Paulus87 wrote:
You're comparing a limp membrane with a rigid panel absorber. Here's some good reading:
Yes correct sorry, the types slipped my mind - thanks for the links they made some good reading
https://www.acousticsciences.com/art-no ... bass-trapsWhat stood out for me in this article is the adjusted formulas for Limp Vs Rigid Panel which takes the center frequency up by 1/3 for the rigid panel absorbers.
The adjustment for porous absorption which takes the center frequency down but widens the Q and reduces its efficiency.
The further adjustment for air volume - by adjusting the W X L by a small amount further reduces its center frequency.
Paulus87 wrote:
Your walls may already be an effective membrane for the range of frequencies you're trying to target, if they are either lossy or a MAM system (Last gearslutz link).
This isn't possible for me as my walls are cement plastered brick.
Paulus87 wrote:
Personally, I would be tempted to try some VPRs if you can afford it. You can read about them in the gearslutz link above. Very broad range of absorption and they only take up ~200mm.
Having looked into this it seems like a very viable option - I have come to the conclusion that too thick an MDF panel would probably be too rigid and ineffective. the steel would provide a much higher density at these lower thicknesses - I got some quotes today for 304 Grade Stainless Steel 1.5mm, 2mm, 3mm, 4.5mm - its not cheap, but it works out a similar price as the thicker MDF anyway, and certainly more cost effective, accessible, and denser than MLV. So everything is pointing in this direction for me right now. Is there a particular grade of Stainless Steel that is used? Also when it comes to the thickness of the steel I presume there would be an upper limit (wondering if 3mm is too thick?)
Paulus87 wrote:
MDF panels will resonate, but beware imperfect processing can cause differences in stated properties and actual e.g. surface density. If you decided to go this route then perhaps you could use some very sticky double sided foam tape to seal the front panel to the cabinet and use screws to "tune" it once it's installed. I think they might be expensive, risky and inefficient though compared to the other methods above.
Yes this is true and would hold true for the VPR Steel Panel too - my work around is to get the pieces first, weigh them and then calculate the depth required based on measured weight/density.
The part that I am struggling the most with now is the design & calculation parameters because all of the calculators vary so vastly in their calculations?!
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A.) According to the calculator found at
http://forum.studiotips.com/viewtopic.php?f=1&t=3497 (Membrane Absorbers.xlsx) - this has various options for membrane types (MDF, Steel, Gypsum etc - I don't see limp options):
270mm Deep, 25mm 703, 2mm Steel Panel Sheet (15.6kg/m2) = 24.36Hz (0.9 coef.)
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B.) Using the formulas at
https://www.acousticsciences.com/art-no ... bass-traps The results differ vastly except for the limp membrane with absorption. Based on these Results it Seems that the previous Calculator is working on Limp Membrane Calculation. (A similar Depth of 275mm yields the same result for the limp formula. Accounting for panel absorption using the adjusted formula yields the need for a 500mm deep trap with insulation.)
270mm Deep:
Limp Membrane:f = 170 / (SQRT(3.19 * 10.63)) = 29.2 Hz (No porous Absorption)
f = 143 / (SQRT(3.19 * 10.63)) =
24.6 Hz (Formula Adjusted for porous absorption)
Rigid Panel:f = 228 / (SQRT(3.19 * 10.63)) = 39.2 Hz (No porous Absorption)
f = 192 / (SQRT(3.19 * 10.63)) = 33.0 Hz (Formula Adjusted for porous absorption)
500mm Deep
Limp Membrane:f = 143 / (SQRT(3.19 * 19.7)) = 18.0 Hz (Formula Adjusted for porous absorption)
Rigid Panel:f = 192 / (SQRT(3.19 * 19.7)) =
24.2 Hz (Formula Adjusted for porous absorption)
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C.) Acoustic Modelling http://www.acousticmodelling.com/multi.phpThis calculator only provides calculation for a limp membrane...
Using the initial spec: 270mm Deep, 25mm 703, 2mm Steel Panel Sheet (15.6kg/m2)
Provided: 28.6 Hz (0.05 Coef.) (basically useless)
To adjust the paramters to get 24.2 Hz (0.98 Coef.) I had to do 165 porous + 165 Air = 330mm
A.) says 270mm Deep
B.) says 500mm Deep
C.) Says 330mm Deep
Which do you use? Which do you choose? With such big variance between all the formula's and calculations how on earth is someone supposed to build these with fairly predictable results when they all say such different things?
It seems thats perhaps B.) is the most accurate because it adjusts for the Rigid Panel & The Insulation? and is seems that A.) is based on a limp membrane even though the selected option is "2mm Steel" and C.) explicity mentions "limp membrane"