My DIY bass traps project

I just made a bunch of DIY bass traps using OC703 (all 4" thick, 9 traps @ 2x6 and 5 traps @ 2x4) and am posting some design and cost info in case it can help anyone out.

Pictures: http://ltibbits.public.iastate.edu/downloads/SonarForum/tutorials/bass-traps/photos/

Basic design


Information
To create the frames I glued and screwed the corners together, pre-drilling the holes to prevent cracking the wood. Then I put L-brackets in every corner to improve the frame strength.

Next I cut and stapled utility muslin cloth to the front of the frame.

Then I cut and inserted the rigid fiberglass. Each 2x4 OC703 sheet was either cut in half or had 1.5" removed in order to fit into the frame. The "scrap" OC703 was inserted into 1 of the 2x4 traps along with a full 2x4 sheet.

I made 4 of the 4' traps and 2 of the 6' traps into 3 sets of go-betweens by attaching hinges to the frames. This allows them to be adjustable and free standing without creating any type of "feet" to support them. Plus I can remove the hinge pins and separate them if necessary.

Finally I stapled more muslin to the back to cover everything up and cut off the excess cloth. For most of the traps I did not try to hide the staples on the back since the traps will be leaning against the wall most of the time. On the traps that are go-betweens I cut and stapled some thin 1" x 0.25" white plastic trim over the staples to make things look a bit nicer.

The traps look pretty good overall to me and each trap took roughly 1 hour to build. The only downside is that the boards were not perfectly flat/straight and I didn't use any cross bracing so some of the traps are a bit warped. When the top is flat against the wall one of the bottom corners will sit farther out from the wall than the other, but I don't think that's too big of deal. Also, I used the ultra cheap muslin so you can see the frame and fiberglass through the cloth. You could spend 2x-3x the amount on cloth and get better quality muslin that would look a bit better.

Cost analysis for DIY bass traps
9 traps @ 2x6 4" thick
5 traps @ 2x4 4" thick

Item                              Unit Cost  Qty   Subtotal
 OC703 2x4x2" (cost/tax/shipping)  $11.77      36    $423.72
 L-Brackets                         $0.64      56     $35.98
 Wood 1x4 @ 12 ft                   $3.59      17     $60.95
 Cloth (Utility Muslin-sq yd)       $1.17      53     $62.01
 1 1/4" Drywall screws (box)        $2.75       1      $2.75
 Wood Glue                          $3.50       1      $3.50
 Trim Strips (per foot)             $0.43      80     $34.40
 Hinges                             $3.00       6     $18.00
 Staples (box)                      $3.75       2      $7.50
 Sales Tax (not incl. OC703)           7%       1     $15.23

Total Cost: $664.04
Avg 2x6 Trap Cost: $53.84
Avg 2x4 Trap Cost: $35.89


Edit:
Some other notes: I got the OC703 through a place called API Distribution (www.apidistribution.com) out of St Paul and they have offices in North and South Dakota and Minnesota. Shipping only cost $38 at 220 miles away (about $1/sheet) and it arrived in 2 days. I literally called 8 insulation and acoustical material suppliers and none of them had heard of OC703 so I'm glad I found this company and it was way cheaper than other internet suppliers.

Yeah...nice job Altima. I'm sure those will work great.
I plan on making some one day.
This VID is a similar yet different technique.

Is that a Helmholtz resonator?

anyone else wanna chime in?

Isn't the idea of the plywood that it resonates at a low frequency and the sealed or tuned air chamber behind it dampens the energy?


The reflection of high frequencies is more or less equal to the existing wall material anyway... so you absorb the problem freqeuncies while not killing the hi end... seems lke a great idea.

I never liked the idea of hanging fiberglass panels enclosed in a fabric that is transparent to sound in an open living space where people breath. Fiberglass is friable, it will come loose in microscopic particles when hit by sound waves and float in the air and get pushed around by sound waves and get into your respiratory system.
I'm sure they work well but many years in the building trades and being exposed to insulation debris cause my danger lights to go on every time I hear about these particular absorbers.

I never liked the idea of hanging fiberglass panels enclosed in a fabric that is transparent to sound in an open living space where people breath. Fiberglass is friable, it will come loose in microscopic particles when hit by sound waves and float in the air and get pushed around by sound waves and get into your respiratory system.
I'm sure they work well but many years in the building trades and being exposed to insulation debris cause my danger lights to go on every time I hear about these particular absorbers.

Craig.... what about that cigarette smoke from the cigarette in your hand? I hear tell that stuff is dangerous too. Each puff contains a heaping helping of carcinogins and other nasty things......

The plywood does resonate and dampen bass frequencies... just not all. The air space behind the plywood can be tuned or sealed and it will dampen the plywoods movement. Stuffing the air space with absorptive stuffing will make it functionally act as if it is a larger volume and it will offer smoother and greater dampening. Stuffing it with hi density rockwool will maximize that effect. Additionally frequencies that pass through the plywood will be absorbed by the rock wool as if they were merely hanging there without the plywood. As I understand it the rock wool does double duty.

That's my understanding, I welcome corrections.

best regards,
mike

ORIGINAL: Beagle

Ok, so help me out here, cause I've been planning on building some bass traps when I remodel my studio room (waiting on at least one of the two kids left at home to leave, actually!). anyway - my question is about this plywood and 703 combination. I thought that the 703 was supposed to go in front and that there was nothing in back of the trap so that the vibrations of the sound waves would be "trapped" between the 703 and the wall. if you're saying that a piece of plywood goes in FRONT and the 703 goes in BACK, then the 703 is flat against the wall, right? how does that trap the bass waves?

edit: OR are you saying that the plywood goes in front of the 703, but they BOTH go in the front of the trap with nothing against the wall?

Hi, Beagle,

As was mentioned above by Marlboroman 23, go check out Ethan Winer's site. There's enough info about bass trapping and mid/high freq. trapping to keep one busy for a few days. He used to have instructions on how to build his sealed panel bass absorbers on the site, as well.

Not sure if the instructions are still there. But, from what I recall, the idea behind having a bass trap using the sealed plywood enclosure with some absorbent material inside is basically as Mike suggested. The panel vibrates, converting the bass energy to heat. The absorbent material inside damps the internal vibration. However, the absorbent material should not touch the plywood face because it would dampen the vibration of the panel, which is counter-productive. This type of bass trap works on bands of frequencies that are determined by the volume of the sealed enclosure. Therefore, one might want to build a series of traps of differing volumes to absorb a wide range of bass freq's. Winer suggests using three different traps of different volumes to cover the low bass to the high bass freq's. This sort of bass trap will, indeed, reflect mid/high freq's, so you'd want to use 703 absorbers as well to absorb the non-bass freq's if you want/need to.

If you want to use 703 to make your bass traps so that they'll absorb mids/highs, too, you can put 4"-6" of 703 across the corners of your room (or the junction of wall/ceiling or floor/wall). Or you can literally fill the corners of your room with 703 by cutting it into triangles and stacking them from floor to ceiling. The longer bass freq's are absorbed by the thickness of the 703 and the fact that the 703 is away from the wall, which makes it more effective on longer wavelengths.

The above is a *very* simplistic sort of overview of the two concepts. I do not claim to be an expert in this field and I'm only repeating what I've learned from reading extensively about room treatment. For more detailed info, check out Winer's site and other sites that deal with acoustical room treatment.

Best regards,
Bill

Ok - but clarify for me where the plywood and 703 go in the trap. are they sandwiched at the front of the trap with the big air gap behind them

Yes!!!!!!!

That's how they work, and I've built mine based on designs in SOS and on John L Sayers excellent site.

very clear now. thanks guys.

yep! that's what I was trying to wrap my brain around! the drawings mike and marlboro man (sorry - i did not see the link - that would have answered my questions sooner!) supplied solved it!

When considering a reflective surface it's important to identify which frequencies are being reflected.

A bass trap that reflects, for example, content at 2kHz is still a bass trap.

In fact it may turn out to be a more suitable bass trap than one that also sucks up all your energy at 2kHz.

A quick look at the sound absorption charts of hi density fiber will show that it abosrbs way more mids and treble than bass. So you always run the risk of sucking all the life out of the room before you get the bass under control.

best,
mike

Hi Craig,
First let me empahsize that those are not my drawings. I do make technical drawings... but those are not mine. I believe they are from Ethan Winer's website.

You can certainly use Auralex as the stuffing but I would suggest you take a look at the acoustical properties of the Auralex. It should be a simple matter to confirm that the sound foam is almost useless in the low frequencies. That's why someone like myself can categorically say things like "foam makes a bad bass trap".

After you have established volume/thickness and placement in proximity to a wall you can predict what frequencies will most likely be absorbed. You need really thick material to absorb low frequencies.

Then you look at the materials absorptive qualities to determine how much of the identified frequency will be absorbed.

The process of absorption is really one of friction and heat release. That's why dense fibrous structures make good absorbers. They have millions of reflective surfaces and each fiber absorbs some air movement as the energy is converted to movement in the fibers and is finally converted to heat. Up to a point, the higher density simply provides more surfaces and more heat absorption. Foam is not dense compared to 8lb/cu foot rock wool.

I know you have concerns about fiberglass. Fair enough! I always like to point out that real sheep wool is a fantastic sound product. It's just incredibly expensive in the quantities one needs. For the very same reason wool makes a superior sweater garment to other materials it makes a great sound absorber.

The low bass panel resonators in the drawing are another technique for absorbing bass. Before the idea of hi density rock wool became popular most bass traps were tuned panel resonators, Helmholtz resonators, or a combination of both.

The popularity of dense rock wool absorbers seems to be linked to the easy and inexpensive availability of the material and the decidedly non technical nature of making and installing them (as opposed to actually tuning a Helmholtz resonator).

Foam on the other hand seems to have become popular because it looks cool. I say that because if you look at the absorptive qualities foam has to offer and if you are seriously thinking in terms of "tuning" a room you'll probably conclude, as I have, that foam is most suitable to tame the distinctive hi frequency slap back such as as is experienced in a tiled bathroom with a marble floor. Of course hi density rock wool does a better job at this as well... but f you really want to use foam in it's best way... think about using it to work with frequencies over 500Hz.

Most foam installations are not in this type of setting.
Most foam installations serve to kill any hope of having a nice diffuse sound in the upper mids.
Most foam installations seem primarily designed to provide a cool visual effect.
BUMMER.

So as you are learning, one can easily get started with bass traps but at some point it's best to start thinking about the overall room sound and then if you really need to tame the upper mids you can judiciously start adding foam SPARINGLY and in specific locations until you end up with the balance you desire.

Starting out by killing the mids while allowing the bass to get sloppy is not helpful... which is why I advise people to avoid foam until they are well acquainted with their room.

very best regards,
mike

One thing I've read/heard repeatedly from Ethan is that for small rooms (< 2500 cu ft IIRC) absorption is generally preferable to diffusion because of the need to control ringing and echo. My room is on the cusp of that being 23x13.5x7.75 = 2406 cu ft.

I did notice something interesting in one of Ethan's articles was that my fiberglass-only trap is a velocity absorber whereas the sealed plywood trap is a pressure absorber. Ethan also suggests that both types of traps should be used for best results by alternating them as you go around the room. It sounds like the pressure absorber will work better than a velocity absorber when placed against a wall therefore it might be better for very small rooms. More info here: http://www.ethanwiner.com/acoustics.html

I'm finally getting a chance to set my traps up, but the only thing in the room now are my drums and the traps. Hopefully I can get my desk, computers, and monitors set up this weekend. After walking around clapping I can still hear a slight ringing or flutter echo. Since by big traps are 6' high and the ceiling is 7'9" that leaves a gap at the top where sound can reflect. The ceiling is entirely untreated. I'm starting to think that I'll have to set them on something to reach the ceiling since there's less chance for reflection across the floor since the floor has carpet plus 7/16" padding under it.