Tim's Limp Mass Bass Absorbers
Tim Farrant
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#511
17th December 2012
Old 17th December 2012
  #511
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Thread Starter
Quote:
Originally Posted by Bilou View Post
Hi,

I just made a SketchUp model of Tim's example 1.
(Tim, correct me if I made mistakes in it)

I find a little bit strange to turn the 50Hz traps toward the floor, as 50Hz is the length room mode. Wouldn't it be more effective if turned toward the back of the room ?

As many room have doors in corners, do you think there might be some stereo or phase issues treating only 3 corners ?
Remember at low frequencies it's not like shining a torch, so the panels do not need to "face" the offending direction. What you have drawn is essentially the idea, but do not conceal the face of the trap with another box, as you have shown in your drawing behind the speakers. You need slide the A traps on the front along a little to expose the face. This method effectively "kills" the corner.
Tim Farrant
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#512
17th December 2012
Old 17th December 2012
  #512
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Thread Starter
Quote:
Originally Posted by jeremycox View Post
Im not sure if its as straight forward as this. You need to get the traps in the room and move them around while measuring to get the best results.
Yes, it is as simple as this, just because it's "acoustics" doesn't mean it needs to be complicated! The idea is to replace the hard surface of the corner with a floppy surface.
#513
17th December 2012
Old 17th December 2012
  #513
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When you say 'kill', do you mean the corner is left empty behind the 2 boxes ?

In my design, the boxes are touching each other, but in fact I always screw a 1x1cm wood stud on the front panel sides to stretch the fabric not touching the membrane... In this case, will this work ? Could it be better than spacing the traps ?
#514
17th December 2012
Old 17th December 2012
  #514
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AwwDeOhh's Avatar
 

Quote:
Originally Posted by Tim Farrant View Post
Yes, it is as simple as this, just because it's "acoustics" doesn't mean it needs to be complicated! ...
Blasphemy!

Quote
1
#515
17th December 2012
Old 17th December 2012
  #515
Quote:
Originally Posted by Tim Farrant View Post
Yes, it is as simple as this, just because it's "acoustics" doesn't mean it needs to be complicated! The idea is to replace the hard surface of the corner with a floppy surface.
I agree with this sentiment. I just meant that you might get better results by trying to drive the room modes with a speaker and moving the traps around. Either way, this process doesnt need to be too complicated.
Tim Farrant
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#516
18th December 2012
Old 18th December 2012
  #516
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Thread Starter
Quote:
Originally Posted by Bilou View Post
When you say 'kill', do you mean the corner is left empty behind the 2 boxes ?

In my design, the boxes are touching each other, but in fact I always screw a 1x1cm wood stud on the front panel sides to stretch the fabric not touching the membrane... In this case, will this work ? Could it be better than spacing the traps ?
Yes, the edges of the boxes touch each other and the corner behind is a void. You could fill it with fluffy absorbent if you wish. I do not quite follow you about the 1x1 timber, but it does not matter if the fabric touches the membrane
Quote
1
#517
18th December 2012
Old 18th December 2012
  #517
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(sorry, poor English)

Hi Tim,

I corrected the SketchUp to your recommendations.

But, as the corner is the best place for pressure, I was wondering if it wasn't better to place one of the traps in the corner, and the other one space 1 or 2cm. Will this prevent the first to vibrate correctly ?

Attached Thumbnails
Tim's Limp Mass Bass Absorbers-room-1-3d-v2.jpg   Tim's Limp Mass Bass Absorbers-mounting-1.jpg   Tim's Limp Mass Bass Absorbers-mounting-2.jpg  
Attached Files
File Type: skp Room 1 - 3D.skp (1.45 MB, 29 views)
#518
18th December 2012
Old 18th December 2012
  #518
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Naturally, I'd recommend testing both of your sketch orientations and listening (would not expect a huge difference) and also try stacking them.
#519
18th December 2012
Old 18th December 2012
  #519
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Quote:
Originally Posted by Syncamorea View Post
Naturally, I'd recommend testing both of your sketch orientations and listening (would not expect a huge difference) and also try stacking them.
Plus one. I would think in most cases stacking would be the way to go. Every room and problem can change things so always best to test to make sure.
rez
#520
19th December 2012
Old 19th December 2012
  #520
rez
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Quote:
Originally Posted by Syncamorea View Post
Do you know or can you measure the density?

Do you know the composition?
sorry i do not know the exact composition, but it looks, smells and feels like rubber. the density is estimated around 700kg / m3
it is a thing called "bautenschutzmatte" in german, which means something like "building protection layer" and it is meant to protect concrete floor from heavy (vibrating) machinery.

peace, rez
#521
21st December 2012
Old 21st December 2012
  #521
Gear maniac
 

700 kg/m³ seems low for solid rubber, or is it the cellular type? Does it float in water (1000 kg/m³)?

If rubber and such a low density it is probably very flexible = good but the surface weight might have to be increased. You could use double adhesive tape and add a steel, plastic or MDF sheet to the front, leaving about 50 mm rubber rim outside the added sheet. (This higher surface weight would need a shallower box depth for the same lower frequency, while at the same time giving a very flexible membrane front)
#522
9th January 2013
Old 9th January 2013
  #522
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plumma's Avatar
 

Hybrid Corner bass trap

Gents,

Let me know if you want me to post this as a new topic...

I have been reading Tim's post about sealed limp mass membrane bass traps I and would like to build some.

However as I already have some high density Rockwool slabs, which I understand would be too dense for Tim's traps I have proposed a hybrid design and wanted to know your thoughts.


The plan was to build triangular (cross section) sealed traps behind the existing slabs that straddle the corners of the studio.

In the drawing I would seal top and bottom with wood.

Please see pic.

My question to you is as the cavity space is now a triangle as opposed to your rectangular trap how would I work out what frequencies it would attenuate?

Do you think I can also 'tune' them, by adjusting the length of the trap or putting a wood infill to reduce the depth of the cavity.


Also for the materials can you confirm if this MLV will do the jod:-

http://www.siderise.co.uk/pdf/produc...rier_188_1.pdf

And what low density rock wool should I use inside the traps, i.e whats the max density I should use?


Hope you can help.

Matt
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-corner-bass-trap.jpg  
#523
9th January 2013
Old 9th January 2013
  #523
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Quote:
Originally Posted by plumma View Post
Also for the materials can you confirm if this MLV will do the jod:-

http://www.siderise.co.uk/pdf/produc...rier_188_1.pdf
That looks fine. You can do the math to estimate the resonant frequency and if you need to go lower, look and see if they sell a thicker product.
#524
9th January 2013
Old 9th January 2013
  #524
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Thanks and how would I work out the frequencies that my bass trap would affect.

It's triangular and Tim's formula is for a rectangle shape, so it would be total guess work?
#525
10th January 2013
Old 10th January 2013
  #525
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It's all based on volume. You need to calculate the volume inside the triangular area... i've long since forgot my algebra formulas; but i'm sure you can find it or a calculator online.
compare your volume to the corresponding volume of the rectangle build.. adjust dimensions accordingly.

I don't know if or how having the variable depth will effect performance otherwise.
#526
10th January 2013
Old 10th January 2013
  #526
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Lepper's Avatar
 

This thread needs to be sticky.
#527
4th February 2013
Old 4th February 2013
  #527
Quote:
Originally Posted by Tim Farrant View Post

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.

Cheers
Tim.
If these are my axial modes then which frequencies do you think I should build for?
If I'm interpreting Bob Golds calc correctly then I should do one around 50hz, one at 90hz and one around 130hz. Is this right?
I will be also testing with sine waves but wanted to get a head start.
Thanks for the designs :-)
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-brixtonstudio_modes.jpg   Tim's Limp Mass Bass Absorbers-brixtonstudio_modes2.jpg  
#528
5th February 2013
Old 5th February 2013
  #528
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Quote:
Originally Posted by allawishus View Post
If these are my axial modes then which frequencies do you think I should build for?
If I'm interpreting Bob Golds calc correctly then I should do one around 50hz, one at 90hz and one around 130hz. Is this right?
I will be also testing with sine waves but wanted to get a head start.
Thanks for the designs :-)
Bob calc dosen't take into account for leakage from walls, doors, windows. Stiffnes in walls if soft/hard. The calc is for an ideal room wich nobody has.

To predict using this calc to build membrane abs is for me a dead end. One member tried 19 of them with poor/no result at all.

Mesurment is the way to go and will take in all anomalies that can be seen in that particular room. Then maybe when analysing this data one can build a membrane and attack the problem in question. But I dought that membrame is a good solution for attacking first axial modal resonance.

So if measurment is not done before and after then we all can speculate.
#529
5th February 2013
Old 5th February 2013
  #529
Thanks.
As I said, I will be testing the room. I'm just trying to get an idea. Tim who supplied the design uses a calculator to spec his rooms and I assume it works for him or he wouldn't be in business.
#530
19th February 2013
Old 19th February 2013
  #530
Gear maniac
 

hi
why not try this Modex Corner - Acoustic Products
How do I get it its have the same design as tim's but the "absorber"(i think its 100+ rockwool or the same)is Positioned in the middle of the box ?and have air after it
so Sound strikes the membrane which sympathetically vibrates converting the sound pressure to air motion. The air loses velocity in the air gap between the limp mass to the rockwool then the Speed ??of sound waves slow down again when hitting the compressed absorbent(From what I understand a compressed absorbs slow more to sound waves then Less dense(denser=more air flow resistance kns/m4 or kps/m2)
and then more air to the Hard border (wood or the wall) and some of the Remaining sound waves go back and absorbed again by the absorber
what do you think?
Tim Farrant
Thread Starter
#531
2nd March 2013
Old 2nd March 2013
  #531
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Thread Starter
Quote:
Originally Posted by plumma View Post
Gents,
Let me know if you want me to post this as a new topic...
I have been reading Tim's post about sealed limp mass membrane bass traps I and would like to build some.
However as I already have some high density Rockwool slabs, which I understand would be too dense for Tim's traps I have proposed a hybrid design and wanted to know your thoughts.
The plan was to build triangular (cross section) sealed traps behind the existing slabs that straddle the corners of the studio.
In the drawing I would seal top and bottom with wood.
Please see pic.
My question to you is as the cavity space is now a triangle as opposed to your rectangular trap how would I work out what frequencies it would attenuate?
Do you think I can also 'tune' them, by adjusting the length of the trap or putting a wood infill to reduce the depth of the cavity.
Also for the materials can you confirm if this MLV will do the jod:-
http://www.siderise.co.uk/pdf/produc...rier_188_1.pdf
And what low density rock wool should I use inside the traps, i.e whats the max density I should use?
Hope you can help.

Matt
So long as the cavity behind the MLV layer is sealed, it should do something, but you need to have the MLV layer "limp" not squashed up against dense fibre glass.
#532
3rd March 2013
Old 3rd March 2013
  #532
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Gdupproductions's Avatar
 

Quote:
Originally Posted by Tim Farrant View Post
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.
Hey'yo Tim thansk to shine in !!!.
I'd like to build one like this and see if it works

Yet Can i use ANYTHING as long as the box is air tight ?
and the membrane what thickness does it specificly have to be ?,
What materials ?, and density ?.

Also is it neccesary to stradle these fellas across the corner ?,
for safety reasons i'd like to screw them (lol ) against the wall with chemicall anchors. To be hypocrit actually...my 2 back traps are actually hanging with 2 corner hooks and a braided rope above my bed .


Thanks for sharing, and i'm realy curious if it's going to affect my room in a positive way !.
#533
4th March 2013
Old 4th March 2013
  #533
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Bilou's Avatar
 

Quote:
Originally Posted by Gdupproductions View Post
Hey'yo Tim thansk to shine in !!!.
I'd like to build one like this and see if it works

Yet Can i use ANYTHING as long as the box is air tight ?
and the membrane what thickness does it specificly have to be ?,
What materials ?, and density ?.

Also is it neccesary to stradle these fellas across the corner ?,
for safety reasons i'd like to screw them (lol ) against the wall with chemicall anchors. To be hypocrit actually...my 2 back traps are actually hanging with 2 corner hooks and a braided rope above my bed .


Thanks for sharing, and i'm realy curious if it's going to affect my room in a positive way !.
Don't make just one, it won't show much things...

The box has to be air tight but also need to be thick enough. We don't want the box to vibrate. I personally use 15 to 19mm MDF for sides and 10 to 15mm for back.

The membrane density has to be choosen depending on your target frequency. There is the formula in Tim's PDF.
The common material is Mass Loaded Vinyl.

You can screw the bass traps to the wall, as long as they stay air tight.
#534
22nd March 2013
Old 22nd March 2013
  #534
Gear interested
 

Hello!

I guess this type of construction with the MVL not sealed and just hanging would not be very efficient/go low enough?

Studio SOS: Building a DIY Vocal Booth

the issue I have is a strong resonance at 69 and 43 hz that I failed to correct with 10 BBC modules due to poor construction.. my fault.
#535
22nd March 2013
Old 22nd March 2013
  #535
Gear interested
 

This is a great, great thread. Wonderful stuff!
#536
24th March 2013
Old 24th March 2013
  #536
Newbie question here, what would work better in this case?

stradling corners? or paralel to one surface?
(using the same amount of traps)
#537
26th March 2013
Old 26th March 2013
  #537
Gear addict
 

Quote:
Originally Posted by boggy View Post
It is not stiffer than 4mm bitumen limp mass membrane, which can do more than "something" here:



And AFAIK, vinyl flooring material is denser than bitumen (higher surface density with thinner membrane)


Wow - great thread - it took me a few days to digest it all!

I'm hunting down a bad null centered around 85Hz. If I could find a good material source without breaking the bank here in NYC I'd give these a shot.

How about something like this roofing material?

Its $80 at lowes and seems rubbery. At least in its rolled up form, can't really tell how limp it is.
APP (atactic polypropylene) membranes
Modified with plastic

3.5mm thick

It's weighs 84 lbs for for 96 sqft (.875 lb/sqft)


(its the "smooth " one)

APP | Grip Rite Building Tools and Supplies
#538
5th April 2013
Old 5th April 2013
  #538
Gear addict
 

Quote:
Originally Posted by jeff_free69 View Post
Wow - great thread - it took me a few days to digest it all!

I'm hunting down a bad null centered around 85Hz. If I could find a good material source without breaking the bank here in NYC I'd give these a shot.

How about something like this roofing material?

Its $80 at lowes and seems rubbery. At least in its rolled up form, can't really tell how limp it is.
APP (atactic polypropylene) membranes
Modified with plastic

3.5mm thick

It's weighs 84 lbs for for 96 sqft (.875 lb/sqft)


(its the "smooth " one)

APP | Grip Rite Building Tools and Supplies
Can't really bump a sticky (its already at the top)..

but...

Any LIMP experts out there care to weigh in on this material?
#539
9th April 2013
Old 9th April 2013
  #539
Gear interested
 

Hi,
I have a big modes on 32 hz and 48hz. I think to use a limb mass bass absorber to treat it. Can I use for membrane a HDF /High Density Fibreboard/ 9mm or 12mm or plywood 9 mm or 12 mm in box /1200mm x 410mm x 300mm/ ?

Which is better HDF or plywood?

Which material is best to use for this absorber ?
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-waterfall.png  
#540
9th April 2013
Old 9th April 2013
  #540
Gear maniac
 
Bilou's Avatar
 

For very low frequency like that, MLV seems to be a really better material, with a high density like 10kg/m2.
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