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DIY absorption panels

Jimmy,

Hard to believe, but compressing 8" of loosely woven fiberglass batting down to 2" will not give you the same absorption properties of a 2" thick rigid fiberglass panel like OC 703 or 3" or Roxul Safe n Sound. I know because I started off doing almost exactly what you did (I took two layers of R13 and compressed it down to 3") but just couldn't stand reading everywhere that the 703 would be better so I broke down and picked up some 703 and it did indeed prove to be a more effective absorber. This was most evident in the lower midrange/upper bass regions. I went from huge, 8' x 2.5' panels built in a very similar fashion to yours to 4'x2' panels of 2" 703 and midrange clarity and upper bass definition improved over the big ass R13 panels I made. HF and upper mid attenuation were about the same with either panel. I also liked the fact that the smaller panels allowed me keep more of the wall untreated so I kept the room somewhat "live". How many and where the panels should go is room dependent and experimentation goes a long way. The initial treatments should almost always be what you've pretty much done....panels on front wall behind speakers and at midpoint on wall between speakers, at sidewall and ceiling first reflection and on rear wall behind listening position. From there I think methodology wrt to additional treatment becomes empirical.

The airspace behind the panel is beneficial because as the sound waves that are of sufficient length to penetrated the asborber reflect off of the boundary layer (wall) behind the absorber they will come back into the room and get attenuated by the absorber the second time they attempt to pass through. This applies almost exclusively to sound waves in the mid and upper bass regions. A thicker panel on the wall will have pretty much the same effect as a panel half as thick with airspace behind it. A thicker panel with more airspace behind it will absorb frequencies lower in bandwidth than if it were thinner, on the wall, or both.

If you're going to go to put the effort into making more panels I strongly encourage you to get on Lowes website and order a bale of Roxul SafenSound. It's easy, cheap ($50 for 8 2'x4'x3" panels), and it only took about a week for the order to arrive at the nearby Lowe's. I firmly believe $50 spent on a bale of SafenSound and a day out in the garage spent building eight panels will yield an improvement in SQ for virtually ANY system (that resides in an untreated room) far above what a switch to high dollar, boutique cables or interconnects can yield.

- Michael
 
I see Mike. I still can't understand about the compression of the filling. If the standard type of absorption panels are nothing more than compressed fiberglass, how can I compressing the fiberglass myself be a wrong mistake? The wave will still be absorbed by the same material and in the same consistency - I believe? My panels have a pegboard back, from which I can easily hang on the wall, so I am not benefiting from the wave bouncing back from the wall and being absorbed twice - I get that. I may make a nice wood frame that will allow me to raise it a couple of inches if you feel it's necessary. Also, I built my panels with 1x3 boards, pegboard for the rear, squeeze in 8" of soundproofing fiberglass, then I used very thin chicken wire to cover and hold the fiberglass inside the 3" box. Very effective, and it gives the panel a nice "bowing" shape where the center of the panel is raised an extra 1" from the sides. I then covered the inside with a cotton cloth to keep the fiberglass inside, then I covered the entire frame with a nice colored cloth. They look great, are light weight, and are easy to hang. Plus, I got the effect I was after - taming mid bass hump, taming very high end, and thus vocals improved dramatically because of this. The entire room also sounds "quieter" for a lack of a better word.

Because of the placement of my speakers and the screen, the panels are not directly behind the speakers. Compromises :) I can accept that. But I also feel that the area underneath the speakers must also be absorbed if this is to be done right. The center speaker must get the same treatment too, but this time aesthetics have to come into play. This panel is being built at 1' tall and 10 feet wide - basically the length of the screen. The total panels, I think, will look great (again aesthetics) and will absorb most waves behind the speakers. This *should* allow only the direct 1st waves to hit the listening area and bring an even higher level of speaker accuracy. The last 2x4 panel to the right of the rack is the first reflection point on that wall and is a given. I have a large sofa and heavy drapes on the rear of the room, so I think I don't have to do anything there.

Impressions? Suggestions? Do you need pics of the room?

JN

Rear of the room:

IMAG0401.jpg
 
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No suggestions but I will say the room looks good.

I would not worry about the type of insulation. My panels are built with 1x4's ripped down to 1 3/4" and stuffed with R13. They took my basement room with thin paneling walls from totally unlistenable to very good. I do have about a dozen 2'x4' panels in the 11'x20' room though. Still need to do some bigger bass traps as the bass can get a little wooly sometimes.

Actually, I do have a suggestion. One of the keys to getting my room dialed in was to put some panels on the ceiling. I have one 2x4 panel about mid way between the speakers and the listening position and 2 right over my chair. While I'm not sure adding the second one right over head made much difference, the first 2 were very important. I do have low a ceiling height of about 6' 8".
 
Jimmy,

I can't give you an explanation with scientific data other than what can be extrapolated from the charts that show absorption coefficients for different types of insulation and building materials. My guess is that we aren't capable of compressing the fibers to the level of density a machine can. Having seen the cross section of 703 and Roxul there is no doubt the manufacturer can pack a hell of a lot more fibers in 2" than you or I can by hand.

Now, all that having been said, if you're happy with what you're hearing then I wouldn't worry about it. I'm not saying the R13 panels are not effective. But I will say, as I said before, I liked the results I heard in my listening space when going from R13 panels to 703 and/or Roxul panels. The soundstage became better defined, mid bass and lower mid better defined and clearer, and there was just a better overall presentation of the music. My good friend and listening buddy made the same observation in a back to back test with him listening and me moving panels. Two weeks later he was building 703 panels to replace the R13 panels he had built about the same time I did mine. But again, if you like the results you're hearing, have a plan you believe will improve upon what you've already done, and don't really have a desire to deviate from those plans for reasons that are unique to you, then press on soldier and enjoy the fruits of your efforts. One thing is for sure....when it comes to room treatments something is better than nothing. I'd say you're well beyond just "something". A good percentage of this audio game is mental anyway. If you're predisposed to think something is gonna sound like **** before you've even tried it, it probably will once you do.....and visa-versa. :D
 
Very well said. Thanks Mike. I will stick with my plan since I already have some great response from these panels. On my downstairs 2 channel system I will be going your way since it will be a clean start.

Again, thanks a bunch for your help and effort on this subject. It sure has taught me a few things.

JN
 
This type of question comes up often. I need to do an infographic that explains, simply, how absorption works.

A sound absorber doesn't work to absorb sound in the way that a paper towel absorbs water. It doesn't trap sound, we just use that terminology for convenience. An absorber actually acts more like a dashpot (shock absorber), in that it uses friction and has greater effect at higher velocities.

What? By velocity I mean the speed of the air molecules moving through it. The absorber works through friction. It creates a "torturous path" for the air molecules to pass through as they move back and forth with the sound wave. Every time the air molecules have to pass through the absorber they lose energy due to friction. The faster they try to move through, the more energy they lose.

So this should answer both questions.

Fiberglass that's meant to absorb sound is spun to an ideal density for slowing sound waves down. If you compress it too much, it becomes less absorptive and more reflective, meaning sound waves will bounce off the surface of the absorber rather than penetrate inside and get sucked up.

It also tells you why you want airspace behind an absorber to make it more effective against lower frequencies. It's not because there's air behind the absorber, it's because the outer boundary of the absorber has been moved further away from the wall. A 4" absorber should be just as (if not more) absorptive than a 2" absorber with a 2" air space behind it, because in both cases the boundary of the absorber is 2" from the wall. So why is further from the wall better?

Imagine the air molecules that are sitting right next to a wall. They're stuck there, because if they moved away from the wall they'd leave a vacuum behind them. When a sound wave comes over into that wall, the neighboring air molecules are pushed into the ones near the wall, causing an increase in pressure at that location. Those air molecules push back and the other molecules move away.

At the wall, the velocity of the air molecules is always 0. Therefore maximum velocity of air molecules due to the sound wave occurs 1/4 of a wavelength away from the wall. How far is 1/4 wavelength? Depends on the frequency of interest. The lower the frequency the longer the wavelength.

That is why thicker absorbers work better to absorb lower frequency sound. The maximum velocity of a low frequency sound wave occurs further from the wall, so you need your absorber to be further from the wall to act on it.

I really do need to do some kind of illustration or animation. This is actually a really easy concept to understand, it just takes a lot of words to explain it.
 
Good explanation. It's very clear now. OK. So back to my panels. The material I bought is "sound proofing fiberglass type insulation" from Lowes. It is 8 inches thick and is meant to be used inside walls between studs. So that space is 3 inches deep. To me that means that since it is designed to work in walls 3 inches deep then compressing it down is necessary, per manufactures instructions. I did not get the pink Owens fiberglass insulation. This one is white and it says it's specifically for sound proofing.

What do you think? Also, I just can't make panels any thicker than what they are now. Any bigger and they will become an eyesore.

JN
 
That type of insulation is, as you know, meant to go in wall cavities. Its function is to absorb sound inside the wall cavity to prevent it from transferring room to room. It's similar to the job that surface treatment fiberglass does, but not identical.

I don't know how well that type of insulation would do as a wall treatment if you squished it down to less than half of its thickness. I've never seen data for that and I doubt any lab has ever bothered to measure it. It might work well, it might not. I don't know.

I'd say take it back if you can and order some semi-rigid fiberglass that's made for this purpose, such as Owens Corning 703 (or any of the equivalents from OC's competitors). If you can't take it back, then give it a try, I guess. I don't know what other options you have.
 
JimmyNeutron said:
Good explanation. It's very clear now. OK. So back to my panels. The material I bought is "sound proofing fiberglass type insulation" from Lowes. It is 8 inches thick and is meant to be used inside walls between studs. So that space is 3 inches deep. To me that means that since it is designed to work in walls 3 inches deep then compressing it down is necessary, per manufactures instructions. I did not get the pink Owens fiberglass insulation. This one is white and it says it's specifically for sound proofing.

What do you think? Also, I just can't make panels any thicker than what they are now. Any bigger and they will become an eyesore.

JN
Click to expand...

If this is still bothering you, Jimmy, then you should consider this. The manufacturer of the R13 fiberglass you purchased ( white JM or pink Owens or whoever's) can call their fiberglass batting "sound proofing" so long as it shows some ability to perform in that capacity. What you don't get from them (that I'm aware of) is technical data that shows just how effective a sound absorber their fiberglass batting is at specific frequencies. What they do give you wrt a performance value is it's thermal insulation value....the "R" value. That is specifically what that material is designed to do....insulate against the transfer of heat and the "13" (or whatever value you may have purchased) is the numerical value that represents it's level of performance in that category. The more benefits their product can offer the more marketable it becomes. Does it reduce the transfer of sound/noise through walls? Sure it does, and because of this they will absolutely use that as a marketing tag. It's like saying a hoodie from Wal-Mart will keep you warm in cold weather. It's true, it will keep you warm compared to if you don't have a jacket on at all. But what kind of performance will the hoodie offer compared to say a Montbell Thermawrap jacket at temps around or below freezing and winds in excess of 20mph? The wind will cut right through that hoodie, but the performance jacket will do a exponentially greater job at blocking out the cold. See where I'm goin?

And consider this as well. The fiberglass batting is considerably less expensive than rigid fiberglass or rockwool. The people who manufacture and sell purpose built absorption panels for commercial applications put a priority on performance and cost (in that order) when developing their product. First and foremost they need the panel to actually do what they say it will do....absorb sound waves. Secondary to that is it needs to be a relatively affordable material so they can offer it at a price point that is reasonable to the consumer. I know of not one single manufacturer of commercial grade absorption panels who uses fiberglass batting as their absorption medium. Rigid fiberglass and rockwool are the only materials I've ever seen used. They could certainly offer a much less expensive panel if they used fiberglass batting, but they have determined through extensive testing that the tradeoff in performance is not worth the savings. For a little more money the rigid fiberglass and/or rockwool offers a proportionally greater return in performance.

And you won't have to have panels that are any thicker than what you have now to get better performance (unless you keep using fiberglass batting). Like I said before, 2" of OC 703 or 3" or Roxul SafenSound rockwool is all you need to improve upon the batting you've been using.

- Michael
 
I'll get yelled at for resurrecting this great ole' thread. Owens Corning has 703 and 705 rigid fiberglass panels. Has anyone experimented with them for effectiveness? Is one more effective than the other? And then there's ROXUL ROCKWOOL mineral wool Board 60. Given the price difference, it's worth knowing.
 
All types of fiberglass and rockwool can be used, with different levels of absorption. The denser the material, the more sturdy it is, making the construction easier.
This link gives some comparative absorption results by frequency, material and panel thickness.
http://www.bobgolds.com/AbsorptionCoefficients.htm
I would not consider a 1 or 2" panel - the absorption is little and it just consumes space that could be used more effectively. I would aim for 4" panel, which typically means two layers of material per panel.
Ideally, I would also mount the panels at a distance from the wall. The worst case scenario would be to hang them touching the wall. You would be gaining a couple of inches of space but wasting more than 50% of effectiveness. Best to mount them at a distance of 4" from the wall.

The last few rows in the first table show measurements at 16" distance from the wall. At that distance, it becomes clear that the differences between different densities become insignificant. Even, with wall mount measurements are not that big. So you can deduce that at a 4" distance, you don't really care about density of material. Except when comparing prices, and it's worth noting price difference between them.

The same link has results for Roxul rockwool materials and others. I prefer rockwool (mineral wool) because it's cheaper (at least when I went shopping) and doesn't have glass fibers that are irritating to the touch and breathing.
 
Shalmaneser said:
I'll get yelled at for resurrecting this great ole' thread. Owens Corning has 703 and 705 rigid fiberglass panels. Has anyone experimented with them for effectiveness? Is one more effective than the other? And then there's ROXUL ROCKWOOL mineral wool Board 60. Given the price difference, it's worth knowing.
Click to expand...

I use both 703 and rock wool. 2” 703 will perform like 3” rock wool, so if I needed a thinner panel (such as for the portable panels I built) or maximum absorption (super chunk bass traps I built), I chose 703. Everywhere else was rock wool because of cost and availability.

I agree with most of what Sasi says, but I absolutely would not forego creating a room treatment scenario just because you’re not using panels at least 4” thick. I’ve realized easily audible improvements in fidelity by using fiberglass panels as thin as 1” to eliminate echos in the room and tame early reflections. Those 1” (or even 2”) panels don’t do much in the way of absorbing low frequency energy if mounted directly on wall, but eliminating or reducing room echos and taming early reflections creates significant improvements in terms of room acoustic response and in turn fidelity of music playback.

Study that absorption coefficient chart closely...it removes a lot of guess work and is a fantastic tool for creating an effective room acoustic treatment scenario.

Michael
 
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