# Hard or Soft Covering for DIY Bass Traps



## adogand6kids

Most of the comercially available bass traps are fabric covered. Also, most of what I have found as examples of DIY bass traps are also fabric covered fiberglass. In my basement family room / music room / home theater I would like to install some floor to ceiling DIY bass traps in the wall/wall and wall/ceiling corners. However, as you may be able to guess from my log-in name, I have a fairly big and busy family. I am concerned that something sturdier than cloth covered fiberglass is needed. Instead of coverning the rigid fiberglass with cloth, could I use something like eighth inch plywood? From what I understand of sound wave physics (which isn't a lot) the bass waves should travel through the thin plywood and then the fiberglass behind the plywood would do its magic. The plywood in front of the fiberglass would be more resistant to child induced trauma and less likely to be chewed by the dogs. It would also add the convenience of being able to be finished to match the wall, making it less obtrusive (higher WAF). I understand that the plywood would keep these corner traps from being broad band - since it will reflect back the higher frequencies - but I am only looking for bass trapping from these. I have other things planned for first reflections. Does anyone have any input here? Am I correct in thinking this will work for a bass trap, or am I all wrong?


----------



## bpape

Covering fiberglass with plywood will kill the broadband nature of the treatments. You can build tuned membrane absorbers that way but they'll be way less efficient per square foot. Please post your room dimensions and usage and I'll try to help as much as possible.

Bryan


----------



## adogand6kids

_Covering fiberglass with plywood will kill the broadband nature of the treatments_

Can you give me an idea of what frequency range you think will still be absorbed by the plywood covered corner traps? Do you think that frequencies of 400 Hz and below will be absorbed?


----------



## bpape

When you use a plywood face, what you're doing is moving toward a membrane type absorber. The cavity behind it will need to be sealed air tight and the plywood will need to be at least 1/2" away from the fiberglass. What frequency range it will absorb is determined by the depth of the cavity and the mass of the panel on the front. You can expect about 2 octaves total and rolling off on either side.

Bryan


----------



## adogand6kids

Thank you Bryan for the feedback. I have read Ethan Winer's acoustics page with his description of the membrane or pressure based traps that he built for his home. That is what started me thinking about hard coverings for corner bass traps. His designs used more robust plywood (1/4 inch and 1/2 inch). I was wondering if using a thinner plywood would allow a corner trap design to still function as a velocity based absorber - but be more resistant to child induced damage. I was hoping to avoid the need to caulk and seal everything to try to make it airtight as is needed for the pressure based membrane absorber. My thought was that from 300 or 400 Hz and down, the sound waves would mostly pass right through the 1/8 inch plywood. I would then place 4 to 6 inches of rigid fiberglass behind the plywood, straddling the corner. Since Ethan's lab test found that FRK faced fiberglass absorbed low frequencies better than bare fiberglass, I was hoping for a similar type result from having "plywood faced" fiberglass. Of course, this is just speculation on my part and I don't know of anyone trying this. That is why I am throwing the idea out to you all. I was hoping for you and others with more real world knowledge than I have to look at the concept and let me know if the concept could work (i.e. will sound waves of low frequency pass through thin plywood) and also what sort of effect seems reasonable to expect (am I looking at absorption from 300 Hz and below, or 500 Hz and below...). Most importantly, if the whole thing sounds nuts - I want you folks to tell me now before I build anything.

Thanks again for the input and your patience with my imagination.


----------



## bpape

Anything that is going to be thick and rigid enough to avoid the poking fingers is going to have to act like a sealed membrane absorber. It will only work over a couple of octaves and will not be a velocity absorber but a pressure absorber. In order for it to be a broadband velocity absorber it must allow air in. If you're only exposing a tiny bit of it, you'll only get a tiny bit of absorbtion - sorry.

If you need to keep prying fingers out, may I suggest something more like steel window screen in front of the fiberglass. 

Bryan


----------



## adogand6kids

So, when Ethan Winer studied the FRK faced fiberglass, does that mean it was acting more like a membrane absorber - since air can't pass through the foil reinforced paper? Or, am I wrong and the FRK facing actually does allow air through? I was just making assumptions about it based on the pictures. I don't own and FRK faced fiberglass. The article I am referring to can be found at http://www.ethanwiner.com/density/density.html


----------



## Ethan Winer

adogand6kids said:


> So, when Ethan Winer studied the FRK faced fiberglass, does that mean it was acting more like a membrane absorber - since air can't pass through the foil reinforced paper?


Exactly. FRK stands for _Foil Reinforced Kraft (paper)_ and it is not porous.

--Ethan


----------



## Guest

adogand6kids said:


> ...I have a fairly big and busy family. I am concerned that something sturdier than cloth covered fiberglass is needed.


What about using a perforated metal shield? I think you could contact a local metal worker's shop or steel company and find someone who sells it. I was thinking about using some myself in some panels I made over the weekend, although I ended up using fabric for the time being. I found an article recently that related to using perf metal and its benefits.


----------



## bpape

Using a perf metal will not allow the membrane effect and only block parts of the broadband material behind. It's great for keeping prying fingers out but that's about it.

Bryan


----------



## Guest

bpape said:


> Using a perf metal will not allow the membrane effect and only block parts of the broadband material behind. It's great for keeping prying fingers out but that's about it.
> 
> Bryan


Bryan:

I'm sorry, but I'm not following you. Assume you do not want a membrane effect, wouldn't the perf metal allow the soundwaves through, much like fabric? (what you don't know about me is that my front mains are a pair of old A/D/S L990s with the metal grills)

Or I guess, what I am saying is that your reply sounded critical of the idea, I'm just not getting the "why" of it.

Thanks for your thoughts,

Trent


----------



## adogand6kids

Here's a question to consider for Bryan Pape, Ethan Winer and anyone else in the know. Regardless of the physics behind the absorption, according to Ethan's article posted above, putting a non-porous material in front of the fiberglass made for better overall absorption. What are people's opinions on whether the thin plywood would have an effect similar to the FRK facing? Am I right or wrong in expecting a similar beneficial effect on low frequency absorption and reduced overall ringing?


----------



## bpape

Trent. I'm not being critical - sorry if it came across that way. The mesh will cause some reflections at higher frequencies but overall it can work very well.

Wade

Even thin plywood will not function the same way as FSK will The idea is that the membrane is bonded to the fiberglass and the flexibility of the fiberglass acts as a dampener with a hump in absorbtion - where depends on the density of the material behind and the thickness.

The mass of the plywood in comparison to the mass/spring/compression of the fiberglass is simply too great. In addition, it would also reflect a LOT more down into the mids where a lot of it would normally pass right through the paper. 

Bryan


----------



## Guest

bpape said:


> Trent. I'm not being critical - sorry if it came across that way.


Bryan - no worries - I meant critical as in "critical listening" "critiquing" or "wary skepticism" -- not that you were criticizing me or the idea. Ah the limitations of the Internet...

Thanks for all of your time and comments.

Edit - ok, I guess I did say that you sounded critical of the idea -- but in my line of work (let's just say the legal field) that's perfectly o.k. Some people have a hard time separating criticism of their idea from criticism of them personally - I'm not one of them.


----------



## Ethan Winer

Wade,

> What are people's opinions on whether the thin plywood would have an effect similar to the FRK facing? <

Do you want opinions or do you want hard facts? :huh:

Seriously, the problem with "what if" questions like this is it's impossible to know without building eight or more traps, and paying a lab a grand to test them.

If you do that, please let me know what you find out! :kiss:

:bigsmile:

--Ethan


----------



## Guest

adogand6kids:

Here's another thought that you might try. I built some panels over the weekend, but didn't like the idea of just fabric holding in the fiberglass, so I went to Home Depot to see what I could find. I ended up stapling screen door mesh to front and back faces of the panels to hold in the fiberglass. It comes in 36" and 48" rolls, up to 25', and in a couple of different types. One is fairly stiff and the other is more pliable and fabric-like. It was a little more work to put it on prior to the fabric, but ultimately, I like what it is doing. Plus, it is much more porous than perforated metal, so I guess my untested theory is that it won't have much effect on the soundwaves hitting it (****, it's more porous than burlap).

Anyway, you could give the stiffer type a try - just staple, nail, use firring strips, whatever, to hold it firmly to one side, then stretch it across and do the same thing, then apply your nicer looking fabric or treatment over the top.


----------



## bpape

That's a great way to do it. Just make sure to use the steel screening and not the nylon. 

Bryan


----------



## Guest

bpape said:


> That's a great way to do it. Just make sure to use the steel screening and not the nylon.
> 
> Bryan


OK, I'll bite: Why not the nylon?

Trent


----------



## Bob_99

I'd like a piece of that action. Why not the nylon???

Bob


----------



## Ethan Winer

Bob_99 said:


> I'd like a piece of that action. Why not the nylon???
> 
> Bob


Me three.


----------



## Bob in St. Louis

jaegertc said:


> OK, I'll bite: Why not the nylon?


Rips easier? :dontknow: (Less kid proof)

Bob


----------



## bpape

Nylon will stretch and decay over time. Steel will stay taught and last basically forever and keep things nice and tight.

Bryan


----------



## Guest

bpape said:


> Nylon will stretch and decay over time. Steel will stay taught and last basically forever and keep things nice and tight.
> 
> Bryan


Nothing acoustically different, though, of which you are aware?


----------



## bpape

Shouldn't be. The steel might be a tad more reflective but it will be trivial. 

Bryan


----------

