought the foam wasn't so good realtive to the FG?

I think the point of the test was to show that the DIY SSC (Studiotips Super Chunk) performed nearly identical to a commercial bass trap.

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post #1625 of 11856 Old 09-11-2006, 07:59 AM
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A quick question for the experts. I have just completed framing out my basement and had to box-in two I-beams with the heating ducts in between (8' span). This "box " hangs approx 1' down from the ceiling,and is 14' wide (width of room) The two faces of the box are approx 12" high, 6" deep from face of 2x4's to the I-beam. Is it of any benefit to stuff this opening with Safe and Sound and cover with GOM and use as two bass traps, even though the trap wouldn't straddle the ceiling/box intersection? I plan on two super chunk style bass traps from floor to ceiling at the back of room (room is 29' long) and wondered if the 2 faces of the box would add anything or should I just drywall over them. Thanks
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post #1626 of 11856 Old 09-14-2006, 03:28 PM
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I have a question on my wall treatment bottom half.

I am using a 1" acoustical cotton behind GOM with 1 1/4" frames to keep it a little off the wall.

After I put the frames up it seems like the padding is not touching the GOM and I am afraid that it is laying up against the wall.

My concern is that when you push on the GOM it is just tight fabric and you have to push down hard to feel the padding.

I know most of the HT's that I have been in when you push on the walls it is like a firm mattress like feel. What am I going to experience in the future with sound and GOM issues..?

thank you
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post #1627 of 11856 Old 09-14-2006, 07:49 PM
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If you're using 1" cotton with only 1/4" behind it, it really doesn't matter. That 1/4" won't make diddly difference. If you want a nice padded feel (for those that must poke walls...), put a 1/4" slat across the back at 1/4, 1/2, and 3/4 of the height of the panel.

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post #1628 of 11856 Old 09-14-2006, 10:14 PM
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I went the other route. Since fiberglass panels measure nearly exactly 1" thick...I made "picture frames"on my walls using long pieces of 1.5" x 8' x .75" boards with 1.5"x 1.5" x 1/4" cut spacers\\shims. Instead of running the 1/4" material the full length behind the .75" material, I only placed the spacers\\shims where I nailed gunned through the into a stud. On pieces that didn't have a stud to nail, I put in drywall anchors and used screws. This required drilling through holes in the wood, but it worked almost as well as the nail gun nails into studs.

After the frame was up and the fiberglass installed, I then stretched GOM from frame piece to frame piece over the fiberglass. For each wall, I ran two pieces of GOM the of the length of the wall room. (Two pieces since GOM is 64" wide and my room was 96" high.) This left GOM "seams" at each frame piece which I covered with stained wood "chair rail" pieces, floor board, and crown molding.

Whatever you do...do NOT try and use cotton batting in places you don't have acoustical cotton. The batting will NEVER maintain the same "thickness" and your eye will ALWAYS know where the two materials are different. If I had to do it over, I would simply put drywall or particle board in the places I didn't use fiberglass to maintain the proper depth.
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post #1629 of 11856 Old 09-18-2006, 07:50 PM
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After calling a dozen+ "local" (within 100 miles) HVAC distributors, I've had no luck finding Linacoustic, Insulshield, etc. I do have a line on "Knauf Wall & Ceiling Liner M", but all I know of its properties are a 1.5 PCF and a NRC of 0.70.

Q1. Does anybody know if this is a suitable replacement for Linacoustic, 703, etc?

Q2. I've asked this dealer (as well as an e-mail to Knauf) what the Absorption Coefficients are for this material, as they aren't posted on the website. Of course, I don't know what do with this info when/if I get it. If I compare this data with comparable product data from http://www.bobgolds.com/AbsorptionCoefficients.htm, what tolerances are acceptable?

Thanks,
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post #1630 of 11856 Old 09-19-2006, 06:47 AM
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Hi James,

This is a tough call, because "suitable replacement" is hard to define. Most home theaters are (unfortunately) not designed to particular acoustical specifications. If I had designed and modeled a theater which required 2.25-3 pcf fiberglass, then 1.5 pcf would not be a suitable replacement.

Knauf's published absorption coefficients for this product are:

1.5 PCF 1" .18 .36 .59 .86 .95 .90

However, there is also a 2 pcf version with the following published absorption coeffficients:

2.0 PCF 1" .25 .35 .69 .89 .96 1.01

If your supplier can get the 2 pcf Knauf, I would use that instead. BTW, I don't rely on any single set of reported absorption coefficient measurements, since the reverberation room method has a pretty high margin of error. We have our own proprietary absorption coefficent database.

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post #1631 of 11856 Old 09-22-2006, 10:19 AM
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This is a question on the general principles of sound absorbtion, as I find I'm usually better able to understand real-world situations by first understanding the underlying scientific/physical mechanisms in play.

I'm wondering what is it about a material that makes it able to absorb a sound wave. It is known that 3" of pink fiberglass will pretty much absorb all of the sound at 10,000 Hz, but very little at 40 Hz. Why is this? What is it about the higher frequencies that make them able to be absorbed by insulation (or ANY material for that matter), and what is it about lower frequency sound that enables it to pass right through insulation without being touched?

These questions drive a desire for me to understand how to effectively treat a room for appropriate bass atenuation, and it comes as a result of a conversation I has with a dealer about room accoustics. Intuitively it made sense to me that a thicker material might be needed in order to absorb a longer wavelength (lower frequency) sound wave. So I was asking the dealer how much of a corner will a bass trap that's effective down to 50Hz take up? Like, will a triangle of pink insulation in the corner that spans a distance 18" out from the corner on each wall work?

The response I got is that sound absorbtion is much more dependent on mass than shape. So I got to wondering, could a 1 or 2" thick panel in the corner work as a bass trap if it was heavy enough? Can a 1" thick layer of sand (being very heavy) over a wall effectively absorb the low frequency sound hitting the wall? One of the heaviest foams I know of comes in the form of those blue foldout gymnastic mats... Maybe that material could make an effective low frequency absorbtion material...
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post #1632 of 11856 Old 09-22-2006, 11:01 AM
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Quote:
Originally Posted by Felgar View Post

I'm wondering what is it about a material that makes it able to absorb a sound wave. It is known that 3" of pink fiberglass will pretty much absorb all of the sound at 10,000 Hz, but very little at 40 Hz. Why is this?

There's more going on than the following description, but a simple explanation of absorption in porous material like fiberglass involves material thickness vs. wavelength. A material can only absorb sound if its depth is at least some reasonable fraction of a wavelength (like 1/8).

To get the wavelength in feet of any frequency, divide 1000 (speed of sound) by that frequency. So a 10,000 Hz wave has a wavelength of 1/10 ft ~= 1". 3" fiberglass is 3 wavelengths thick, and has no trouble absorbing this sound.

A 40 Hz wave has a wavelength of around 1000/40 = 25 feet = 300". The 3" fiberglass is only 1/100 of this wavelength, so it can do very little to absorb it.

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post #1633 of 11856 Old 09-22-2006, 11:28 AM
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Ok, that's exactly what I was wondering, and is also more in line with what my intuition was telling me originally... So lets say pink fiberglass can absorb frequencies of wavelengths up to about 8 times its thickness. So 3" might absorb well until about a 24" wavelength, or a frequency of 500 Hz. Would putting it on a wall work to double the effectiveness because the wave has to travel in and then back out after reflecting off the wall?

And can I assume that different materials will have different properties for effectiveness? Like, maybe the special insulation that's supposed to improve sound isolation may be able to absorb 10 or 12 times it's thickness instead of just 8?

And of course, is there any documentation that can be found on what materials work in what way?
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post #1634 of 11856 Old 09-22-2006, 01:44 PM
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Felgar,

To expand on Terry's explanation a bit:

The reason fiberglass absorbs best when a substantial portion of the wave fits within its thickness is because "porous" absorbers like fiberglass act on wave velocity. If you picture a sine wave in your mind, the portion where the wave is all the way at the top or bottom has more velocity (greater absolute level) than the portions near the center.

When a sound wave hits a room boundary the velocity is zero at that point, and rises as you get farther from the wall. At 100 Hz the wave achieves maximum velocity 34 inches away from the wall. At 1000 Hz the maximum is only 3.4 inches away. So fiberglass four inches thick appied to the wall will absorb 1000 Hz completely, but at 100 Hz the wave has barely gotten started, so to speak, so there's not much to absorb.

I think of porous absorbers as working sort of like a person trying to run through chest-high quicksand. If you don't try to walk too fast you can slog your way through it. But the faster you try to go, the more friction is created and the harder it is to proceed. Fiberglass (and acoustic foam and rock wool etc) all work by creating the same sort of friction, but in this case for sound waves.

> Would putting it on a wall work to double the effectiveness because the wave has to travel in and then back out after reflecting off the wall? <<br />
No. If you think about it, travelling through the material twice happens no matter how thick the material is or how far it's spaced away from the wall.

--Ethan
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post #1635 of 11856 Old 09-22-2006, 06:43 PM
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Ethan -

You could have saved some typing by quoting your own excellent article on the subject . Also, I found this article describing different types of absorbers, including porous.

Let me add a couple of speculations:

- I think you actually mean the particle velocity not the wave velocity, no ? The distinction being that the wave velocity is an intrinsic property of the propagation medium (it is a constant for a given material, temperature, etc) whereas the particle velocity is extrinsic (it varies in space and time) and describes how fast the air molecules themselves are moving.

- The problem with using a high density material is getting the sound to get into it in the first place. If it is too dense then too much of the sound will be reflected from the front surface and it won't matter how good an absorber it is.

I say speculation above because I am still in the process of digesting all the information in this thread myself.

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post #1636 of 11856 Old 09-23-2006, 06:09 AM
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Quote:
Originally Posted by kiwishred View Post

Let me add a couple of speculations:

- I think you actually mean the particle velocity not the wave velocity, no ? The distinction being that the wave velocity is an intrinsic property of the propagation medium (it is a constant for a given material, temperature, etc) whereas the particle velocity is extrinsic (it varies in space and time) and describes how fast the air molecules themselves are moving.

- The problem with using a high density material is getting the sound to get into it in the first place. If it is too dense then too much of the sound will be reflected from the front surface and it won't matter how good an absorber it is.

I say speculation above because I am still in the process of digesting all the information in this thread myself.

Brent

Correct, Brent! Higher density means higher flow resistance. The exact relationship depends on the material, in ways that we don't fully understand but can at least measure. For a material of a certain flow resistance, there is an optimum absorption thickness for a given frequency. Much thinner, and the porous absorber is not very effective due to wavelength size, as you already know. Much thicker, and the higher flow resistance (higher acoustical impedance) kicks in and prevents some of the wave from getting into the material to be absorbed.

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post #1637 of 11856 Old 09-23-2006, 06:46 AM
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Quote:
Originally Posted by Terry Montlick View Post

Correct, Brent! Higher density means higher flow resistance.

Eureka! That's right, we are talking about moving air molecules! Slow the air molecules, attenuate the frequency.

"The future is already here, it's just not evenly distributed." W. Gibson

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post #1638 of 11856 Old 09-24-2006, 03:56 PM
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Wow, really long thread, and I am trying to get through it all.

My room is 22' long and 18.5 wide, but it isn't square. At the 16' mark the rest of the rear is "half a hexagon", like the top half of a stop sign.

What do you do for a room like that. I have 1" channels built in to all the moulding and around the windows to put in panels or whatever, just have no clue how/what I need.

I think it sounds way good now, what will putting the extra money in do for me.

If it matters, I have Martin Logan SL3's up front and Aerius in the rear.
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post #1639 of 11856 Old 09-24-2006, 10:31 PM
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My room is identically shaped but slightly smaller.

For my room, I built a 10" riser in the "back\\square" portion of the room and built my screen (with curtians) in front of the half hexagon making the room appear square. This allowed me to put all my front speakers behind the screen. My sound stage is nearly perfect as I have all speakers at the exact same height at midpoint of the screen.

Acoustically I treated the room as follows:

Entire hexagon area and side walls up to the first row of seats: fully lined with 1" OC703. I lined the ceiling behind the screen as well.

Side walls after first row of seats and rear wall: 50% coverage with 1" OC703. The other 50% areas I used cotton batting to fill in the 1" space. I SERIOUSLY regret this however as the batting pushes against the GOM covering it producing a "bubble" effect. Even stretching the material as best I could did not remove this bubble and some minors puckers.

Bass traps: I put "super chunk" style traps (made of OC703) in the ceiling to wall corners behind the screen. I also built a very large, free standing frame to hold 12, OC705 sheets. (2 sections at 6" thick each...held 4" off the wall). The back piece of 705 has FSK facing the front wall.

I also put a 8' long "super chuck" style trap behind the rear row along the back wall\\ riser floor corner.

All in all, I would have liked to have a little more bass trapping in the back corners (especially the tri-corners), but I just didn't like how they looked. The bass response is actually nearly perfect in the back row. There is a slight room mode in the front row giving a hump around 100hz. Its not horrible, and I may look into controlling it with a BFD.

I can share picks of how I did all the acoustic stuff if you wish...just let me know.

Hope this helps.
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post #1640 of 11856 Old 09-25-2006, 06:23 AM
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Quote:
Originally Posted by Caspyr View Post

Wow, really long thread, and I am trying to get through it all.

My room is 22' long and 18.5 wide, but it isn't square. At the 16' mark the rest of the rear is "half a hexagon", like the top half of a stop sign.

What do you do for a room like that. I have 1" channels built in to all the moulding and around the windows to put in panels or whatever, just have no clue how/what I need.

I think it sounds way good now, what will putting the extra money in do for me.

If it matters, I have Martin Logan SL3's up front and Aerius in the rear.

Hopefully, you'll get the brians, the terrys and the ethans to comment, but from my limited knowledge I'd say that there's no way to "predict" how it will sound, and therefore know what treatments to install. All of the spreadsheets and calcs I've seen are for rectangles and - horrors! - squares. More sophisticated acoustical modeling is no doubt available, but not as a free or relatively free download. The best recommendation may be for you to consult/hire a professional to analyze the room. (This advice is from someone who's a devout DIY'er.)

As for the windows, I will go out on a limb and say that the more complete your light control is, the better it will be for the video part of the theater.

"The future is already here, it's just not evenly distributed." W. Gibson

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post #1641 of 11856 Old 09-25-2006, 08:49 AM
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I can share picks of how I did all the acoustic stuff if you wish...just let me know.

I would love to see any pics. The hex part of my room is the back, two Berk's on a platform in the center of it. I would have to totally rebuild to put the screen back there, and I think losing that extra 5 feet would make th room to small for my 123" screen.

Quote:


As for the windows, I will go out on a limb and say that the more complete your light control is, the better it will be for the video part of the theater.

There are five total windows, double wide on the side walls, and singles on each of the "hex" walls, all completley light controlled. Black fabric covered panels that fit fit snugly into the window frames. It's wired for curtains, but I like the look of the panels and they work great.

Light control is "perfect" right now, as a flat black room, you are blind in the room if the lights are out and the projector isn't on.
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post #1642 of 11856 Old 09-25-2006, 09:46 AM
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Caspyr,

> What do you do for a room like that. <<br />
You'd place bass traps in the corner sections as usual. Even if the angles aren't 90 degrees, bass waves still collect there. In fact, a concave shape is bad generally (focuses sound back at you), so you'll need broadband absorption there, not just bass trapping.

> I think it sounds way good now, what will putting the extra money in do for me. <<br />
Make it sound way better.

Seriously, most people who have never experienced the improvement acoustic treatment makes are blown away when they hear if for the first time. Now all of a sudden you can actually hear the pitch and articulation of every bass note. And when the first reflection points are treated the sound opens up to be much wider and, even more important, much clearer.

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post #1643 of 11856 Old 09-25-2006, 10:40 AM
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You'd place bass traps in the corner sections as usual.

Either corner in front would be tough to do anything. Looking at the screen the left side is the entry. The door is about 2 ft back from the screen edge. The other side is the audio cabinet doors.
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post #1644 of 11856 Old 09-25-2006, 12:41 PM
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Quote:
Originally Posted by Caspyr View Post

Light control is "perfect" right now, as a flat black room, you are blind in the room if the lights are out and the projector isn't on.

I'd say you've got that taken care of.

"The future is already here, it's just not evenly distributed." W. Gibson

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post #1645 of 11856 Old 09-26-2006, 01:18 PM
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Caspyr,

> Either corner in front would be tough to do anything. <<br />
Every (rectangular) room has 12 corners, including where walls meet other walls, plus where they meet the ceiling and floor.

--Ethan
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post #1646 of 11856 Old 09-26-2006, 01:55 PM
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Quote:
Originally Posted by Ethan Winer View Post

Caspyr,

> Either corner in front would be tough to do anything. <<br />
Every (rectangular) room has 12 corners, including where walls meet other walls, plus where they meet the ceiling and floor.

--Ethan

I *think* he was referring to "corner" as in "Go sit in the corner." Gravity being what it it, it tends to limit the number of corners one can actually sit in.

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post #1647 of 11856 Old 09-27-2006, 12:31 PM
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Quote:
Originally Posted by pepar View Post

I *think* he was referring to "corner" as in "Go sit in the corner." Gravity being what it it, it tends to limit the number of corners one can actually sit in.

Huh? Here's what I read:

"You'd place bass traps in the corner sections as usual."

"Either corner in front would be tough to do anything."

What did you read?
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post #1648 of 11856 Old 09-27-2006, 12:35 PM
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Quote:
Originally Posted by Ethan Winer View Post

Huh? Here's what I read:

"You'd place bass traps in the corner sections as usual."

"Either corner in front would be tough to do anything."

What did you read?

HEY, look over there!

"The future is already here, it's just not evenly distributed." W. Gibson

"I like the future, I'm in it." F. Theater
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post #1649 of 11856 Old 09-27-2006, 01:30 PM
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What I am trying to say is that at the left of the screen, there is no corner. Currently about 2' behind the sceen on the left side is a curtain that is the entrance. The left side wall heads straight down past the screen, then a curtain, then the hallway to the rec room/bar area.

Plus, I was thinking of "corners" being where walls meet. I was not thinking top corner/bottom corner where the walls meet ceiling/floor. I am a 1k brain, to me a room has 4 corners not 8

That's why I need places like this, gotta love the amount of information you can get from all you guys. Thanks !!
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post #1650 of 11856 Old 09-28-2006, 12:05 AM
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In the spirit of giving back something to this great thread, here are construction details for my absorbers (ideas borrowed from here and Bob Golds).

I chose to go with an open frame construction for reasons of (a) light weight, (b) open sides for greater total area of fiberglass exposed, & (c) ease of custom-tailoring thickness. For room aesthetics reasons, total thickness was limited to 3. I used a stack of three 1 sheets of 3 pfc rigid fiberglass in each frame. However, the construction method would also easily allow 2 of fiberglass with a 1 gap to wall (or any other desired combination). While the third sheet might not increase acoustic absorption much, it apparently does no harm and only adds a relatively small incremental cost ($3.52).


Top and bottom frames constructed from 3/4*1/2 hemlock and clamped square. Top frame used shoe moulding and mitred corners to provide a rounded front edge. Bottom frame used rectangular molding and butted corner joints.


Nailing and gluing the top and bottom frames together. Pillars are 2 1/8 for 3 1/8 total thickness.


Drilled guide holes for the #17 * 1 wire nails used to tack the mitred top frame pieces onto 3/4" square pillars using a cut-off #18 * 1 nail (an old trick my dad taught me ).


Front views of finished raw frame (left) and completed, covered, frame (right).


Speaker cloth stretched and stapled onto frame following instructions here. Cloth is actually a lot darker than it appears in this photograph. Yellow stuff is the "rigid fiberglass" acoustic absorber.


BOM (per absorber):
- 4' * 2' * 1 Johns Manville 3 pfc rigid fiberglass sheets. Source: E J Bartells, 700 Powell Ave SW, Renton, WA. 425-228-8807. Cost: 3 sheets @ $3.52 = $10.56.
- 3/4 * 1/2 hemlock 1/4 rounded shoe for front frame. Source: Lowes. Cost: 2 8' lengths at $3.36 each = $6.72.
- 3/4 * 1/2 hemlock rectangle (for back frame and 6 mid posts). Source: Lowes. Cost: 2 8' lengths at $3.36 each = $6.72.
- 3/4 * 3/4 hemlock square (for pillars). Source Dunlumber. Cost 2' @ 85 c/foot = $1.70.
- 60 wide black speaker cloth. Source Jo-Ann. Cost: 1 yd @ $6.80/yd = $6.80.
- miscellaneous (nails, glue, sandpaper, etc) $0.50.

Total cost per absorber = $33. This includes some waste as could only buy the molding in 8' lengths.

Total time per absorber (once in production mode & not counting time for glue to dry !) approx 2 - 3 hrs. So, I probably spent less time making them than reading about how to make them plus the multiple trips to various stores buy materials .


Observations & tips:

- Top and bottom frames were laid out on a sheet of 3/4 MDF to provide a nice flat surface as the glue set. Didn't have any significant problems with warping.

- I mitred the top frames and made them out of shoe moulding to provide a nice rounded appearance. The rounding was hard to actually notice once the black grill cloth was on the frame. Despite this, and the fact that this approach requires twice as many cuts, I still think it was worth the extra effort.

- All cuts were made by hand using a 16 point dovetail saw and mitre box. All lengths were marked using pre-cut wood pieces rather than a tape measure. This really helped speed & consistency.

- Added 1/4 clearance for fiberglass so that total outside dimensions were 49 3/4 * 25 3/4 * 3 1/8.

- Didn't bother painting the wood. The bare frames and the (yellow) fiberglass are not visible through the speaker cloth.

- I was pleased with the way the finished absorbers turns out. Not too obvious that they are DIY. The 3/4" * 1/2" hemlock provided a nice balance between rigidity and weight (although something a bit beefier would be required for frames any larger than these).


Hope the above is useful to other would-be DIY'ers and thanks to other contributors to this thread

Brent
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