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Tim's Limp Mass Bass Absorbers
Old 17th March 2017
  #871
Gear Guru
 
DanDan's Avatar
Promising

Well done, it does look like success. Could you post the .mdat file here plse.
We could probably illustrate or prove the action better with some tweaking of the graph controls.

DD
Old 21st March 2017
  #872
Gear Head
 

Am about to make some LMAs and had a couple of questions.

Has anyone discovered an optimal density for the absorbent material in the box? I have loads of RW45 left over but am wondering if using a material this dense would compromise efficacy?

Can the boxes be bigger than the recommended 60cm x 40cm? If it doesn't affect performance it would be more useful to build boxes for the spaces I have.
Old 6th April 2017
  #873
Gear Head
 

Quote:
Originally Posted by YesSir View Post
maybe it really just needs more of them.. when i built my 24" x 19" test absorber it also showed almost no effect in the measurement. and i now have 4 with size 47" x 31.5" which is equal to the mass of 16 Tim's LMAs.
For positioning, play a sine wave on your targeted frequency (the 60 Hz in your case) with the tone generator, then go from corner to corner and stick your head in them, and where it is the loudest you put the LMAs.
I'm wondering if my first set didn't come out right. Although I didn't tension the material on purpose, I attached it with the box laying down. This caused it to sag a little in the box and it feels somewhat stiff.

I just completed 4 more in which I attached the material while the boxes were standing up. This causes the material to feel looser. I'll compare them and if these work out better I'll cut the MLV out of the old boxes and attach new MLV using my new method

Should know in the next few days...
Old 7th April 2017
  #874
Gear Nut
 
Freddie Flame's Avatar
 

If I would make a couple of 40 hz and a 80 hz traps as big as 50x200 cm (20x80 inches).
Would they be noticibly less effective compared to a lot of smaller ones, 40x60cm, covering the same space?
I´m gonna put them all on my front wall.
Old 7th April 2017
  #875
Gear Nut
 
Freddie Flame's Avatar
 

Also: Can I put a thick layer (1 foot) of rigid fibre glass in front of all the MLV traps, with an air gap, for absorbing higher freg?
Or would that lower the efficiancy of the MLV traps?
Old 8th April 2017
  #876
Here for the gear
 

Hello,

Since I'm trying to get my bass problems into order, I also came over this thread.
I am also thinking of doing LMA's, but I have a huge problem with my lowest mode, that is 26Hz.
Now here is the question concerning LMA:
To get to this frequency with an LMA, one would have to get a really heavy membrane. The only (wood) material I know of having enough weight is MDF at 19mm.
I suspect MDF 19mm would be way too stiff, correct?

If yes, any alternatives to battle 26Hz? Except filling the front wall of the room with 60cm Rockwool (my sub is behind me).
Old 10th April 2017
  #877
Gear Head
 

Quote:
Originally Posted by kosta88 View Post
Hello,

Since I'm trying to get my bass problems into order, I also came over this thread.
I am also thinking of doing LMA's, but I have a huge problem with my lowest mode, that is 26Hz.
Now here is the question concerning LMA:
To get to this frequency with an LMA, one would have to get a really heavy membrane. The only (wood) material I know of having enough weight is MDF at 19mm.
I suspect MDF 19mm would be way too stiff, correct?

If yes, any alternatives to battle 26Hz? Except filling the front wall of the room with 60cm Rockwool (my sub is behind me).

No. 19mm MDF is the way to go!
Make it big and tunable. Staple the MLV on the face of the box. And youre good to go.
Old 10th April 2017
  #878
Here for the gear
 

Quote:
Originally Posted by Claudio Ribeiro View Post
No. 19mm MDF is the way to go!
Make it big and tunable. Staple the MLV on the face of the box. And youre good to go.
Thanks for the answer.
How big? The position is what dictates how big... I was thinking about 1x1m, but that depends on position...
So what do you mean "on the face of the box"? I should place it in front of the subwoofer?
And how to make it tunable?
Old 10th April 2017
  #879
Gear Head
 

Quote:
Originally Posted by Claudio Ribeiro View Post
First of all, i would like to give a special thanks to Tim Farrant for all the info you made available on the subject. Also thanks to all the nice input from the participants.

My room dimensions are with a 2.1 speaker system:
L 3.15 W 2.85 H 2.73

I've made 2 TLMBA 60x40cm, 19mm mdf, 16mm depth, entended to deal with the first mode in my room 61.2hz. I used a 6kg/m2 membrane

Being afraid of some kind of error in calculation, i've manage to make the depth with 25,5 cm so i can make fine adjustments.
The dumping material was rockwool 45kg 6 cm on the back.
The position of the TLMBA is side/frontwall's floor.

The results were very good. 3db's each = +/- 6 db's attenuation

After the last release of roomeq rew v5.1 beta 21 I'm been tweaking speakers and sub positions. What a can say is that i've found the sweet spot for them.


Next move:
In a week i will receive a usb calibrated measure microphone and comprove the new placement with measures.
Start adding absorber material on the walls to see if it match the predicted values in few room slim option.
Build 1 more TLMBA or if the 113 mode don't desapear with absorbent material build 3 more.

Thanks once again.
Tell me what you think

ps. the empty room measures have different spl. sorry newbie here

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.

As soon as i get the new mic i'll do a bunch of new accurate measures to clear things up.

To see all information available on the membrane i've used, click on my first post in the membrane. there's a link on it.
I've paid 38€ 100X800cm. Pretty cheap, no?!



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

Quote:
Originally Posted by DanDan View Post
The level difference makes comparison difficult but I think your original assessment of about 7dB was about right Claudio.
Attachment 406642
The RT60 also does indeed show great improvement. As do the Waterfalls when they are sort of 'equalised' .
I think your (rare) success is probably due to your very clever tuning in situ.
Well done.
Attachment 406640

Attachment 406641

DD

Quote:
Originally Posted by kosta88 View Post
Thanks for the answer.
How big? The position is what dictates how big... I was thinking about 1x1m, but that depends on position...
So what do you mean "on the face of the box"? I should place it in front of the subwoofer?
And how to make it tunable?

Space limitation's you mean. For such frequencies go at lest with 120x80~100cm

https://www.gearslutz.com/board/10236831-post702.html


https://www.gearslutz.com/board/10239199-post705.htmlhttps://www.gearslutz.com/board/10239199-post705.html
Old 10th April 2017
  #880
Gear Head
 

Old 10th April 2017
  #881
Here for the gear
 

Old 10th April 2017
  #882
Gear Head
 

Quote:
Originally Posted by kosta88 View Post
Claudio, I think we are misunderstanding each other:
When I said MDF 19mm, I meant the MDF as a MEMBRANE. You also write you used 9kg/m2 membrane, but the link is dead.
I misunderstood you. I've used a 6kg/m2 MLV.
Old 13th April 2017
  #883
Gear Nut
 
Freddie Flame's Avatar
 

The only kind of MLV material I can find in Sweden is "Auralex sheetblok", which their support calls "Mineral filled EVA" when asked.

Auralex Acoustics | SheetBlok™ Sound Barrier

Anybody knows if it will work well?
I do not understand the density either: *+1#/sq. ft.
What does that mean? Can anybody translate it to kg per square meter?
Old 13th April 2017
  #884
NLP
Gear Addict
 
NLP's Avatar
 

I think cca. 4.88kg/m2.
Old 13th April 2017
  #885
Gear Nut
 
Freddie Flame's Avatar
 

13 mm drywall has almost twice the mass compared to the Auralex sheetblok. 9kg/m2 vs 5kg/m2.
Is drywall a good alternative to MLV when it comes to lower freq?
Old 13th April 2017
  #886
Gear Addict
 

Freddie, there is no need to go looking for MLV. You can just as well use rubber sheeting. What matters is the mass / m². Another thing is the "limpness". If you knock on a larger sheet of drywall, MDF or plywood, you will hear a ringing tone / resonance. If you knock on large sheet of rubber or MLV, you will hear more of a "thud", no resonance of its own to speak of, as the material has a much better inner damping. Another good thing is if the membrane is as free to move back and forth as possible. In a pistonic way, -like the membrane on a bass driver. With hard sheets, screw fixed along the edges, you will have less of pistonic movement. Less movement = less effectiveness.

(Make a testbox and measure how it performs frequency wise before you make the actual absorber boxes for your room.)
Old 15th April 2017
  #887
Gear Nut
 
Freddie Flame's Avatar
 

According to this article one should change the formula by ingreasing the numerator with 34% when using plywood instead of MLV.
Is that correct?

| Acoustic Sciences Corporation
Old 21st April 2017
  #888
Here for the gear
 

mdat, finally

Quote:
Originally Posted by DanDan View Post
Well done, it does look like success. Could you post the .mdat file here plse.
We could probably illustrate or prove the action better with some tweaking of the graph controls.

DD
hey DanDan, here they are (bit delayed, sorry for that).
Curious to see what you read out of them!

Cheers
Attached Files
File Type: mdat Empty.mdat (147.1 KB, 22 views) File Type: mdat 4 LMA.mdat (145.4 KB, 22 views)
Old 21st April 2017
  #889
Gear Guru
 
DanDan's Avatar
Great

Great result.
DD
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-good.jpg  
Old 22nd April 2017
  #890
Gear Head
 

Quote:
Originally Posted by YesSir View Post
hey DanDan, here they are (bit delayed, sorry for that).
Curious to see what you read out of them!

Cheers
Looks really great.
Could you post some pictures of your build?
Would be interesting to know how you mounted the limp membrane.
Old 25th April 2017
  #891
Here for the gear
 

Hi!

I'm in the process of building some LMA's.

I've made the boxes from 16mm MDF, MLV = 5kg/m2, depth 10.3cm, and the dimensions is 100x45cm, since that fits my studio.
Now i'm unsure if the 100x45cm membrane is to big, or if i have to divide the box into 2x 50x45cm?
and if so, is it necessary to divide the whole box, or can i just divide the MLV with a strip of MDF, and keep the box as one volume?

I've also bought some heavy MLV, 15kg/m2, which i intent to use as an absorber tuned at 32Hz. The sheets are 125x150cm - would you use them as one big membrane, or divide it into 2 times 125x75cm ?

Br
Kenneth
Old 5th May 2017
  #892
Here for the gear
 

Hey there

Has anyone (in the UK) had any luck building these traps using tecsound 100?

I have a target node of 37.5 Hz and I'd like the traps not to dominate the space.

Tecsound 100 is 10kg/m2, so using Tim's spreadsheet, I can get away with a 25cm enclosure. However I'm concerned at that thickness the MLV becomes a bit stiff for the purpose, or maybe thats an advantage when targeting such a low frequency...

Who knows? Probably Donald Trump, but he won't answer my tweets.

Also any comments on the density of rockwool which works best on the back panel appreciated. I'm guessing porous rather than thick.

Last edited by ollytheoctopus; 5th May 2017 at 07:16 PM..
Old 9th May 2017
  #893
Here for the gear
 

Rubber Limp Mass mounting

Quote:
Originally Posted by JochenM View Post
Looks really great.
Could you post some pictures of your build?
Would be interesting to know how you mounted the limp membrane.
Hi Jochen, here you go! There's a little sketch i drawed, for better understanding of the process.(second pic shows the finished LMAs, 41 Hz Box on top of 39 Hz in both front corners, between the ER Broadband Abs. Best position for my room...)

I used a permanently flexible glue, used in car and ship building, in german we call it "karosseriekleber", and some really big clamps (45+ cm). Note that i mounted the Limp mass directly onto the box and not into an inner frame, like Tim suggests. The size of the box is crucial, no possibilities to adjust depth after the wooden box is assembled!
Start with cutting the Limp Mass sheet a bit bigger than the box, you can easily cut it at the borders after the mounting if you are using SBR-rubber.
Then, fixate the Limp mass with a straight wooden slat and two clamps onto the box, so that the top area stays free for glueing.
Apply the glue to the area on the box above the first slat that holds the Limp mass, and clamp the rubber to the glued area with a second slat.
It is very important that you don't apply too much pressure with the clamps to the glued part, cause the glue has to build a proper layer in the sandwich between the rubber and the frame and should not be squeezed out again (glue layer should be at least 1 mm thick after clamping). Then, let it dry for at least 12 hrs.
After that, continue with the sides one by one (no need to fixate the rubber anymore, just do step two, glueing and gentle clamping with a slat), and finally, turn the box upside down for the last, bottom part.
As you can see, this is quite a time-consuming way to do it, glueing took me 2 weeks for 4 boxes, but it was worth it...
hope that explains my process well, feel free to ask questions!

Cheers
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-img_20170424_014112.jpg   Tim's Limp Mass Bass Absorbers-img_20170422_174749.jpg  
Old 9th May 2017
  #894
Here for the gear
 

Quote:
Originally Posted by DanDan View Post
Great result.
DD
Thanks, DanDan!

Cheers
Old 14th May 2017
  #895
Here for the gear
Following an early point Rod Gervais made about manufactures test data (and the lack thereof) here's my 2c's worth...

A moot point. Experimental validity and reliability can only be assured in mutually agreed standardised test environments. That way evidence supporting and refuting performance characteristics can be viewed with confidence.

But, 'no plan survives in the trenches' so for those without a formal acoustics training and who want to do go the DIY route, it is important to gain a good working knowledge of acoustics, spend time researching and planning the out the room design, combine room analysis, rules of thumb such as the 38% 'rule' or SBIR and mode analysis tools and explore the results from other examples and take advantage of the incredible level of advice and teaching available from the pro's. Then it's buy/build, experiment and test-retest in situ.

Of course, that's why we end up with all these extended discussions: lack of empirical measurement and standardisation, subject complexity, differing expert views, and lots of naïve, passionate (pathological?) motivation by do-it-yourselfers in non-standardised contexts.

How hard can it be!!! Gotta-love-it!

Thanks to all

G
Old 14th May 2017
  #896
Here for the gear
Following an early point Rod Gervais made about manufactures test data (and the lack thereof) here's my 2c's worth...


A moot point. Experimental validity and reliability can only be assured in mutually agreed standardised test environments. That way evidence supporting and refuting performance characteristics can be viewed with confidence.

But, 'no plan survives in the trenches' so for those without a formal acoustics training and who want to do go the DIY route, it is important to gain a good working knowledge of acoustics, spend time researching and planning the out the room design, combine room analysis, rules of thumb such as the 38% 'rule' or SBIR and mode analysis tools and explore the results from other examples and take advantage of the incredible level of advice and teaching available from the pro's. Then it's buy/build, experiment and test-retest in situ.

Of course, that's why we end up with all these extended discussions: lack of empirical measurement and standardisation, subject complexity, differing expert views, and lots of naïve, passionate (pathological?) motivation by do-it-yourselfers in non-standardised contexts.

How hard can it be!!! Gotta-love-it!

Thanks to all

G[/QUOTE]
Old 14th May 2017
  #897
Here for the gear
A comment on empirical data and context

Following an early point Rod Gervais made about manufactures test data (and the lack thereof) here's my 2c's worth...

(Can't insert the quote, !)

A moot point. Experimental validity and reliability can only be assured in mutually agreed standardised test environments. That way evidence supporting and refuting performance characteristics can be viewed with confidence.

But, 'no plan survives in the trenches' so for those without a formal acoustics training and who want to do go the DIY route, it is important to gain a good working knowledge of acoustics, spend time researching and planning the out the room design, combine room analysis, rules of thumb such as the 38% 'rule' or SBIR and mode analysis tools and explore the results from other examples and take advantage of the incredible level of advice and teaching available from the pro's. Then it's buy/build, experiment and test-retest in situ.

Of course, that's why we end up with all these extended discussions: lack of empirical measurement and standardisation, subject complexity, differing expert views, and lots of naïve, passionate (pathological?) motivation by do-it-yourselfers in non-standardised contexts.

How hard can it be!!! Gotta-love-it!

Thanks to all

G
Old 5th June 2017
  #898
Here for the gear
 

Quote:
Originally Posted by garyhb View Post
Following an early point Rod Gervais made about manufactures test data (and the lack thereof) here's my 2c's worth...

G
A moot point, indeed.. no hard feelings, but what are you trying to share with us? (and why three times in a row? )
Old 2 weeks ago
  #899
Gear Maniac
 

Well, this has certainly been interesting, but there is something that I have been wondering about.

What happens a few years down the road when gravity and other environmental factors have had its way with the membrane?
Old 1 week ago
  #900
Quote:
Originally Posted by FLZapped View Post
Well, this has certainly been interesting, but there is something that I have been wondering about.
This is a very good question; especially if one has the intention of hanging one these vertically (I have a pointy ceiling and wish to face the membrane directly downward). Will MLV stretch over time? Seems likely.
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