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Limp Mass Bass Trap - suitable materials?
Old 16th November 2013
  #1
Gear Nut
 

Limp Mass Bass Trap - suitable materials?

I'm reading the book MIXING SECRETS for the Small Studio by Mike Senior.

He discusses a type of "last ditch attempt" bass trap he calls a "Limp Mass Bass Trap":

"This involves loosely hanging a large, heavy and impermeable sheet a little distance from one room boundary, where it can damp low frequency air movements.

Some kind of rubberized barrier matting with a density of around 5kg/m2 (5 kilograms per meter squared) is a common choice, and it helps if it has reinforced backing so you can hang it by its edge at the ceiling without it tearing or deforming under its own considerable weight.

Failing that, old carpet can deliver a similar result if mounted the same way, and may be cheaper if purchased as remnants."

What's a low-cost material for this application that's relatively easy to find in the United States?


Or have you had good results using carpet remnants instead?


Thanks
Old 16th November 2013
  #2
Gear Nut
 

I just read a couple of the earliest and latest pages of the sticky, "Tim's Limp Mass Bass Traps".

I think guys here just LOVE to do woodworking projects. heh

I don't know if Mike Senior is right, but he suggests simply hanging a sheet of suitable material over a pipe suspended a few inches below the ceiling - like a shower curtain hanging from the curtain rod.
No boxes or frames.

No worries about anything being airtight.

No absorptive material specific to the limp mass sheet.
Senior's approach is to make the hanging bar moveable relative to the wall behind it. So you can experiment with varying distances. His approach is in response to particularly hard-to-tame low frequencies, and he assumes the limp sheet will be the size of an entire wall, or nearly that size (since low frequency waves are gigantic).

Who's right? Mike or Tim?
Old 2nd March 2014
  #3
Gear Addict
 

Bumping this in hopes to see some discussion on this, and what material(s) would be appropriate.
Old 2nd March 2014
  #4
Lives for gear
 

Quote:
Originally Posted by ManyHats View Post
Senior's approach is to make the hanging bar moveable relative to the wall behind it. So you can experiment with varying distances. His approach is in response to particularly hard-to-tame low frequencies, and he assumes the limp sheet will be the size of an entire wall, or nearly that size (since low frequency waves are gigantic).

Who's right? Mike or Tim?
I think the key word here is "experiment". If you hang a large sheet of carpet, rubber underlay, mass loaded vinyl or freshly made pasta over a pipe and "vary the distance of the sheet" from a boundary you will probably measure some acoustic effect on the room response. The problem that we are faced with is finding repeatable, predictable results that are proven to have a particular effect. There are very few measurements presented on this forum that could be considered as real performance data, that has been independently verified regarding limp mass absorbers, despite the fact that clearly many participants have solved particular problems with their own rooms, and in general the theory behind the design is based on scientific principles

I don't think it's a question of "who's right", one is an attempt at a narrow-band "resonant" absorber whereas the other is not. It is not really a valid comparison although both will have some acoustic effect.
Old 3rd March 2014
  #5
Gear Nut
 

Well said, Icecube1.

I'm using ARC 2 Advanced Room Correction software in addition to physical acoustical treatments. Am thinking about moving my recording endeavors from a small room with gypsum board walls into the larger basement space, which involves two adjacent cement block walls (they meet at a corner) and two gypsum board walls.

The "room" is not entirely contained; it's not a rectangle - it juts into other open spaces such as a laundry area and a furnace area. I'm worried that the disparate reflectivity of the two pairs of wall surfaces and the "uncontained geometry" may cause unexpected results.

I'm wondering what the effect might be of closing off the back end of the room (blocking off the "uncontained geometry") with some sort of limp mass wall might be. I could do it with minimal construction.

Your thoughts, guesses and/or knowledgeable insights WELCOME.
Old 3rd March 2014
  #6
Lives for gear
 

Ok, this post is really becoming more involved than your original question.

I think it's fair to say that more info is needed for anyone to comment constructively. As a bare minimum, dimensions (L,W, H) of both spaces for comparison and a plan drawing of the basement layout.

Also, have you had any experience of taking measurements with something like REW - Room EQ Wizard Room Acoustics Software?
Before you decide what treatment you may need you have to have an idea of what you have to start with!

As a general observation, bigger room volumes are a better starting point for potential mix rooms. However, your 2 concrete walls would be more "rigid" than gypsum (plasterboard) walls. Whilst high mass, rigid walls are good in terms of "isolation", very rigid boundaries make low frequency control more difficult.
Old 3rd March 2014
  #7
Gear Nut
 

Quote:
Originally Posted by Icecube1 View Post
Ok, this post is really becoming more involved than your original question. I think it's fair to say that more info is needed for anyone to comment constructively. As a bare minimum, dimensions (L,W, H) of both spaces for comparison and a plan drawing of the basement layout.
You're right. I withdraw the question. I was being lazy in not starting a new thread with dimensions and floor plan, as you suggest. Great minds do think alike. If I do need to make the switch, I'll post as you outlined. I'll be working with others soon, rather than just me, ergo the interest in a larger space.

Quote:
Originally Posted by Icecube1 View Post
Also, have you had any experience of taking measurements with something like REW - Room EQ Wizard Room Acoustics Software?...you may need you have to have an idea of what you have to start with!
Point taken. No experience with REW, but I find the ARC2 plugin provides useful feedback. I run the calibration and review the results. I move some traps and panels and run the calibration again: the new results depict the aural changes resulting from the physical changes. Not exact, but illustrative and helpful.

Quote:
Originally Posted by Icecube1 View Post
As a general observation, bigger room volumes are a better starting point for potential mix rooms. However, your 2 concrete walls would be more "rigid" than gypsum (plasterboard) walls. Whilst high mass, rigid walls are good in terms of "isolation", very rigid boundaries make low frequency control more difficult.
My thoughts exactly. I figure my absorptive panels will be stretched thin in the larger space. I imagine locating most of them on the concrete block walls and hope to make up some of that lost ground by implementing a cheap, easily moveable limp mass barrier as a rear wall (of sorts).

Thanks, Ic1. I appreciate being able to kick this around first.
Old 3rd March 2014
  #8
Lives for gear
 

No problem!

When I mentioned "experimentation" in an earlier post above it was not meant to be derogative. Indeed one of the useful qualities of this forum is the large number of keen, dedicated, audio enthusiasts that are prepared to experiment, with some scientific guidance from others. The pros, that freely give out advice, of course have to be more cautious in a sense, only giving tried and tested advice, learnt from academic study and experience. If you make your living designing to a spec you can't (often) afford to push the boundaries with experimentation. BUT to make your experimentation useful to others it needs to be done in a methodical, repeatable way. So, if you do experiment try and document through measurement and careful regard to before and after results. That way everyone benefits. Good luck.
Old 3rd March 2014
  #9
Gear Nut
 

Icecube1 ==

With that post, I think you qualify as the official "Voice of Reason" around here. heh

Thanks, and will do.
Old 9th April 2019
  #10
Gear Head
 
twiy's Avatar
 

Did any one ever try this? Mike's book is so popular but I've never seen one photo of such a sheet of matting hanging in a room. Would love to know results.
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