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Panel/Membrane Bass Trap question
Old 13th November 2009
  #1
Gear Head
 

Panel/Membrane Bass Trap question

I've been studying posts here, a few texts online and a book on acoustics and I am still unclear on a bass trapping construction issue. If anyone could give me a quick tip before I start building I would love a second opinion.

(quick background)

My control room has a very irregular shape and unfortunately has about six corners for low end to develop. (its in a basement) my ceiling is dropped with rockwool and wall board over top. Two of my Walls are also framed a couple inches off the foundation with suspended rock wool, also covered with wallboard. the back wall is concrete foundation, with a staircase.

I did scope sweep in two locations (listening position, back corner) and I attached the results here for reference if needed.

The higher frequency irregularites i think I can deal with relitive ease, perhaps one of those aurelex packages or something.

My main issue is in the very low frequencies, I definitely have a bass resonance problem. This is my biggest mix problem in practice as well.

So with that background, if I am not mistaken, I looking to effectively trap around 40-50hz as much as possible and probably a bit of 200hz for good measure.

So a diaphram/panel style resonator seems to be the way to go. My three questions (finally) are.

Since a half inch plywood front resonator on a 20" box will absorb around 40hz roughly, would that extend to mean that a 3/4" plywood front on a 13inch box could do the same, roughly?

I understand the concept that by corner mounting the trap there will be a variety of depths from the barrier to the front panel, thereby increasing the range of absorbtion. However if I permanently mount the traps like that, nobody will be able to get gear down the stairs. If I make two flat panel traps and mount them into the walls along the corners I am afraid I won't get that super low end absorbtion.

If I make a sealed airtight long triangle shaped box that fits the dimensions of the corners exactly but is not attached to the wall, (standing in the corners), would this work? The bass trap could almost be touching the wall.

the longest distance inside would be around 13inches so I figure it could work pretty well, especially with a 3/4" plywood panel.

last one

I am aware of how to properly mount a piece of rigid fiberglass inside the trap but is there a real acoustic advantage to filling the rest of the space up with rockwool or something like it? Would that simply help it to absorb more at higher frequencies, or would it at all comprimise the effectiveness at the lowest frequencies?

As far as the 200hz problem, that seems like normal panel traps will do the trick and that process is well documented.

Lastly if anyone thinks that a Helmholtz resonator would be a better plan I am all ears, but it seems a little more complicated and easier to screwup.

Thanks in advance for any info
Attached Thumbnails
Panel/Membrane Bass Trap question-listening-position.jpg   Panel/Membrane Bass Trap question-back-corner.jpg  
Old 14th November 2009
  #2
Lives for gear
 

Since no one's responded yet, I'll chime in with my two cents.

Quote:
Originally Posted by Delusionboy View Post
Since a half inch plywood front resonator on a 20" box will absorb around 40hz roughly, would that extend to mean that a 3/4" plywood front on a 13inch box could do the same, roughly?
Not necessarily. There are formulas that could tell you...don't remember specifically for the panel resonator. They're simple formulas, but also apparently not very accurate due to the vagaries of the specific membrane attachment method, properties, etc. I've never messed with panel resonators because they strike me as being inelegant.

Quote:
Originally Posted by Delusionboy View Post
I understand the concept that by corner mounting the trap there will be a variety of depths from the barrier to the front panel, thereby increasing the range of absorbtion. However if I permanently mount the traps like that, nobody will be able to get gear down the stairs. If I make two flat panel traps and mount them into the walls along the corners I am afraid I won't get that super low end absorbtion.
For a sealed panel resonator, the determining factor in terms of the contribution of the sealed compartment to frequency and absorption is its volume. The varying-depths (corner-placement) aspect of panel placement would apply for a porous absorber in a different way from that for a panel absorber. In fact, most panel absorbers I've seen are shallow and flat, and meant to be attached flat against the wall.

Quote:
Originally Posted by Delusionboy View Post
I am aware of how to properly mount a piece of rigid fiberglass inside the trap but is there a real acoustic advantage to filling the rest of the space up with rockwool or something like it? Would that simply help it to absorb more at higher frequencies, or would it at all comprimise the effectiveness at the lowest frequencies?
If you're talking about the fibrous material you're placing inside a membrane absorber to damp its resonance, the specific density, thickness and placement within the trap all contribute very strongly to the effectiveness of the device.

Quote:
Originally Posted by Delusionboy View Post
Lastly if anyone thinks that a Helmholtz resonator would be a better plan I am all ears, but it seems a little more complicated and easier to screwup.
I personally think a Helmholtz resonator is always a great way to go for low frequencies, but mostly because I'm obsessed with them for their own sake. If you're not into big projects and lots of measurements and corrections and so on, probably not worth your time. On the other hand, in my experience the formulas match the measured performance of the devices very well. They're extremely sensitive to minuscule errors in measurement or fabrication, and tend to need adjustment no matter how smart and careful you are. But I personally think they're fun to build and cool looking, and they're effective if you have enough of them in the right spot.
Old 14th November 2009
  #3
Gear Guru
 
Ethan Winer's Avatar
 

Lightbulb

Just a few random comments:

A third-octave display tells nothing useful about the LF response in the room. You need to measure at 1 or 2 Hz resolution minimum to see the true extent of the peaks and nulls. You really should be using room software because that also shows time-based issues such as modal ringing:

ETF, Windows, $150
FuzzMeasure, Mac, $150
Room EQ Wizard, Windows and Linux, Freeware
Using ETF explains how I use ETF, but the principles apply to all such programs.
Comparison of Ten Measuring Microphones

I wouldn't worry so much about 40 Hz and below. The bass range from about 60 to 80 Hz and higher is far more important for hearing well and making portable mixes.

--Ethan
Old 14th November 2009
  #4
Gear Guru
Difficult

RealTraps used to make large panel traps. Many of their newer products still have an element of membrane involved. Modex is a membrane design. GIK don't seem to use membranes. All of them work well.
There is a balancing act with wooden panel traps. The are sealed so air does not pass into them. Thus they are very ineffective at all frequencies other than around the resonant area. Their tuning centre can change over time with wood drying out, shrinkage and so on. I was very attracted by a Helmholtz design with spectacular test results in the Master Handbook of Acoustics. I duplicated the device and got no significant result. The formula for resonance was also incorrect. Acoustics Forum • View topic - Helmholtz formula, is this incorrect?
Overall I have to recommend the usual suspects. StudioTips SuperChunk, or the commercial products which have published verifiable test results. You will need lots to get any worthwhile result in a concrete basement, so go at it hard.

Best, DD
Old 14th November 2009
  #5
Lives for gear
 
PaulP's Avatar
 

Quote:
Originally Posted by Delusionboy View Post
Since a half inch plywood front resonator on a 20" box will absorb around 40hz roughly, would that extend to mean that a 3/4" plywood front on a 13inch box could do the same, roughly?
Half inch or thicker seems awfully thick to me. I think 3/4" is used to turn
off an absorber. If you haven't already seen these may I recommend :
http://downloads.bbc.co.uk/rd/pubs/reports/1980-12.pdf
and
http://downloads.bbc.co.uk/rd/pubs/reports/1992-10.pdf
There's lots more interesting stuff in the BBC archives :
BBC - R&D - Publications - RD Reports
Quote:
If I make a sealed airtight long triangle shaped box that fits the dimensions of the corners exactly but is not attached to the wall, (standing in the corners), would this work? The bass trap could almost be touching the wall.
I'm sure this will work well and is what I also plan on doing. A big advantage
is that you can just pick them up and go if ever you decide to move.
I've been toying with the idea of using two stacked modules in a corner,
the lower one being a panel absorber and the upper one a regular filled
corner absorber.

Paul P
Old 17th November 2009
  #6
Gear Head
 

nack to the research

Thanks for all the great replies! Very helpful. I see i might have been going at it a bit wrong, as I suspected, so I will process more of this info that you've all thrown out there and see where I'm at in a few days.

thanks again
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