DIY absorbers

[bwolf]

I read the article posted about the DIY absorbers. Very informative. My question is this...what if I don't use the rigid fiberglass in the absober? Does this render the absorber useless? I have access to thick upholstery grade fabric to cover the absorber and inside the 1-2" thick frame, I would use something like cotton batting you can find at any craft store. I'm trying to save a little money as I'm building my home theater and was curious if I could skip the cost and trouble of locating and purchasing rigid fiberglass for the project. Just curious if my poor man's absorber would do any good. Thanks.

 

[warnerwh]

Yes an absorber with just fiberglass packed in with some batting over it to keep it in will help significantly. The main difference being that the rigid fiberglass would absorb lower frequencies. If you want to do them like you say then do so as I believe you'll be pleased with the results.

 

[bpape]

You can even compress normal wall insulation into something of a similar density of 703 board. But, by the time you figure out the cost of how much you'll need and all the hassle of compressing it, trying to keep it in that state so it looks decent, etc., $10 for a 2x4' piece of 2" 703 doesn't seem out of line at all.

The other option is to just tolerate them being thicker. In order to do broadband (reach down into the bass - say 50Hz up), there are 3 things that have an effect:

1. Thickness of the absorber
2. Density of the absorber
3. Position of the front of the panel in relation to a room boundary or boundaries.

If you don't want the density, you can compensate with more thickness and different positioning. 6-12" of fluffy fiberglass actually makes quite a good bass/broadband absorber. Very smooth absorbtion curve of close to 1 from about 60Hz up.

Now the question becomes do you have/will you tolerate 6-12" thick panels. Or, is it better to get some 2" 3lb or 6 lb material and either double it up or mount it 2" off the wall.

 

[bwolf]

Would an absorber with no fiberglass in it do anything? I was just curious if I only used cotton batting inside if it would do anyting for the acoustics. If I do resort back to the condensed fiberglass board, how much does that cost? I was thinking it was somewhat expensive, but maybe I'm wrong...

 

[bpape]

Most batting has such little gas resistivity that it would take a LOT of thickness and a lot of compression to make it work into the bass.

The rigid fiberglass isn't all that bad price wise. There is also Acoustical Cotton if you don't want the hassles and itch of fiberglasss - but it's a bit more expensive - but a dream to work with.

The last option is rockwool. In many places, this is the least expensive option. I'll warn you up front though that it's by far the nastiest to work with and sheds at the drop of a hat when you're working with it.

If you're extremely budget conscious, and want something that's relatively easy to deal with, I'd go with OC 703 board or an equivalent.

 

[WmAx]

Most batting has such little gas resistivity that it would take a LOT of thickness and a lot of compression to make it work into the bass.

The rigid fiberglass isn't all that bad price wise. There is also Acoustical Cotton if you don't want the hassles and itch of fiberglasss - but it's a bit more expensive - but a dream to work with.

The last option is rockwool. In many places, this is the least expensive option. I'll warn you up front though that it's by far the nastiest to work with and sheds at the drop of a hat when you're working with it.

If you're extremely budget conscious, and want something that's relatively easy to deal with, I'd go with OC 703 board or an equivalent.

Additional possible alternative: cellulose wall/ceiling insulation(the kind that you use a blower to install). However, I've never seen this material measured in this application. Also, it might be tricky to get good looking panels with minimal labor as compared to the rigid fiberglass.

The rigid fiberglass would definately be the easiest thing to work with for this application.

Just an idea...

-Chris

 

[Glenn Kuras]

I can not stress enough how much better your room will sound if the acoustics are done correctly. I can send you many emails from people that have never understood how good there room could sound by working with the acoustics. Dollar for dollar acoustics is your best investment for your room.
Bryan is right about the 703, but just make sure you make the panels 4" thick, straddling all corners and or spacing them off the wall 4". Really for bass trapping you want to us 705, but if money is short then that is that.


Glenn
GIK Acoustics
www.gikacoustics.com

 

[bpape]

You can certainly use 705 board for bass absorbtion. The extra density in small thicknesses will help. From a cost perspective, you can get 3 2" pieces of 703 for less than 2 2" pieces of 705. Personally, I'll take 6" of 703.

Remember, it's a combination of thickness, density, and location from the boundary that control effectiveness of a bass absorber. They all work together. As Glenn said, spacing can also help. 2" of 703 2" off the wall will do a better job in the bottom than 2" of 705 flat on the wall.

As for using 4" all around - well I'd want to run an analysis of the space and see what it really needs first. No sense in spending money where you don't need to. I mean if you can get by with 2" spaced off 2" on the reflection points, that frees up money for thicker absrobtion in the corners to do a better job in the bottom end.

 

[Glenn Kuras]

Bryan has a good point about shooting your room to see what is needed. In my experence I have found that most rooms need 6 to 7 4" panels. Covering all corners and spacing others off the wall. The rest of the panels can be 2" thick.
I think it is wise not to just spend money, but when it comes to your room sound your rooms acoustics are just as important as the equipment you have. Remember that when you spend thousands on your equipment and only hundreds on your room. I tell people all the time that the best equipment I have in my room are my acoustics.


Glenn
GIK Acoustics
www.gikacoustics.com

 

[jsw]

does anybody know if syrofoam 3/4 inch with some blackout material plus some cotton fabric would work to assorb echos.

 

[bpape]

Styrofoam is not an effective absorber for this application. The blackout material by itself would probably do something from 5kHz up but that's about it.

 

[AverageJoe]

Would anything be gained/lossed from covering an entire room with rigid fiberglass during construction? For example, could I get away with using 1" material instead of 2"? Would there be a benefit to using the fiberglass panels directly over the studs on my interior walls instead of over sheetrock, and then covering everything with cloth?
In my case, the room would be 14' x 16' with two exterior walls and large windows at one end (with drapes). Since construction hasn't started yet, I have a few options (but not enough knowledge, as usual).

 

[bpape]

You really don't want to do that for a few reasons.

1. Without the drywall layer, you'll lose that bit of bass absorbtion from an insulated drywall wall acting like a membrane. Also, you'd lose isolation.

2. The room would be VERY, VERY overly dead in the mids and highs and still boomy in the bottom.

3. There is a target decay time for each size room based on it's usage. Having all 1" or all ANYTHING covering all of the walls would result in way too low decay times in some areas and still being too high in others.

 

[AverageJoe]

Thanks very much. I appreciate the info.

One of the things I wanted to avoid was hanging small panels on the walls. It's just a personal preference, but I'd rather have something floor-to-ceiling or wall-to-wall, even if it's a narrow band around the room (or narrow rows up the walls). As you mentioned, the material isn't overly expensive. Any thoughts on this approach?

 

[bpape]

Then figure out what you really need and make 703 panels or 705 panels for those places. For the rest, just make plain panels out of wood and cover them the same as the absorbtive ones. Nobody but you will know they're different and you'll have the look you want.

 

[AverageJoe]

Picture me slapping my forehead saying, "Why didn't I think of that?"

Anyway, thanks for a great idea. That should work perfectly.

 

[Tomorrow]

Picture me slapping my forehead saying, "Why didn't I think of that?".

It runs in the neighborhood!

 

[fatroz]

I am looking to reduce flutter echoes and reverb in my living room. The room has a vaulted ceiling. So where the tv and speakers sit the ceiling is about eight feet high and point toward the back of the room where the ceiling is about eighteen feet high(roughly). The ceiling gradually goes up and peaks at the end of the room. Would it be best to put absorbers on the left and right walls next to the speakers or place them high up where a lot of the echoes are happening? Thanks if anyone can help.

 

[Ethan Winer]

> Would it be best to put absorbers on the left and right walls next to the speakers or place them high up where a lot of the echoes are happening? <

Aside from bass trapping, which you didn't mention but is important, absorption is needed at the first reflection points. In your room this is on the side walls, but more like halfway between you and the speakers versus right next to the speakers. You will also need absorption on any large areas of bare parallel surface, such as the upper portion of the walls. Clap your hands and listen for the characteristic "boing" sound.

The drawing below shows the approximate position of first reflection panels in a 2-channel room with parallel walls.

--Ethan

<img src="http://www.realtraps.com/art_room-setup1.gif">

 

[fatroz]

That helps a lot. It seems like initially putting absorbers between myself and the speakers is going to help the most. Unfortunatly with high ceilings you get large walls with lots of parallel surface area. Which, of course, creates a lot of reverb. For me, the difficulty is just getting up on those walls. I live in an apt so I don't own an extension ladder. Plus I can't permanently mount anything, so I dont want to go overboard, but still get some help since the room really needs it.