Tim's Limp Mass Bass Absorbers - Gearslutz.com
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Tim Farrant
#1
6th July 2012
6th July 2012
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Tim's Limp Mass Bass Absorbers

Over the years I have had great success fixing small room bass problems with limp mass absorbers. As there are so many threads here about this very problem, I have posted my design here for you to use. It's not that flash, but they do work if you follow the rules. The most common question is going to be, "how many do I need?". In most smallish rooms (like this one Time to give up on the 80Hz null? ) sometimes only 4 absorbers per problem frequency mounted at the corners will be effective and you can always add more if needed.

Cheers
Tim.

Edit, I have added a Excel frequency calculator, enter your mass and depth into the red areas and it will spit out approx frequency.
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limpmassbass1.pdf (461.7 KB, 3326 views) limpmasscalc.xls (15.5 KB, 1832 views)
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Last edited by Tim Farrant; 6th July 2012 at 07:16 AM.. Reason: added calculator
#2
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Jeez! This is PERFECT! Thank you!!
kasmira
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Thanks Tim! Just one question, why should the absorbers be so small?
Tim Farrant
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I read somewhere that they should be small compared to the wavelength of the resonant freq, but for the life of me I cannot recall where, it might have been Michael Rettinger. I have found in practice that making small traps of different depths and mounting them in an alternating pattern is very effective. The build up of modes is in the corners of the room so making a huge membrane that covers an entire wall is unnecessary, not to mention expensive.
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It would be interesting to see before and after measurement when these devices are installed.

But, how does one now if you have hit the right problematic freq in a room?
kasmira
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As these are not tuneable you have to build them the correct size and depth to achieve the correct frequency of operation. Another reason why sometimes its easier to buy than DIY - companies who build them will test them to get the exact frequencies they absorb with the materials they build with.

Edit: sorry I thought this was a different thread. You calculate the frequency by depth of the panel and the mass of the membrane. The calculator Tim posted will calculate this for you. Also, limp mass membrane panels have a wide Q so they are easy to use in most rooms. Only bummer is you need more of them than you would Helmholtz or panel resonators.

Tune-able would be nice, but I've yet to see a tuneable absorber that works well for clearing up resonances.

Helmholtz have too thin of a Q to be practical for my room, and I don't need a panel that also absorbs harmonics of the target freq, although this feature might be very useful in other rooms.

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I understand how to calculate the Limp mass absorber. But, if a measure a room at LP and have peak at 30Hz and deep at 50Hz and a peak at 80Hz, and so on, you get the picture. Which one schould I choose to build?
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Quote:
Originally Posted by Mctwins
I understand how to calculate the Limp mass absorber. But, if a measure a room at LP and have peak at 30Hz and deep at 50Hz and a peak at 80Hz, and so on, you get the picture. Which one schould I choose to build?
This is what I did not understand for a long time. Here is Jens' answer, which cleared up a lot of things for me:

How to treat nulls

Therefore you would be needing at least three panels, one for 30Hz, 50Hz, and 80Hz. And you would place them in the room where too much of these frequencies exists. In Jens' words: "Place them at the relevant surface."

@Tim: Thank you for this very useful and informative thread!
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Quote:
Originally Posted by seen-da-sizer
This is what I did not understand for a long time. Here is Jens' answer, which cleared up a lot of things for me:

How to treat nulls

Therefore you would be needing at least three panels, one for 30Hz, 50Hz, and 80Hz. And you would place them in the room where too much of these frequencies exists. In Jens' words: "Place them at the relevant surface."

@Tim: Thank you for this very useful and informative thread!
Ok, I build three panels and place them where, what surfaces? Just put them randomly in the room and just azuming it is working. How do I find where those freq exists in a room?

Or, do you mean I schould put them at my LP, where I have my three freq.

kasmira
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Quote:
Originally Posted by Mctwins
How do I find where those freq exists in a room?
Say if you have a 55hz resonance or null in your listening position, you can use a tone generator / sine wave generator to play a signal at 55hz. You can use an SPL meter to find the loudest position in the room at that frequency (or you can just use your ears). Adding an absorber at this position will help to eliminate the problem.

You should actually place Varitunes similarly. Placing panels where they are convenient won't really always work, but large traps like the Varitunes work at a lot of positions in most scenarios anyways (similarly with a lot of large absorbers)
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Quote:
Originally Posted by kasmira
Say if you have a 55hz resonance or null in your listening position, you can use a tone generator / sine wave generator to play a signal at 55hz. You can use an SPL meter to find the loudest position in the room at that frequency (or you can just use your ears). Adding an absorber at this position will help to eliminate the problem.

You should actually place Varitunes similarly. Placing panels where they are convenient won't really always work, but large traps like the Varitunes work at a lot of positions in most scenarios anyways (similarly with a lot of large absorbers)
We are not talking about Varitunes here, please don't go there. Let's stay focus on Limp Mass Absorber.

Let's be specific here, I don't understand why you involve 55Hz resonance or null. I said, I have according to my measurement a peak 30Hz, null 50Hz, and peak 80Hz at LP, period.
kasmira
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Quote:
Originally Posted by Mctwins
We are not talking about Varitunes here, please don't go there. Let's stay focus on Limp Mass Absorber.

Let's be specific here, I don't understand why you involve 55Hz resonance or null. I said, I have according to my measurement a peak 30Hz, null 50Hz, and peak 80Hz at LP, period.
Relax man, I was just suggesting that any type of absorber should be placed where the problem frequency has the highest SPL for maximum dampening. Well, at least for pressure based absorbers (like Helmholtz, panel traps, limp mass traps, etc)

I only gave 55hz as a random reference. Read my post again but replace my 55hz with 30, 50, and 80. The same technique works for any frequency. Nulls can be caused by a multitude of things, though. If it is boomy in an area other than your LP this could cause phase issues at the LP resulting in a null. In this case I would place an absorber in the boomy area in the room of the null frequency. Otherwise experimentation of panel placement between the speaker and its boundaries if SBIR is the culprit. If room modes are to blame, then they should be placed in the plane that is causing the null. Etc, etc.

All types of pressure based absorbers should be placed with the same thing in mind: their effecive frequency and the area in the room with high SPL measurements in that frequency.

(i think, please correct me if I'm wrong anywhere)

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hsal
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Quote:
We are not talking about Varitunes here, please don't go there. Let's stay focus on Limp Mass Absorber.

Kasmiras example was relevant since we are talking about pressure based absorbers.
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Quote:
Originally Posted by Tim Farrant
Over the years I have had great success fixing small room bass problems with limp mass absorbers. As there are so many threads here about this very problem, I have posted my design here for you to use. It's not that flash, but they do work if you follow the rules. The most common question is going to be, "how many do I need?". In most smallish rooms (like this one Time to give up on the 80Hz null? ) sometimes only 4 absorbers per problem frequency mounted at the corners will be effective and you can always add more if needed.

Cheers
Tim.

Edit, I have added a Excel frequency calculator, enter your mass and depth into the red areas and it will spit out approx frequency.
Ok then, I will ask questions to Tim then.

If I would have that room according to that link you are providing, what size would I build the trap and what is the target freq and how many do I need?
gullfo
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just some ideas on assembling one of these. if you use a pair of 1x1 frames and lay out the MLV between them, putting tension on the panel is easier, less likely to tear the rubber since you screw through the wood frames which provide a good amount of friction. then add the cloth and mount that assembly within the larger frame. trim the cloth and rubber to the 1x1 frame with a razor knife.
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DanDan
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Welcome

Great practical info, thanks Tim and Glenn.
This particular design doesn't seem to happen around here much.
I guess that BBC research discourages us from investigating the design.
As when any device or technique is advocated, it would be nice to see some examples of success. Do either of you, or anyone, have Before and After measures using these traps?
I would add to the recommendation about using sine waves to find the best spots to treat. Sometimes the middle of a boundary is hotter than the corner.
I am intrigued about the recommendation for these very small sizes though. Tim, if you could find your info source on that, I would greatly appreciate it.
EDIT, I note the BBC size is not particularly large either.
DD
8th July 2012
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Mctwins.
I have no idea who you are or what is happening to you..
But there is something very wrong here.

First.
Please try to show some respect for the people who do this for a living.
They give a lot of their time here and everyone know there are no magic secrets in this world.
You are interested in these subjects and are very eager to make your room as good as it can get.
I personally believe you think you have found the holy grail..
But that will never be the case..

Imagine how many people who have worked with these subjects a lifetime.
There are no magic products or secrets.
The holy grail will never exist..

If you think your room are very special I can tell you it is not.

Try to discuss these subjects with an open mind.
You have no secret weapon/answer and neither does anyone else.

Welcome to the club.....

Peace
gullfo
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DD - i don't have results in isolation by unit but i have a task to do some before and after with slats so i'll put it on my list of to-dos for one of my next design build-out.
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Quote:
Originally Posted by gullfo
just some ideas on assembling one of these. if you use a pair of 1x1 frames and lay out the MLV between them, putting tension on the panel is easier, less likely to tear the rubber since you screw through the wood frames which provide a good amount of friction. then add the cloth and mount that assembly within the larger frame. trim the cloth and rubber to the 1x1 frame with a razor knife.
Looking at the schematic you posted showing a luan back, didn't the BBC report say that they didn't have good success with a thin back and a more rigid back was needed? I read four or five BBC reports together-I might be mixed up.
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Quote:
Originally Posted by 2manyrocks
Looking at the schematic you posted showing a luan back, didn't the BBC report say that they didn't have good success with a thin back and a more rigid back was needed? I read four or five BBC reports together-I might be mixed up.
I am guessing you will be OK, when you mount the panel flat onto a wall. Also Glenn's design posted here is only 5" (6"-1" frame) deep and therefore goes not go as low frequency wise, unless you use some very heavy MLV.

Another difference I noticed is that the BBC (DD's link) filled the entire trap with fiber, which takes away the possibility for the wave to work against some dampening material. Therefore the "vibration" are transferred almost direct onto the back-wall of these traps.

Glenn's is a good design, and way better construction than "stapling" the membrane onto a frame. And you're always free to make your own adjustments to your needs. Me personally would replace the rigid panel with some less dense fiber (like pink fluffy, or R30; see Tim's PDF) in addition to a sturdier back. And I would use MDF for anything else but the membrane and fabric frame.

@Glenn: Thank you for posting. I am seriously thinking of building one (at least a prototype) or more of those.
Rod Gervais
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Tim, question here......

When you install the limp mass - do you tension it to any degree?

Or, to put it another way, exactly how limp is limp?

Rod
gullfo
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Quote:
Originally Posted by 2manyrocks
Looking at the schematic you posted showing a luan back, didn't the BBC report say that they didn't have good success with a thin back and a more rigid back was needed?
it's an example and depends on how you mount them. same for the depth. you could easily go deeper, or use denser MLV (or replace with thin wood or metal panels) and exchange the rigid for softer insulation etc.

you definitely want to run the gamut of calculations to make sure it will be in the ballpark when you build it and recognize the tension on the MLV changes the response of the device. in most cases you want the MLV to be tight enough that you get a nice thump like a LF kick drum.

for the back, if they are mounted on the wall or ceiling, then you don't really need the back. if you're putting them into a suspended ceiling grid or otherwise not against a solid surface you may find a denser backing panel is needed.

i would not staple MLV as it tends to rip fairly easily when you have limited surface contact (why i suggest the stacked frames). another option is to use the roofing nails with the rubber washers but this would usually be when attaching to 2-by lumber etc.
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If you make a "triangle" shaped frame like Primacoustic, for corner mounting, do you think the back MDF is necessary ?

For BBC question, if I remember correctly, they recommend a minimum of 10mm MDF back to avoid resonances problems.
MTB Vince
#24
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Quote:
Originally Posted by Rod Gervais
Tim, question here......

When you install the limp mass - do you tension it to any degree?

Or, to put it another way, exactly how limp is limp?

Rod
Great question Rod. I'm wondering exactly the same thing.

I recently bought a 25'x4' roll of 1 lb MLV and was hoping to experiment with adding it to the rear air space of my existing fibrous traps like the Primacoustic bass traps and I thought might also take a stab at this recipe. If I read the manufacturer's description and related threads correctly, Primacoustic describes the MLV being attached so as to be suspended limply but I'd love a better description from a user who has assembled one of the Primacoustic traps.

Happy Trails!
MTB Vince
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Quote:
Originally Posted by gullfo
it's an example and depends on how you mount them. same for the depth. you could easily go deeper, or use denser MLV (or replace with thin wood or metal panels) and exchange the rigid for softer insulation etc.

you definitely want to run the gamut of calculations to make sure it will be in the ballpark when you build it and recognize the tension on the MLV changes the response of the device. in most cases you want the MLV to be tight enough that you get a nice thump like a LF kick drum.

for the back, if they are mounted on the wall or ceiling, then you don't really need the back. if you're putting them into a suspended ceiling grid or otherwise not against a solid surface you may find a denser backing panel is needed.

i would not staple MLV as it tends to rip fairly easily when you have limited surface contact (why i suggest the stacked frames). another option is to use the roofing nails with the rubber washers but this would usually be when attaching to 2-by lumber etc.
Thanks.

If tearing the membrane is a concern, rolling it over the edges of the frame and attaching from the sides instead of the front would allow the friction of the edge to bear more of the tension instead of the staples. Roofing membranes are pulled over a roof edge to shed water, but it is also serves a mechanical purpose, too.
#26
9th July 2012
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Quote:
Originally Posted by Rod Gervais
Tim, question here......

When you install the limp mass - do you tension it to any degree?

Or, to put it another way, exactly how limp is limp?

Rod
Good question! (but a tricky one!)

And an excelent description by Gulfo

Exactlay how limp? This is a generic question because it deals with all membranes be it gypsum walls, ply, MDF, rubber, asphalt, plastic, limp mass and similar.

For the material only my guess is:
If you like an efficiant absorber find a material with an optimum relationship (key word) between limp/stiff and inherent mechanical loss factor. This job is hopefully allready done by the designer if you buy a comercial product. (A lot of practical testing involved)

Plywood, steel or rubber etc. are mehanically very different. Just a clue…

(I am not a physicist or structural engineer so I can’t give you figures or formulas. Young’s module etc.

Then we want to dampen the movement in a linear way by placing a porus absorber at some distance from the moving membrane inside this air tight box. Some designers say: very close and allmost fill the box, others say half of the box, low density mineral wool.

Jan Voetman (father of the SLAM absorber) Soft Layer Absorbent Membrane
says: Don’t stretch it. (Private communication) The tuning frequency will go up. (just like a kick drum) SLAM is meant to be "broadband" below 500 Hz

Best
UnderTow
#27
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The Primacoustic bass trap membrane is limp, no tension whatsoever. It is even slightly wider than the internal width of the enclosure so it isn't quite straight. It bulges a bit. I took pictures of the inside of one. Now I just have to remember where I put them...

Alistair
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UnderTow
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Found the pics. They are not very clear so I'm not sure this helps much be here goes.

Alistair
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Tim Farrant
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I'll try and answer the questions but first I should point out that acoustic design is not my main line of work. I became interested in it years back when control rooms that I worked in sounded so bad. I have done my own research and come up with a few ideas, I am not a qualified acoustics engineer and simply offer you what I have found to work in the rooms I have designed.

Using the limp mass seems to work well and takes up a lot less space than deep traps. I remember installing an array of these into the rear of a control room at Marmalade Studios. The builders were still hammering in the room and you could easily hear the bass reverb reduce as the boxes were covered with the membrane.

To be honest, I have never measured the response of a room in my life, waste of time IMO, too many variables. My method is to calculate the room modes (as best you can if the room is not symmetrical) and build the absorbers to target the dominant axial modes. This generally works. If you can do a tone sweep as has been suggested, this may be another way to find the target frequencies, but at the end of the day, the Q of the limp mass absorber is going to be fairly broad so an absorber tuned to 80Hz is still going to have an effect at other frequencies, so it's not that critical.

The beauty of limp mass is being able to absorb 50Hz with a unit only 1 ft deep.

To answer Rod's question, the membrane is not tensioned at all. I guess if you did, this would raise the Q of the unit rendering it more like a drum.

As to the quantity of absorbers required, I would probably decide this based on the room proportions and construction. If the room had favorable dimensions and smooth modes then it would require less absorption. If the room had very solid walls (ie for sound isolation) then more absorbers would be required as any diaphramatic action from the wall would be reduced. There is no easy answer to this question unfortunately. What is important to try and install them at all wall/wall/ceiling corners of the room to keep it even.

I also have a theory that whilst Helmholtz resonators are useful in the mid band, their use at low frequencies is questionable. I have read that 13th century Swedes used to use "jars" and "sound pots" to prolong the reverbration of dead theatres. These are basically high Q Helmholtz resonators. To my mind, having something that is singing in tune with what you are trying to eliminate does not make sense and something inert like the limp mass is preferred.

Cheers
Tim.
boggy
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Quote:
Originally Posted by Tim Farrant
....... I guess if you did, this would raise the Q of the unit rendering it more like a drum.
Yes, and tensioning will raise (change) frequency too...

Question:
Which type of (porous absorber)* you use in this traps?

Thank you very much for sharing your plans.

EDIT: *I mean "fiberglass" (type)
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