Bass traps use a "membrane" - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 16 Old 08-11-2005, 07:55 PM - Thread Starter
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Whether bass traps are mounted as a soffit or in the corners of walls, a term sometimes used relating to bass traps is "membrane." Please describe what that means, what materials are used, and how it is constructed into a bass trap.
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post #2 of 16 Old 08-11-2005, 09:57 PM
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takeaim:

Although some people prefer 2' to 3' deep porous (fiberglass/rockwool/cotton/polyester) absorbtion (e.g. Paul Woodlock, Mark Edmonds, etc) to absorb bass, I believe that it's more common to use resonant absorbers to absorb bass. You've probably read about Dennis Erskine using them in columns and soffits and risers. Partially because it takes up less floor space when combined with 1" porous around the room. The disadvantage of resonant absorbers is that they shake the whole wall at that frequency, possibly making that frequency transmit through the wall reducing soundproofing.

"To get broadband passive absorption across the frequencies of most interest to human acoustic design, usually requires a combination of resonant and porous absorption." (Trevor Cox in Room Acoustics 2004-5)

There are several different types of resonant (non porous) absorbers: membrane, slat helmholtz, perferated helmholtz, single port helmholtz. (beware there are lots of wrong Slat Helmholtz formulas on the www -- the correct one can be found here http://forum.studiotips.com/viewtopic.php?t=94 )

Anyway, on to membrane formula:

A example membrane trap would be an airtight box with a 4'x8'x3/4" sheet of plywood on a 2x4 frame about 10" deep, with more plywood on the sides and back of the frame, with fiberglass/rockwool inside, not touching the front membrane. You can put horizontal bracing from front to back (the 10" dimension) near the sides but not under the membrane (the front 4'x8' panel).

http://www.bobgolds.com/BassTrap_JeffCooper_pg127.jpg

Membrane trap formula: d = 28900 / (M * f^2)
d = depth of airspace in inches
M = surface density of panel, lb / ft^2
f = peak absorbing frequency

M for 1/2" plywood is about 1.375 lb / ft^2
M for 3/4" plywood is about 2.06 lb / ft^2

Let's try to design one for 38hz.

d = 28900 / (2.06 * (38^2)
d = 9.7"

Umm handy. We can make the sides out of 2x10's with that dimension, instead of 2x4's and plywood sides.


Let's try to design one for 43hz.

d = 28900 / (2.06 * (43^2)
d = 7.6"

That's a 2x8, with 3/4" plywood membrane.


Let's try to design one for 50hz.

d = 28900 / (2.06 * (50^2)
d = 5.6"

That's a 2x6, with 3/4" plywood membrane.


Let's try to design one for 63hz.

d = 28900 / (2.06 * (63^2)
d = 3.5"

That's a 2x4, with 3/4" plywood membrane.


Let's try to design one for 75hz.

d = 28900 / (1.375 * (75^2)
d = 3.5"

That's a 2x4, with 1/2" plywood membrane.


Let's try to design one for 97hz.

d = 28900 / (2.06 * (97^2)
d = 1.5"

That's a 2x2, or a 2x4 on it's narrow side, with 3/4" plywood membrane.


Let's try to design one for 118hz.

d = 28900 / (1.375 * (118^2)
d = 1.5"

That's a 2x2, or a 2x4 on it's narrow side, with 1/2" plywood membrane.


The key point is the density of the panel, so weigh the plywood when you get it home, and cut the side frames to give the correct depth.

Building these things to a target frequency apparently is difficult, as the real world for a variety of reason's doesn't match the formula. Also there's the problem of resonance/ring back into the room, i.e. that it'll not only absorb at that frequency (a good thing) but might even ring at that frequency (a bad thing). The fiberglass inside should help damp the ringing, but it widens the Q of the absorber and reduces its ability to absorb.

For example, as you make them smaller area than a 4'x8' sheet, structural stiffness becomes more of a factor. Obviously a 4"x4"x9.7" isn't going to resonate at 38hz. But as for just where a smaller surface area starts deviating significantly from the formula, I don't know. I believe a 4x8 sheet is safe to use the formula, especially in the 50hz to 100hz range. And you probably need a significant surface area anyway to deal get enough sabins of absorbtion out of it anyway.

The lower the frequency, the less likely you'll be able to build one. I know of at least one acoustician (Eric Desart) who tried to make a 31hz absorber panel, and wrote "Be careful with traps going that low in frequency [e.g. 20hz, 30hz]. Not only the resonance frequency is important but also the damping. Such devices very easily can cause secondary reverb curves, radiating sound back into the room at a minus 20 to minus 30 dB level. ... The resonance frequency itself is only ONE SINGLE parameter. I once tried to correct a 31 Hz problem with a panel resonator (horizontal). I could tune the panel resonator to the EXACT frequency. But I couldn't get the internal damping high enough. And when I got it high enough it lost its efficiency. So even with the correct resonance frequency I just got other problems instead. So I stopped and demounted the whole thing (which had cost already time, energy and money)."


If you can find some instructions on how to build an accelerometer tester, you can find out what the resonance frequency is of something you've built.
Or you can purchase resonant absorbers some from companies like RPG who sell traps tuned to a target frequency and Q.

http://www.bobgolds.com/WideQ_NarrowQ.gif

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post #3 of 16 Old 08-12-2005, 06:43 AM
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Good explaination Bob. Don't forget to mention that when adding the absorbtion to the inside, not only is the Q changed, but in practice, the center frequency drops a little bit too.

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post #4 of 16 Old 08-12-2005, 07:11 AM - Thread Starter
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As always, I'm impressed with your answers. Several more questions relating to my smallish 12'w x 16'l x 10'h dedicated HT under construction.
1. The formula deals with the depth of the air tight box. What are the parameters relating to the width of the membrane? Any relationship to the depth?

2. If the fiberglass/rockwool inside the box cannot touch the membrane, how far away from the membrane should the fiberglass be? What if any type of material keeps the fiberglass separated from the membrane?

3. Could not these be used as soffit for bass traps?

4. How would I alter the design "to get broadband passive absorption across the frequencies ..... requires a combination of resonant and porous absorption"? I read something about building a soffit using the box with a membrane of pegboard, which has many holes. I believe it said that the pegboard faces downward, the fiberglass fills up the box, and the holes are covered with porous GOM 701.

5. Which is more important, building a box for one frequency or a broadband box that absorbs less but over a reasonable range of bass frequencies, maybe <200-300 Hz? Above 300 Hz, the combination of Linacoustics/OC 703 and polyester batting come into play for absorption and reflection. Could one soffit box be designed to do both depending upon the porous/non-porous membrane used?

6. If my room is 16' long but the front of the Triad is one foot away from the wall, is my room now 15' long for calculating its resonate frequency, which is 34.4 Hz for 16' or 36.7 Hz for 15'? This seems too theoretical and too close to make any real difference. Using whatever frequency, it looks from your formulas that a depth of about 10 vertical inches of soffit hanging from the ceiling might be acceptable. But how wide? Combination of porous and non-porous membrane? Is there some happy medium for soffit? Do I need soffit on three sides of the HT?
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post #5 of 16 Old 08-12-2005, 09:42 AM
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I would be wary of using 3/4" (or even 1/2") plywood. I've asked this in the past and it seems its too stiff to behave as an approximation of a piston/mass loaded spring. Stick with 1/4" or 1/8" plywood and increase the depth.

Alternatively, try 1psf or 2psf mass loaded vinyl.

Andy K.
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post #6 of 16 Old 08-12-2005, 09:47 AM
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takeaim,

1. The width (and height) of the panel are not relevant to what frequency it absorbs, only the depth. Where is the width and height come into play is how *much* absorption you are getting. So make them as big as you can (or want to aesthetically) fit.

2. An inch separation is sufficient. You will want to use rigid fiberglass so that it doesnt sag/expand over time and touch the front panel.

3. They could indeed.

Andy K.
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post #7 of 16 Old 08-12-2005, 11:15 AM
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the following tips will hlep you get closer to teh predicted center-frequency:

1. use non-rigid panels. 2*1/4" plywood will be 4 times less rigid than 1/2" plywood, for example. MLV is even better, or part of the mass as thin wood, part as MLV. etc . acoustik-mat is non-stiff and heavy and cheap, rubber floor mats

2. build the traps big. the bigger the trap (in ALL dimensions 4' x 4' is better than 2' x 8') the less stiff the mechanical component will be, and the less the mechanical stiffness will affect the location of the center frequency of absorption

3. just plan to test and taper the results. Andy K. did some of this, and once you've built a trap, you can easily tune the center frequency down by adding mass to the center of the panel. in the form of lead slabs or whatever, the center of the panel will give you the most effect


and mostly, i guess, just plan to tune the things. unless you build it big and non-stiff.

:)

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post #8 of 16 Old 08-12-2005, 11:46 AM
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Aims,

> How would I alter the design "to get broadband passive absorption across the frequencies <

You can't. A wood panel trap is a tuned device, and there's no way to make it work well over a wide range. For a room that size I wouldn't even consider that type of trap anyway. All small rooms have problems at all low frequencies, not just those related to the room dimensions. This is why broadband bass traps make the most sense in rooms like yours.

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post #9 of 16 Old 08-12-2005, 01:16 PM
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Thanks for the followup/extra-details everyone.

Brian Ravnaas - do you think it would be a more effective absorber if one were to use two layers of 1/4" with green glue inbetween? Or for that matter several layers of laminate with green glue inbetween?

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post #10 of 16 Old 08-12-2005, 01:52 PM
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takeaim

Quote:
1. The formula deals with the depth of the air tight box. What are the parameters relating to the width of the membrane? Any relationship to the depth?
Answered above by others. Summary: keep it wide enough that it's mostly a membrane, and stiffness is not a factor.

Quote:
2. If the fiberglass/rockwool inside the box cannot touch the membrane, how far away from the membrane should the fiberglass be? What if any type of material keeps the fiberglass separated from the membrane?
Doesn't matter how far away. Rigid fiberglass in a 4' wide box is certainly easy to stuff in to the back and it won't move. But you could tie other stuff to the back with string and eye hooks (not wire). The more absorbtion is placed inside the lower the absorbtion at the peak frequency and wider the Q

Quote:
3. Could not these be used as soffit for bass traps?
Please rephrase.

Quote:
4. How would I alter the design "to get broadband passive absorption across the frequencies ..... requires a combination of resonant and porous absorption"?
You can't alter the design to make it do other things. It has a peak absorbtion at it's tuned frequency, and less at both sides.

Some people put absorbtion/diffusion at their first reflection points (fairly little room surface area).
Others put absorbtion/diffusion at their early reflection points (a little more surface area).

Beyond that people try to treat RT60 and modal response using absorbtion. This takes a reasonable amount of room surface area in 1500ft^3 to 3500ft^3 rooms, say 50% to 75% ish. Depends on the room (size, wall construction, what else is in it like furniture, etc).

In this RT60/modal category there are at least two schools of thought
1) thin 1" absorbtion over most of the walls and resonant bass traps. This is what Trevor Cox above recommended, as do Phillip Newell and Alton Everest, and what I believe Dennis uses frequently (but can't prove at the moment).
2) Thick 4" absorbtion at first reflection points and scattered around to control flutter etc, and broadband diagonal porous corner absorbers to absorb bass/modes. This is more along the lines of what Ethan and Studiotips frequently recommend. It's certainly easy to build (just about idiot proof) and everyone will get lots of sabins of absorbtion out of these things. In the case of studiotips broadband corner absorbtion is strongly recommended for otherwise untreated home recording studio rooms (no carpet, one little wooden chair and no couch, bare walls) as a cost-effective accoustically-effective first treatment for the room.

Resonant absorbers, because they are not broadband, can be used to offset other things in the room that absorb highs but not lows, such as carpet.

It's also, both theoretically and in floor square footage, easier to make a resonant absorber effective at lower frequencies than it is to do it with porous absorbtion.

As for which way you go, that could depend on your skills and WAF and room. Another benefit of resonant absorbers is there's less fiberglass in the room.

If you build resonant absorbers and sell the place, you can expect the new owner to scream at you (the prior owner) for putting up a wall without studs some day when he's mounting shelves.

Quote:
I read something about building a soffit using the box with a membrane of pegboard, which has many holes. I believe it said that the pegboard faces downward, the fiberglass fills up the box, and the holes are covered with porous GOM 701.
That's a perferated helmholtz absorber. Except if you try the formula you'll find that pegboard has the wrong hole sizing and hole spacing to be of any use acoustically. You need a smaller hole area to wood area ratio than pegboard offers.

Quote:
5. Which is more important, building a box for one frequency or a broadband box that absorbs less but over a reasonable range of bass frequencies, maybe <200-300 Hz? Above 300 Hz, the combination of Linacoustics/OC 703 and polyester batting come into play for absorption and reflection. Could one soffit box be designed to do both depending upon the porous/non-porous membrane used?
Neither. The important bit is the average absorbtion of the entire room. Absorbtion is additive. So if you have some high frequency absorbers and some low frequency absorbers, that's the same as having a broadband absorber (assuming it actually does the same high and lows that the prior two do).
The problem is usually devoting enough surface area of the room to the frequencies you want absorbed.
Sure, one could build a soffit such that 50% of it was porous absorber, and 50% of it was resonant absorber -- but relative to what else is in the room, would it balance out the absorbtion across the frequencies or make it worse? Well, that depends on what else is in the room. If you have thin walls and big leather couches and a single ported riser, and no other threatment, then that 50% resonant absorbtion in the soffet may absorb too much bass and not enough highs, so it would have been better to skip the resonant in the soffits ans go with all porous. If you have thick walls and cheap seats on concrete floors with carpet and have put 1" linacoustic everywhere else, then the 50% porous in the soffit may be too much absorbtion in the highs and you should have used more resonant in the soffits.

Quote:
6. If my room is 16' long but the front of the Triad is one foot away from the wall, is my room now 15' long for calculating its resonate frequency, which is 34.4 Hz for 16' or 36.7 Hz for 15'? This seems too theoretical and too close to make any real difference. Using whatever frequency, it looks from your formulas that a depth of about 10 vertical inches of soffit hanging from the ceiling might be acceptable.
Use the room dimensions, not where the speaker is.
If the walls are thin, and the frequency is lowish (< 60hz), then the acoustical dimensions may be even larger than your wall's physical dimensions.

Quote:
But how wide? Combination of porous and non-porous membrane? Is there some happy medium for soffit? Do I need soffit on three sides of the HT?
Wide enough that it's a membrane, and structure isn't stiffining it. As for the others, that depends on your room as to how much surface area of what type of absorbtion you're going to need -- and I have no formulas for how many sabins/ft^2 a given absorber that you might construct would deliver in the 20hz to 120hz range. (I do have some absorbtion coefficient numbers in the 125hz to 4khz range for some constructions).

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post #11 of 16 Old 08-12-2005, 02:20 PM
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Quote:
Originally Posted by BasementBob

Doesn't matter how far away. Rigid fiberglass in a 4' wide box is certainly easy to stuff in to the back and it won't move. But you could tie other stuff to the back with string and eye hooks (not wire). The more absorbtion is placed inside the lower the absorbtion at the peak frequency and wider the Q
Have to disagree with this. The most effective place for absorption is immediately behind the panel. This is where air velocity is at a maximum, and where porous absorbers like fiberglass are most efficient at adding acoustical resistance, which lowers the Q. Unless you are filling up the box with fiberglass (in which case it doesn't matter), the back of the box is the least effective place.

As for lowering the frequency, the position doesn't matter. The fiberglass effectively creates a larger volume for resonance, and so the the place for this volume expansion doesn't matter.

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post #12 of 16 Old 08-12-2005, 03:55 PM
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Terry Montlick:

Interesting. I get your point -- maximizing the effectiveness of the LF in-the-unit absorbtion with distance.

I thought that with helmholtz (slot and perforated) and wideband that one would put the porous right against the front (intimate contact with the panel for helmholtz), but with membrane it shouldn't touch. Some absorbtion is required for damping, so I put it to the back to narrow the Q --

Quote:
This is where air velocity is at a maximum, and where porous absorbers like fiberglass are most efficient at adding acoustical resistance, which lowers the Q.
-- but I'm wondering if I have it backwards.

Looking at Master Handbook Of Acoustics 4th Figure 9-23 and 9-24, it seems the rule for diaphragmatic absorbers is to have 1/4" between the panel and the porous. pg 208 says "A glass or mineral fiber blanket of 1" to 1.5" is glued to the wall surface [back]. An airspace of 1/4" or 1/2" should be maintained between the absorbent and the rear surface of the plywood panel [membrane/diaphram/front]".
Jeff Cooper's gobo design (pg 135) has the absorbent as far away from the membrane as possible (the back).
The example on pg 158 of "Acoustic Absorbers and diffusers" by Trevor Cox and Peter D'Antonio has the absorbant placed closer to the membrane than to the back, but not quite touching, and says (same page) "the absorbant should not be so close as to inhibit movement of the membrane."

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post #13 of 16 Old 08-12-2005, 04:34 PM
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Quote:
Originally Posted by BasementBob
Terry Montlick:

Interesting. I get your point -- maximizing the effectiveness of the LF in-the-unit absorbtion with distance.

I thought that with helmholtz (slot and perforated) and wideband that one would put the porous right against the front (intimate contact with the panel for helmholtz), but with membrane it shouldn't touch.
Correct. The principle is exactly the same for both membrane and Helmholtz absorbers. You put the absorber as far forward as possible. It shouldn't contact the membrane though, because that would interfere with its vibration.

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post #14 of 16 Old 08-12-2005, 07:17 PM
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Quote:
Originally Posted by BasementBob
takeaim

Quote:
Originally Posted by takeaime
6. If my room is 16' long but the front of the Triad is one foot away from the wall, is my room now 15' long for calculating its resonate frequency, which is 34.4 Hz for 16' or 36.7 Hz for 15'? This seems too theoretical and too close to make any real difference. Using whatever frequency, it looks from your formulas that a depth of about 10 vertical inches of soffit hanging from the ceiling might be acceptable.
Use the room dimensions, not where the speaker is. If the walls are thin, and the frequency is lowish (< 60hz), then the acoustical dimensions may be even larger than your wall's physical dimensions.
Hi Bob,

Jim (aka takeaim) neglected to say that he plans on building wall to wall, floor to ceiling cabinetry around his screen. He'll be installing Triad In-Walls into the facing of the cabinets. I believe he told me that he plans on the base of the cabinets will be about 2 feet deep for storage. Based on his comment above, I assume the upper portion of the cabinetry will be only one foot deep.

Larry
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post #15 of 16 Old 08-13-2005, 10:34 AM
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Terry,

> The most effective place for absorption is immediately behind the panel. <

Agreed, and that's exactly why the fiberglass is spaced off the rear wall in the "low bass" version of my wood panel trap plans:

www.ethanwiner.com/basstrap.html

--Ethan
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post #16 of 16 Old 08-15-2005, 04:34 AM
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Quote:
Originally Posted by BasementBob
Thanks for the followup/extra-details everyone.

Brian Ravnaas - do you think it would be a more effective absorber if one were to use two layers of 1/4" with green glue inbetween? Or for that matter several layers of laminate with green glue inbetween?
i don't know the answer to that. it will raise damping of the panel at the primary resonance (how much will depend on configuration, maybe alot), and probably essentially eliminate other resonant peaks.

but effect on absorption i just don't know. would be cheap to test, i guess

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