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Trap material density

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Trap material density

Postby swiss toby » Thu Nov 25, 2004 11:22 am

Here is what I'm sure is a very basic question, but I don't know the answer so I'm going to ask and look stupid anyway! :headbang:

Product A has a density of 50kg/m3, product B has a density of 100kg/m3; is 1 unit of B going to have the same trapping/damping effect as 2 units of A?
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Re: Trap material density

Postby Digipenguin » Thu Nov 25, 2004 4:49 pm

The short answer, no. But I,ve gotta go eat bird with my folks right now so if no one has answered by then, I will give a longer answer when I return.

I can't even think of any links to sites that address that question specifically. Try BobGolds site and mull over the absorption coefficients that should excplain things a little.

GTG
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Re: Trap material density

Postby Scott R. Foster » Fri Nov 26, 2004 3:34 am

Probably not... but its a classic boxers or briefs question and the complete answer is: "Depends".

Assuming that products A & B are porous materials then they absorb sound by converting the energy of the room’s air volume’s vibrations into heat by detouring the vibrations [the sound waves energy] into a path separate from the air volume of the room and keeping it there.

In mineral fiber panels this works through the interaction of the materials interwoven fibers [or the open cells in the case of foam] which create a boundary with many interstices [little holes] for the sound energy to enter and dissapate as heat.

Generally, the absorption characteristic of a broadband absorptive material [either foam or mineral fiber] comes down to a matter of the density of the material [stiffness / elasticity of the material typically being proportional to increases in density - though perhaps not directly proportional] AND the topology of the interstices created by the size and orientation of the individual fibers [cells].

Density and hole size/configuration yield a property called gas flow resistance [will the air being pushed around by the sound waves be able to pass into the material, and if so, how much resistance will the air being so pushed encounter].

The process is not strictly additive so you should not make any assumption that a given volume at a given density will yield results equal to twice the volume at half the density.

More important, you should NOT assume that more density means better absorption. To get a maximum efficiency out of a broadband absorption material these factors must be balanced - too much density and you've filled in all the little holes - lower the density to where the holes get really big and you got no gas flow resistance.

Do as DP suggests and hit Bob Gold's site - but even then, be careful about assumptions. I'd suggest you shop locally and determine price and availability of various materials - use this info to draft a prosposed design, and then seek input from the peanut gallery.

With a little feedback from folks who have done this stuff a few times I doubt you will go far wrong. But informed replies will be more helpful if you give specifics on what the market offers in your locale and how you intend to use it.

Goos Luck!
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Re: Trap material density

Postby Avare » Fri Nov 26, 2004 4:59 am

The question of what density absorbent is best for low LF depends on the thickness of the desired absorrber. The theory has been explained already.

I tried to use one make with the only variation being the density. Hence using Roxul products. Unfortunately the thicknesses tested only went to 75 mm, but the pattern is clear.

If the absorber is to be fairly thin, then the denser the better. The top part of the chart those this.

If the absorber is thicker, denser material reflects the sound. The densist material never reaches 1.00. The bottom part shows this.

So, the thicker the absorber, the less dense the material, the better.


Code: Select all
   rxl 40   rxl 60   rxl 80<br />   25 mm thickness      <br />125   0.07   0.08   0.11<br />250   0.32   0.33   0.31<br />500   0.77   0.78   0.82<br />1000   1.04   1.03   1.01<br />   75 mm thickness      <br />125   0.63   0.78   0.75<br />250   0.95   0.89   0.82<br />500   1.14   1.04   0.89<br />1000   1.01   0.98   0.94


For reference RW2 270 mm thick has an absorption co-efficient greater than 1.0 down to 50 Hz (from Aunt Beeb BBC RD 1992/11 fig 1)

Absorbingly;
Andre
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Re: Trap material density

Postby Digipenguin » Mon Nov 29, 2004 1:05 pm

Ditto. Except they said it with more detail and knowledge than I have. :)


So..................Yeah! What they said.
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