John Sayers' Recording Studio Design Forum Archive

I know this might be a topic still debated...

but if an air-space behind a bass trap causes a dip (lowering the cut-off frequency but also decreasing the absorption for frequencies slightly above), what is the downside to filling all the space? Is it JUST cost of material? Would filling the entire airspace and making the trap thicker (for lower absorption) always give smoother absorption? (like pictured below)

Is there a time when the airspace is ideal?

What about varying the densities? any benefit to that? like such...

Kinda difficult to answer rambling questions. Don't take that personally, as anyone who knows me on these theads knows that I can ramble like a pro.

The advantage of not filling the space is less material (cost).

A completely filled absorber gives the smoothest absorption, all other things kept equal of course.

There is no advantage to varying materials in the absorber. The best is with less dense material. Eg, 703, SAFB and its ilk.


Absorbingly:
Andre

Gathering from everything I've read about bass traps, everyone says the same thing, "If you can afford to fill the whole space, by all means do it."

Otherwise you're trying to tune the trap's distance from the wall to a 1/4 length of the waves you're trying to stop. That can be very tricky, and in the end you probably won't get it just right. If you fill the whole space then you're catching a lot of different waves at their 1/4 length, thus giving you a more effecient and smooth trap.

The thicker the front material, the less this air space matters; and cost is the only reason NOT to completely fill. My gut feel, however, is that if you DO use varying densities I'd normally put the LIGHTER stuff out front; less chance of grazing incidence allowing higher freq's to reflect. The higher density INSIDE may allow more absorption at lower frequencies, but this is (again) just gut feel; no test results to back it up... Steve

Well if we're going off gut feelings now... I have a gut feeling of my own.

I'll start by stating a couple obvious facts.
- The less dense material will absorb the higher frequencies, but will pass the lower freq's.

- The more dense material is going to absorb lower frequencies.

Since bass waves are longer, and you're looking to hopefully catch them at their 1/4 length, it would be advantageous to have the thicker material further off the wall. Higher waves are shorter, so 1/4 wave length is caught closer to the wall.

But this is simply a gut feeling. I personally don't feel there'd any need to vary the style material. I see people cutting 703 into triangles and stacking them in corners all the time. I'd have to imagine that'd end up being a pretty damn smooth bass trap.

Yup; and my "gut feel" would change if more bass absorption was needed instead of smoother response - but by far the simplest, best (not cheapest) is the "superchunk" thang, as you said... Steve

What about when adding something to prevent the higher frequencies from being absorbed?

Does this ruin the smooth bass absorption?

Any recommendations for what this reflective part should be? Is the foil on the FRK version of 703 enough? It will of course have a cover over it. And of course I am only speaking of corner bass traps (not first reflection points).

I'm making a studio live room/control room in one. I want to do a lot of bass trapping, but i'm very concerned with having a room that is too dead. I don't want it to be unpleasent sounding to mix in... and I don't want drums to sound extremely dry.

I know I can move around traps, but I'd rather not. Especially because I think bass trapping could help the drum sounds I get anyway. Also, I will be recording/playing back to get sounds since it is one room. I want to make mic placement judgements based off of my "mix enviornment".

Sorry for the rambling questions... I think of more things to question every time I think I have a simple question.

Your getting into questions THAT CAN NOT BE ANSWERED with the amount of information you have given. Have you dome a room decay rate analysis (calculations, not measurements)? compared room reesponse with industry standard recommendations? Measured the current decay rate? BTW "decay rate" is the more accurate term for room liveness in dead rooms.

Dimensions, construction, and current room treatment details are needed to start to give you accurate answers.

Yep, I wrote "live" "dead" rooms. Not only can I ramble, I can create oxymorons too!

What about when adding something to prevent the higher frequencies from being absorbed?

Does this ruin the smooth bass absorption?

You'not going to like the answer. Yes and no. It depends on what the "something" is and how it is used (I told you you wouldn't like the answer ).

FRK is commonly used for this purpose.

More info please:
Andre

edit 06.05.30 Changed several groups of letters to words and added several words to make sentences be logical.

I have always had a though about adding bass hanger material into the Superchuck type corner trap. MDF, masonite, particle board etc etc.

cut the 703 as well as the hanger material into triangles. Stack them in the corners like a triangle sandwich...703, MDF, 703, MDF,703, etc etc all the way up to the ceiling.

wonder if it would trap more bass?

also though about the DIY tube traps being stuff with 703 instead of leaving them hollow.

are maybe a designer bass hanger?? would look like large wind chime...LOL

I have always had a though about adding bass hanger material into the Superchuck type corner trap. MDF, masonite, particle board etc etc.

cut the 703 as well as the hanger material into triangles. Stack them in the corners like a triangle sandwich...703, MDF, 703, MDF,703, etc etc all the way up to the ceiling.

Yeah - I was pondering that idea Docta'J - weight is a problem, over a period of time the mdf weight would compress the insulation.

I don't think it needs to be as heavy as mdf, I was thinking of cardboard or similar.

cheers
john

Hello,
Generally if I want to absorb bass and not the highs, I use wood slats or squares in front of the bass trap. The cut off frequency for the reflectors are wavelengths 4 times longer than the dimensions. (3" square ~ 1khz). Of course you have to watch the percentage of open vs reflective surface because of the theoretical beginings of a helmholts resonator. I keep it under 35%.

nukmusic wrote:I have always had a though about adding bass hanger material into the Superchuck type corner trap. MDF, masonite, particle board etc etc.

cut the 703 as well as the hanger material into triangles. Stack them in the corners like a triangle sandwich...703, MDF, 703, MDF,703, etc etc all the way up to the ceiling.

wonder if it would trap more bass?

How much bass are you trying to absorb? 10" of 702 like material is flat down to 50 Hz in random incidence absorption. Fig3 in BBC RD 1992-10] shows this.

Andre