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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 3 weeks ago
  #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 3 weeks ago
  #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 3 weeks ago
  #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 2 weeks ago
  #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 2 weeks ago
  #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 2 weeks ago
  #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 2 weeks ago
  #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 2 weeks ago
  #880
Gear Head
 

Old 2 weeks ago
  #881
Here for the gear
 

Old 2 weeks ago
  #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 2 weeks ago
  #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 2 weeks ago
  #884
NLP
Gear Addict
 
NLP's Avatar
 

I think cca. 4.88kg/m2.
Old 2 weeks ago
  #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 2 weeks ago
  #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 1 week ago
  #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 1 week ago
  #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, 5 views) File Type: mdat 4 LMA.mdat (145.4 KB, 6 views)
Old 1 week ago
  #889
Gear Guru
 
DanDan's Avatar
Great

Great result.
DD
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-good.jpg  
Old 6 days ago
  #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 4 days ago
  #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
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