designing bass traps

[nicko]

I want to put traps in a room which is 14x11x8 ft. It’s my bedroom. I am in NSW australia. Im a uni student without the funds to buy premade bass traps.
I’m having a bit of difficulty deciding which diy traps to use.
I know I need broadband absorbtion of the bass spectrum..
I have carpeting, so absorbtion of the highs is not really needed, nor the mids , as far as I know.
I will corner mount the traps on 6 of the room corners.
To achieve broadband absorbtion of the bass spectrum I have divided it into ‘upper bass’ and ‘deep bass’, with the boundary between the two approximately 200-250hz.
As I see it, absorbtion of the upper bass spectrum would be most cheaply achieved by utilizing porous absorbers located in 2 or 3 corners. This will consist of 4 inch thick 36-96kg/m3 40 cm compressed rockwool or fiberglass placed across corners.
Question A
Is this correct?

Question B
Will it be better to completely seal the edges of the walls, or leave a gap at the intersection of 3 walls where the bass can get in and get absorbed by both sides of the panel? I’m leaning towards the former solution for aesthetic and logical reasons, but still it’d be good to know.

Now what to do about deep bass?
Ethan has some panel trap plans on his site…. But even the deep bass one looks like it could be modified to suit lower frequency applications. The existence of a similarly principaled Modex design speaks for that, and they are available tuned down to 40hz. I want to build my own for cost reasons though..

Now its well known that the frequency at which maximum absorbtion occurs in a panel absorber occurs can be calculated by
60/((m.d)^0.5)
M being the mass of the panel in kg/m2 and d being the airspace in metres
However I want the panels to be 8 inches thick at an absolute maximum, and I’d prefer less. This room is after all, a bed room, and guests frequently come over.(what can I say, I’m a teenager). This implies a very heavy ( and if made out of wood) thick membrane.

Question C
I have a hunch that where the thickness of the membrane becomes significant, a 2x14 or 4x11 or whatever surface surface for the trap may prove too small for the formula to hold. Is this correct? Is it advisable to choose a different material for the membrane?


However the useable frequency range of these deep bass traps will be quite small; and a single wood membrane design will not be able to adequately address 50-250hz as far as my intuition tells me. I’ll thus need to take one of two steps:
1. Construct broadband modified membrane traps (realtraps approach)
2. Construct multiple standard membrane traps targeting different frequencies ( modex approach)

Now I’ll address how ill address each approach, and what my perceived pros and cons of each method are. Hopefully you guys can correct my muisconceptions. The ultimate decicion as to which approach to use seems to be determined at this stage by question c.

TO create broadband membrane traps there are 2 approaches I could take

i. Use a hybrid trap design- a perforated membrane on top of a porous absorber…
A con would be that since this is both a velocity and pressure based absorber, I’;d need to mount irt away from the wall AND make it fairly thick! Doh
!
Question: Are these any good? Does anyone have any experience making or testing these?
ii.Use a self damping limp membrane
Examples of possible membranes would be bituminous felt or mass loaded vinyl.
Question D
I believe this self damping action will result in a much wider usable frequency range than a wood membrane. Now rememberr the simplest panel trap doesn’t have any damping material at all in it. It’s a very efficient absorber, but over a narrow frequency range, and has a problem with reradiation of the absorbed frequency. Adding damping material inside the trap, as far as I know increases the usable frequency range f the trap somewhat (or am I getting confused with adding damping material to a helmholtz resonator here?) but decreases absorbtion efficiency somewhat. I have heard that using a self damping material exacerbates each of these effects; creating a broadband absorber which is effective over a much larger range of frequencies.

See the acoustics articles at http://www.soundonsound.com/sos/sep98/a ... tic_3.html
What kind of useful frequency range can I expect to get from such a system? What kind of absorbtion decline can I expect to experience? are these any good? Does anyone have any experience making or testing these?

ii. construct units with different physical membranes within them horizontally and different sealed cavities
Question D Would the cavities for each membrane need to be isolated from each other for this trap to function as a broadband absorber?
Pro: simple construction? Modexx seems to think this method has better efficiency than the above.is this correct? It seems to me the fact that each panel would absorb more per unit area is offset by the fac thtat each panel will have to be smaller.
Con- See question C- wood just might not work at these frequencies!
Question : any comments on this? Am I on the right track?

iii. Using what I’ve seen refered to as a double sided trap. I’m not sure if this has a membrane on either side of the air space, each of a different density, or whether one membrane is placed over the other. Any advice?

Any advice you guys might have as to the validity of my statements would be greatly appreciated
Cheers


One last thing….. where do john sayers design for broadband helmholtz resonators in cornesrs fit into all of this?

[knightfly]

Although you obviously have done some serious study on absorbers/traps, in this case I believe you're drastically over-thinking the situation - if you look at the Bonello distribution for a room that's 14 x 11 x 8 feet, there is no imbalance of modes - that is to say, those dimensions (although small) result in a well-balanced modal distribution, which will give as even a sound as one can expect from a small room.

This means that you do NOT want to target specific frequencies; so the simplest/best approach for that room is simply broadband corner absorption, plus treatment of early reflection points.

Which brings us back to questions A and B - considering the carpet, which will have a fair amount of absorption above 300 hZ or so, the only ceiling treatment you may need would be a "cloud" of rockwool over the mix area (I'm assuming there will be one?) - if you cover very much of the ceilng and walls, you will have too much mid/high frequency absorption for a balanced room.

Since the carpet will absorb somewhat down to around 250 hZ, I would choose slightly heavier density rockwool for your corner traps; more toward your higher value above, like 80-96 kG material; this will have less absorption at higher frequencies for anything but 90 degree incidence, so will keep the room a bit brighter.

Also, in the interest of maintaining maximum usable floor space, I would mount all but two of these corner absorbers at the corners between ceiling and walls, and only put two of them across vertical corners on the end of the room where your speakers will be. This is assuming the possibility that you can keep your mix area centered on one of the walls for best stereo imaging; if this is the case, you would use the two vertical corners that wall is common to, in order to minimise SBIR.

As to sealing corner absorbers - not necessary. They work almost 100% on velocity, so will work sealed or not. They get their wideband response largely from the varying air depth behind the trap.

Frames for these are kind of a two-edged sword; they make the traps look more "finished" and possibly easier to hang, but they also reduce absorption by screening the edges of the material from sound.

For the rear of your room (behind your head as you face the speakers) one usable way of evening out the response and getting more bass trapping is to build a movable trap; just a frame with a base that will let it stand on its own, filled with 4" of rockwool and cloth covered, that can be placed directly behind you while mixing. Spacing from the rear wall determines the depth of absorption somewhat, and the unit can be stored almost flat against a wall when not being used. This absorber should NOT have any solid backing, only gussets in the corners of the frame for rigidity.

HTH... Steve

[powerjoe]

Nicko,

Knightfly has given you very good advice here and it is worth considering!

Beautiful place you live in. I've seen every inch of your country on many occassions and I always leave crying!

For broadband bass absorption in corners, I believe Owens Corning has distribution in your neck of the woods. Look for a reputable insulation company, maybe in Brizzy, and they should have some of the OC 703 available. I think the main distribution point is at Surfer's or up a bit in Rockhampton. But, I am pretty sure it's right in Brisbane. If you can get some of the 703 (or rockwool), slap some breathable fabric around it like swaddling a baby, and you'll have a corner absorber good to 80hz or lower. You can use 1 inch on your early reflection points and this should set you off on a good wicket.

Please report back and let us know how you're going!

Cheers,

Joel DuBay

[bpape]

Very good advice so far. I'll just add one thing.

If you're concerned about overdoing the HF absorbtion, then use a faced absorber (like and FRK on 703 or adhesive and butcher paper on Rockwool) for the corner absorbers. This will still work well as a broadband bass absorber but tame somewhat how much the upper mids and highs are absorbed.

Bryan

[stampen]

As to sealing corner absorbers - not necessary. They work almost 100% on velocity, so will work sealed or not. They get their wideband response largely from the varying air depth behind the trap.

Frames for these are kind of a two-edged sword; they make the traps look more "finished" and possibly easier to hang, but they also reduce absorption by screening the edges of the material from sound.

HTH... Steve

After lurking for years.... It's time.

These kind of corner straddling rigid fiberglass bass traps, do they need to be floor to ceiling?

If so, should they preferably be one piece? IE, if the height of the room is 2,4 meters would two 1.2 meters sheets be OK or should I find longer sheets. 1.2 lenghts are easier to find. Differences between fiber glass and mineral wool?

Thanks
/A

[knightfly]

They don't need to be floor to ceiling, although it would work a little better; because tri-corners are where ALL modes terminate, those are the most useful places for traps. So the bottom and top 1/3 of a corner would be the most effective locations.

However, the middle 1/3 of a corner is typically at head height, and you don't want stray reflections bouncing around there either...

There is no need for any kind of seal, because this type trap works on velocity - as long as the sound must pass THROUGH the absorbent to get to the wall, it will work fine. This means that there is no worry about using one piece or two.

The main difference (aside from color and feel) between rigid fiberglass and rock/mineral/slag wools is typically COST. In most areas, the rigid fiberglass is more expensive and harder to find. Acoustically, similar densities are pretty much indistinguishable fom each other... Steve

[stampen]

Exactly the kind of answer I hoped for. Thank you soo much.

/A

They don't need to be floor to ceiling, although it would work a little better; because tri-corners are where ALL modes terminate, those are the most useful places for traps. So the bottom and top 1/3 of a corner would be the most effective locations.

However, the middle 1/3 of a corner is typically at head height, and you don't want stray reflections bouncing around there either...

There is no need for any kind of seal, because this type trap works on velocity - as long as the sound must pass THROUGH the absorbent to get to the wall, it will work fine. This means that there is no worry about using one piece or two.

The main difference (aside from color and feel) between rigid fiberglass and rock/mineral/slag wools is typically COST. In most areas, the rigid fiberglass is more expensive and harder to find. Acoustically, similar densities are pretty much indistinguishable fom each other... Steve