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Tim's Limp Mass Bass Absorbers
Old 11th March 2018
  #931
Gear Head
 

Hi there,

I thought I share my experience with my first LMA. After reading this hole thread and one on a different forum I was amazed about the power regarding the size this type of absorber has. I got some experience building a wooden panel absorber years ago so I'm not complete new to pressure based absorbers. I decided to build a big one first (120cm*80cm*21cm / 47.24" x 31.49" x 8.26" ). If that fails I could split it into two units. Which I will do...

Room Size is 349 x 409 x 257 (11,45' x 13,41' x 8,43'). Already a good amount of treatment on the front part of the room.

I needed to aim at the 41,1hz mode which bugs me even I equated the peak down. The decay remains of course.
I used the 510 formula not really sure if that would do it better or not. Actually because the depth build was smaller. I glued the rubber to a 1cm frame with Acryl which I wouldn't do again, cause the rubber weight is way too high for the stuff. Took me several days drying and redoing it..


SBR Rubber 9,66kg/m²
18mm mdf wood for the hole frame and backwall
weight is insane


Finally build the thing. With RTA running I knocked on the rubber sheet and got a resonance freq I was aiming to. Great!

Well thats the only great part of the story... To have some room for adjustments I made the frame depth a bit bigger. On some spots in the room which had measurable high pressure (and were spots where I only can suit such big sizzed modul) it made 0 difference changing the build depth from 5cm to 20cm. No luck. I was depressed somehow
The front opening was sealed and the backpanel was so tight I had a hard time to adjust it. So I rule out the air tightness. Only thing could be the limp factor or the low depth of the 510 formular (which differ by around 3cm in depth). In this room its kinda creepy whats going on with the 41hz mode. Sometimes it measures way less even I didn't change anything that crucial.

Anyway. After a long day measuring I gave up. For now the LMA stands in the middle of the backwall which was the only spot which had any change and the change was on good side.

Looking at the spectrogramm or decay there is a change and well its ok. Sounds a tad less on E notes but I would say it sounds like day and night difference. Not the holly gray for sure. I'm going to split the one into two parts now and test on.

After the result is less what I hoped for. How limp is your rubber? Is mine to limp maybe?
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-img_8018.jpg   Tim's Limp Mass Bass Absorbers-img_8021.jpg   Tim's Limp Mass Bass Absorbers-img_8015.jpg   Tim's Limp Mass Bass Absorbers-left_right_wo_lma_3.jpg   Tim's Limp Mass Bass Absorbers-left_right_w_lma_3.jpg  

Tim's Limp Mass Bass Absorbers-left_right_wo_lma_2.jpg   Tim's Limp Mass Bass Absorbers-left_right_w_lma_2.jpg   Tim's Limp Mass Bass Absorbers-left_right_wo_lma_1.jpg   Tim's Limp Mass Bass Absorbers-left_right_w_lma_1.jpg  
Old 17th April 2018
  #932
Here for the gear
 

Forgive me if this has already been asked but what would happen if I used two unbonded layers of MLV in the same box? For example, I am looking at building boxes for 38 hz and 85 hz. I have calculated I can use the same box depth of 8 inches for both frequencies but I will need to use 2.5 lbs/sqft MLV for the 38 hz box and I will need to use 0.5 lb/sqft MLV for the 85 hz box. What would happen if I used 0.5 lb/sqft MLV on the 38 hz box and then put a thin spacer around the perimeter and added 2 lb/sqft MLV over it? Would that work? Or would it end up working for both 42hz [(170/((2*8)^(1/2))] and 85 hz (assuming spacer is very thin)? Either way, this could be a cool benefit right?
Old 18th April 2018
  #933
Lives for gear
 
mirochandler's Avatar
My experience is that too heavy rubber will not work. Maybe for very low freq, but in that case other solutions are better.
I made good results with ~3kg/m² on 20cm depth:
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-unbenannt.jpg  
Old 18th April 2018
  #934
Gear Head
 

Quote:
Originally Posted by mirochandler View Post
My experience is that too heavy rubber will not work. Maybe for very low freq, but in that case other solutions are better.
I made good results with ~3kg/m² on 20cm depth:
Ok. What designfreq. is aimed for and what formular you used to calculate the depth (600 or the 510 value)? Did you place the Limpy in corner?
Old 18th April 2018
  #935
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mirochandler's Avatar
It was not designed for a specific frequency; it was only a test series.
The picture should show that it really works when material-weight and depth is in a good ratio.
Even if a calculator gives a nice result, you have to find a ratio that works.
Same with Helmholtz absorbers.
Old 19th June 2018
  #936
Here for the gear
 

Hi again!

I had great results treating my former room with four 80 x 120 cm LMAs (see earlier in this thread).
However, landlords daughter suddenly wanted to move into our house, and we had to leave (bit of a bummer, finished the studio 6 months earlier
Now i'm building my new CR and since i had to change the LMA tunings anyway and they were rigid, i decided to make them permanently tunable, following claudio's build:

Quote:
Originally Posted by Claudio Ribeiro View Post
Hi nms and DanDan,
Thanks for your input.
Nms, i've made them 25,5 cm deep to be able to play around and fine tune them. The system was simple. 2 screws on the back to pull / push back and forward the rigid back (see photos with location and details)
if we consider the depth with the rigid back, the measure is 9,5 cm wich left us with 16 cm depth.
I've a question wondering in my mind. If the excess depth in the box influence the resonance frequency? If you know some information, good. Otherwise thats what we will find out in the next constructions trimmed closer the the predicted measure and placed as you suggested. Size to.


DanDan,
you're also right. I tried to dial in the numbers but I'm not confident with the results. I'm afraid that the promising report i gave was incorrect. it shows around 2 - 2,5 db attenuation each. FILES
At least looks like in RT30 the values have been enormously reduced, and sounds like.
It's impressive after deal with the first mode, everything sound so much better.
Is there a relation?

Cheers
Right now, i tuned two boxes to 42.5 Hz (the first side wall mode) and i'd like to share the results with you guys. Interesting that, although the 50Hz Mode didn't show itself in the waterfalls as a clear long ringing (not in the listener position nor in the corners where the LMAs are placed, most likely because of interferences with other first modes), the 50 Hz LMAs do a great job on the SPL!

Here are some Pics & Graphs, i will keep you updated on the built of the other LMAs. For modal measurement i used a subwoofer with frequency range 20 -120 Hz, in case you wonder why the SPL suddenly drops at 120

EDIT: The measurement with two LMAs (Red) is corrupt due to change of the sub position.
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-spl-green-no-lma-purple-1-lma-red-2-lma.jpg   Tim's Limp Mass Bass Absorbers-waterfall-no-lma.jpg   Tim's Limp Mass Bass Absorbers-waterfall-1-x-50-hz-lma.jpg   Tim's Limp Mass Bass Absorbers-waterfall-2-x-50hz-lma.jpg   Tim's Limp Mass Bass Absorbers-img_20180619_141457.jpg  


Last edited by YesSir; 11th July 2018 at 07:25 PM.. Reason: Corrupt Measurement (red)
Old 30th June 2018
  #937
Gear Nut
 
jaakkol's Avatar
 

Hi Tim! Thnx for this awesome post, will def try to solve my studios bass problems with this

One question: you mention that the size of the box is not toi relevant. Thing is, I found this limp mass absorber 7,5kg/m2 which they sell in 40x100cm pieces. If say I try to absorb 43hz, could I make two 40x50cm bokses out this one sheet? Would be money efficient

Thnx!

Quote:
Originally Posted by Tim Farrant View Post
Over the years I have had great success fixing smallish 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 instructions. 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? - Gearslutz Pro Audio Community ) 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.

Edit; I get asked a lot of questions about these so here are a few answers to the commonly asked questions and some points to consider.

1] These traps are intended to tame room modes, so measure your room dimensions and calculate the modes. Measuring with microphones does not necessarily identify problematic modes.

2] These traps may not work in small rooms where the largest dimension is less than 4 meters. A lumpy low end frequency response might be caused by other factors.

3] If you are only going to make just 2 traps, make the outside dimensions (area) larger than shown. Ideally, making a lot of smaller traps with different depths is preferred and 2 small traps might do very little. One trap in all 4 ceiling/wall/wall junctions might be a better way to go.

4] The direction the membrane is facing is in the room does not appear to be critical.

5] In small rooms, some experimentation in placement would be required. Try on the floor in the corner behind your monitors.

6] The idea of mounting them into corners in a cluster as I have shown is that the corner is effectively eliminated, it does not exist anymore, and has been replaced by a low frequency diffusive element (the boxes) as well as the membrane absorber.

7] The membrane across the front should remain limp, do not tension it. Any tension will shift the target frequency and raise the Q.
Old 11th July 2018
  #938
Here for the gear
 

Quote:
Originally Posted by jaakkol View Post
Hi Tim! Thnx for this awesome post, will def try to solve my studios bass problems with this

One question: you mention that the size of the box is not toi relevant. Thing is, I found this limp mass absorber 7,5kg/m2 which they sell in 40x100cm pieces. If say I try to absorb 43hz, could I make two 40x50cm bokses out this one sheet? Would be money efficient

Thnx!
Hey jaakol!

I calculated the overall dimension using the same ratio tim did, which is 2/3 (width/height). 80x120 is the same ratio as 40x60, so one of mine has the same surface as four of Tim's LMAs.
I don't know what happens if you use different ratios and smaller sizes than Tim did, but i could imagine that smaller surfaces at a certain point stop to work on lower frequencies.
With "limp mass absorber" i assume you mean the material you use, the limp mass;
if you have difficulties finding the right material, maybe try finding some supply for rubber floor mats / floor protection. I (and others) used SBR rubber mats, important is that it behaves equally limp as rubber does (and that you know the weight).

Cheers

Yessir
Old 15th August 2018
  #939
Gear Head
Quote:
Originally Posted by YesSir View Post
...
I calculated the overall dimension using the same ratio tim did, which is 2/3 (width/height). 80x120 is the same ratio as 40x60, so one of mine has the same surface as four of Tim's LMAs. ...
Congrats on the success with your LMA's. Couple of questions for you.
  1. Since you used a 4-in-1 cabinet design, did you put dividers in the middle to make 4 internal cavities, each being the same size as Tim's?
  2. If so, did you also glue the membrane to the + divider edges as well ?
  3. What is the depth of your lowest LMA's? I have used both Tim's algorithm and the Acoustic Modelling - Home Page calculator and they come up with slightly different depths.

I have 2 room modes compounding at 40Hz that I would like to tame.

Also, FWIW, someone posted a diagram of gluing up the membrane earlier in the thread that might make things go faster. They laid the membrane out flat on the assembly table (surface must be flat). They added glue to the face edges of the box and then lowered the box, facing down on the membrane. One could add weights on the back of the box in leu of clamps during the dry time. The membrane should be flat with no ripples or stretches using this method.

Thanks much and congrats on your success.
Old 15th August 2018
  #940
Gear Head
Anyone what to own up to this build using sheet metal sandwiched between 2 membranes ??? Interesting design, but no results given.



Old 16th August 2018
  #941
Here for the gear
 

Quote:
Originally Posted by ths61 View Post
Congrats on the success with your LMA's. Couple of questions for you.
  1. Since you used a 4-in-1 cabinet design, did you put dividers in the middle to make 4 internal cavities, each being the same size as Tim's?
  2. If so, did you also glue the membrane to the + divider edges as well ?
  3. What is the depth of your lowest LMA's? I have used both Tim's algorithm and the Acoustic Modelling - Home Page calculator and they come up with slightly different depths.

I have 2 room modes compounding at 40Hz that I would like to tame.

Also, FWIW, someone posted a diagram of gluing up the membrane earlier in the thread that might make things go faster. They laid the membrane out flat on the assembly table (surface must be flat). They added glue to the face edges of the box and then lowered the box, facing down on the membrane. One could add weights on the back of the box in leu of clamps during the dry time. The membrane should be flat with no ripples or stretches using this method.

Thanks much and congrats on your success.
Hey there!

1. No, just one big box the size of four. If you consider putting in dividers (should work as well), i suggest you could as well build four small boxes cause they are more flexible Placement-wise.
2. With dividers in there, it would make sense to glue the Membrane to the dividers as well, otherwise they would be senseless.
One Problem i came across while gluing is the gravitary pull of the Membrane, i thought of a divider-solution as well but was finally able to handle it without.
3. Do you mean the formula i am using for depth calculation?
510 / root(kg * depth cm)
This formula only applies if you fill 20% of the cavity with low density rockwool. Without any rockwool, it would be 600 instead of 510.

And for the gluing question: i used a Silicone-type glue that should not be squeezed out when applied, its supposed to build a layer between the wood and the limp mass. Lowering the box onto the glue would apply too much pressure.
Check my earlier post for Details here:Tim's Limp Mass Bass Absorbers

Let me know how it works out!

Cheers
Old 16th August 2018
  #942
Gear Head
Quote:
Originally Posted by YesSir View Post
3. Do you mean the formula i am using for depth calculation?
510 / root(kg * depth cm)
This formula only applies if you fill 20% of the cavity with low density rockwool. Without any rockwool, it would be 600 instead of 510.
Thanks for the answers.

The formulas I was referring to was Tim's original formula and the Multi-layer Absorber modeling calculator that is used/referenced from time to time here on GearSlutz.

http://www.acousticmodelling.com

Also, FWIW, the internal dividers would add some structural consistency to edges to prevent bowing over time.

I was considering making doubles because that size would coincidentally fit in my locations and save some construction time over singles. Quads would be too big for my application.
Old 30th November 2018
  #943
May I revive this thread?
I found this interesting read
https://asa.scitation.org/doi/pdf/10.1121/2.0000816
Old 11th May 2019
  #944
Here for the gear
 

Thinking myself of making some Limp Mass absorbers, (actually trying to figure out a strategy for a rebulkd since I have moved and left my treated room behind) to get generically at frequencies below eg., 150Hz. I was thinking to make a range of them like targeted at 50Hz, 60Hz, 70Hz, 80Hz, 100Hz, 120, 140Hz, maybe some extra few aimed at the theoretcal modes seen in the Amroc table for modes that come up for any one room measurement.

But I noticed at GIK acoustics, that they build theirs aimed at 1/3 octaves:
40, 50, 63 and 80 Hz. http://www.rpgeurope.com/products/pr...dexmodule.html

I can see a list of 1/3 octave frequencies at http://www.unece.org/fileadmin/DAM/t...QRTV-09-05.pdf

What is going on with 1/3 octaves that makes them so important to target?

Thanks
Old 11th May 2019
  #945
Quote:
Originally Posted by romus View Post
Thinking myself of making some Limp Mass absorbers, (actually trying to figure out a strategy for a rebulkd since I have moved and left my treated room behind) to get generically at frequencies below eg., 150Hz. I was thinking to make a range of them like targeted at 50Hz, 60Hz, 70Hz, 80Hz, 100Hz, 120, 140Hz, maybe some extra few aimed at the theoretcal modes seen in the Amroc table for modes that come up for any one room measurement.

But I noticed at GIK acoustics, that they build theirs aimed at 1/3 octaves:
40, 50, 63 and 80 Hz. http://www.rpgeurope.com/products/pr...dexmodule.html

I can see a list of 1/3 octave frequencies at http://www.unece.org/fileadmin/DAM/t...QRTV-09-05.pdf

What is going on with 1/3 octaves that makes them so important to target?

Thanks
Regarding the 1/3 octave spacing might have to do with the limp membranes broad absorption range. So its not so important to hit a small frequency area. In my room i mainly targeted 30, 60 and 90 Hz and it worked pretty well...
Old 12th May 2019
  #946
Here for the gear
 

Quote:
Originally Posted by JuelzB View Post
Regarding the 1/3 octave spacing might have to do with the limp membranes broad absorption range. So its not so important to hit a small frequency area. In my room i mainly targeted 30, 60 and 90 Hz and it worked pretty well...
Ah, yes, I think this might be it -- Tim Farrant said they are effective for half an octive below and one octive above the target frequency, so it you step up in Octaves, then the cross over of the different depths will be distributed evenly (rather than just going mathematically, 40-50-60-70.

The RPG ones are third octave centre frequencies 40, 50, 63 and 80 Hz which is:
40 range 20 - 60
50 range 25 - 75
63 range 32 - 92
80 range 40 - 120
That's a lot of cross over, maybe they concluded this ends up as close enough to get a very even absorption.
Old 13th May 2019
  #947
Here for the gear
 

Limp mass absorber with tensioning

Would this work so you can tension them with some control, for building, but also after measuring your room? It's not a perfect way to do it but it is a simple idea.

The way I am thinking this would work, is:
Screw one top and one side bracket, then tension and hold the other side and top with a clamp, then test if you are happy with the tension setting. Adjust as necessary, then screw in to hold.

Read about tensioning it at this article, under the section, “Adding the damping factor”. So, the MLV should be attached firmly, but not stretched. You should be able to thump the finished panel of MLV with the heel of your hand so there is just the initial overshoot with a very small and slow spring back and then the energy is gone. It’s a thump, not shuddering with vibration, and it’s not saggy; it doesn’t twang, it sounds dead.


Also I read, I think in the BBC literature, that adding dense foam at the join significantly helps the vibration flexibility of the MLV more uniformly, and I think this would work if you are taking the MLV over the edge like the pics.
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-mlv.png   Tim's Limp Mass Bass Absorbers-limp-mass-absorber-tention-2.jpg  
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