Acoustic dampening pads/traps

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I am looking for a way to construct acoustic dampening pads for my home theater, such as those that are placed on the walls of a listening area. I would be interested in constructing a floorstanding trap, but I am very limited in space and positioning, so something thin, wall mountable, and aesthetically pleasing would be my primary choice. Can anyone direct me towards plans or materials for these?
(The ones I poked around with at the local Tweeter felt as though there was a very dense foam behind the fabric, though it was most likely a fiberglass core)


[Edited by Super on 05-29-2001 at 06:55 PM]
You're on the right track...
Some are simply fabric-covered foam. (Hint: The "shark-tooth" foam you get as packing material is a good [and cheap] way to absorb sound. Hit appliance stores and anywhere else you can think of that gets stuff in in big boxes. Look humble. Grovel. Beg. Get shark-tooth foam for free. Get the idea?)
Others are fiberglass pads, but denser than the stuff you'd use to insulate walls. (Hint: Get friendly with your local HVAC fella. They use mats to insulate the duct work and insides of heat pumps. Look humble. Grovel. Beg. Get free fiberglass mats.)
Don't be shy. Use them in combination. A layer of fiberglass backed by a layer of ordinary flat foam. Fabric stores are stuffed to the gills with the flat kind (if you didn't manage to pick up any earlier for free) for little old ladies to make bolster pillows and such. Maybe they'll have scraps. Look know the drill.
Assemble a collection of stuff, then build nice 1x2" (or however deep you want) pine frames around the absorbent material. Stretch some fairly attractive, open weave material over the top and staple at the back.
Presto! Instant sound absorbing pads for next to no money.
As far as theory goes: It takes a quarter-wavelength (half? I think it's quarter...) of sound absorbing material to attenuate a given frequency. A few moments with a calculator will show you that high frequencies are's the low frequencies that are unstoppable.
Low frequencies are a bear to work with. Let's leave that until later. Start with the easy stuff.

I think I'll be able to scrounge up some very thick, dense, egg-crate style foam which should be suitable, and a few fiberglass mats (my pops is a UPS man, and he delivers to a specialty insulation place! :)) He also delivers to a local Jo-Ann Fabrics (if I can't get the free stuff I'll pay the 10 cents a yard for it!)

So the way I see it, I've got the high frequencies down. But how about a bass trap of some kind? My room isn't exactly the ideal listening area, considering its thin sheet-rock walls. Two of my light fixtures vibrated out of the ceiling, and I haven't even added the powered sub yet!
And on top of that, its getting really annoying hearing my computer case rattle against the motherboard inside, and I've got that thing sitting on a 4 inch thick pad! :(

Thanks for the help


[Edited by Super on 05-29-2001 at 09:54 PM]
Don't assume that all the vibration going into your computer is coming up through the floor--a lot is coming straight through the air. Four inches of foam underneath is more than enough. You'll need to fasten the motherboard more securely (spacing it a little away from the case, if you can) or start looking at other options.
As far as bass traps go...*sigh*...
1) They aren't as effective as advertised.
2) They're big, verging on huge, to do a good job on low frequencies.
3) There's as much art as science involved, so it isn't exactly the kind of thing that you can plug in some numbers and crank the handle.
There are odd solutions, such as rubber sheets stretched to resonate at certain frequencies, and 'active' units that put out a counter-tone to cancel the resonance in the room. The cheapest way to go at it is to build a Helmholtz resonator and fill it with sound absorbing material so that it's "lossy," i.e. tends to absorb the frequency in question. The trick here is that you need to know what frequency you're shooting for, as a resonator designed for 80Hz won't be very effective at 50Hz. So the first thing you've got to do is determine where your room resonates. Go ahead and dial 911 *before* doing tests, as you're going to have a heart attack. You'll have at least four or five pretty strong peak/troughs to deal with. Then you start fiddling. And keep fiddling. And keep fiddling. And keep...
Even if you know what frequencies you're aiming for, placement of the sound treatments will take quite a bit of experimentation. Plan on days or weeks, not just an hour.
Me? I use books. Lotsa books. I don't have the time or patience to mess with all the silly little triangular pads in corners, etc. Books are very good at diffusing the mids and highs. To tame bass problems, I move the speakers (subs) around. A good starting point is to place your speakers 1/3 of the way down the length of the room and 1/3 in from the walls (leaving 1/3 between them). That way you're not creating standing waves by being in some even multiple (or fraction) of wavelengths. Experiment a bit from there.
This placement also tends to give you a good start on imaging, assuming that your speakers are capable of such.

I think a Helmholtz resonator, one of the slotted, 6-sided box types would be a good choice, at least for my purposes. I do have a LOT of standing waves in my room, seeing as it is only about 8x7 feet. Any ideas about how I can calculate the frequencies at which these standing waves occur, or where I can find plans to build the box?

Just to torture a friend of mine, I once wrote a computer program to calculate the resonances in his listening room. I set the program for a thousand iterations. I figured that would be enough to reduce him to tears.
Mind you, that's a thousand individual resonances...
It worked. He nearly had a nervous breakdown.
A room where music is playing is a constantly changing, three-dimensional waffle of nodes and anti-nodes. Figuring out which are the most severe...ah, now that's the crux of the matter, as there are a myriad of details other than dimensions of the room that factor in. (The program I wrote only took in to account the dimensions of my buddy's room--not the rigidity of the walls, the effect of the air conditioning ducts, or the effects of the furniture.)
The point is: Don't bother trying to calculate resonances. It's something approaching chaos theory to take all pertinent factors into account.
Measure it.
The cheap way is to buy a Radio Shack dB meter and do frequency sweeps. The better way is to borrow one of those nifty computer programs that will create a waterfall plot of your speakers' response in your room.
Either way...don't say I didn't warn you. Every room with parallel walls (which is most of them, of course) has standing waves. Your room being 8'x7' isn't really relevant in that sense. My listening room is about 14'x21' and I've got a dilly back over in the southeast corner. I don't listen over there, so it doesn't bother me. (Although it's fun to play an organ record and hang out over there, letting the subs do my breathing for me.)

I can't for the life of me, think of a single program name off the top of my head. I don't own one of them. I'll try to remember to grub around and come up with a name or two for you (I'm at work now--it'll have to wait until I get home), but I can't say as any of the ones I've played with struck me as being very logically laid out. Case in point, I had a borrowed portable at the house about three weeks ago trying to do a waterfall plot, just for fun. The program claimed it would do it, but I messed with that thing for four hours and never found any combination of menus that would lead me to the right graphs.
I'm sure the serious 'pro' stuff is easier to use, but will cost an arm and a leg (enough money to buy you a new car).
Poke around...there might be share/freeware stuff out there, particularly in the Linux end of things.

Looks interesting. However, this isn't a matter of speaker placement for me. One of my listening rooms is too small to move anything, and I fall subject to parental control in the other room :) They are even reluctant about putting up dampening pads, but they've got to let me do SOMETHING....

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