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Old 26th May 2011, 05:05 PM   #21
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Originally Posted by markus76 View Post
If one has the space that fluffy stuff is probably the only way to create an effective low frequency absorber out of porous material. But now we're talking about a thickness that is measured in meters, not centimeters.
For constrained spaces other absorption principles are more effective.
The only way? I'm not sure we are comunicating here but the point is that the pillow stuff is very poor for a given thickness. Most decent acoustic materials (fiberglass, rockwool, some foams) aproach 100% absorption when their thicknesses become 1/6th to 1/4 of the wavelength under test. (Look at the data in archetectural acoustics data sheets. Google the "Harvard Wedge" for anechoic chambers.) The polyester will perform well below that. For whatever frequency of cutoff you want the poly will need to be many times thicker. It will never perform as well.

Not an effective absorber by any comparison.

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Old 26th May 2011, 05:50 PM   #22
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Originally Posted by speaker dave View Post
The only way? I'm not sure we are comunicating here but the point is that the pillow stuff is very poor for a given thickness. Most decent acoustic materials (fiberglass, rockwool, some foams) aproach 100% absorption when their thicknesses become 1/6th to 1/4 of the wavelength under test. (Look at the data in archetectural acoustics data sheets. Google the "Harvard Wedge" for anechoic chambers.) The polyester will perform well below that. For whatever frequency of cutoff you want the poly will need to be many times thicker. It will never perform as well.

Not an effective absorber by any comparison.

David S.
Dave, the problem is that a porous low frequency absorber needs to be very thick. One meter or even more. Common fiberglass simply becomes reflective when made thicker. Sound waves need to penetrate the whole absorber. Only fluffy material has these properties. Of course the amount of dissipated energy decreases with lower density. That's the trade off.

Of course you could use multiple layers of denser fiber boards with air gaps in between but does this have any advantage over fluffy fiberglass?

Last edited by markus76; 26th May 2011 at 05:55 PM.
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Old 26th May 2011, 06:15 PM   #23
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I have got rid well enough of it with the tube for my taste (second graph), but it would be nice to know how that works and whether one can transfer that to other speaker principles (boxes).
A theory: The peak is produced like the OB theory says (leave away for now that it's a bit lower than Edge says). The tube forms a halfe-wave resonator with same resonance frequency that absorbs the peak (kind of a TL equivalent of a Helmholtz-Absorber).
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Old 26th May 2011, 06:50 PM   #24
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Markus, didn't you use that pillow stuff in your designs some time ago?
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Old 26th May 2011, 07:09 PM   #25
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Markus, didn't you use that pillow stuff in your designs some time ago?
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Yes. Stuffed my subs with it.
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Old 26th May 2011, 07:19 PM   #26
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Here's the English version: Freund GmbH - All for: Interior Design and Architecture
Too bad it's not American. They have some other cool materials available as well: http://www.freundgmbh.com/en/documen...nels_Flyer.pdf
Nothing like it I know of here.

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Old 26th May 2011, 07:24 PM   #27
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I sometimes use the thickest "pile" carpet scraps I can get my hands on. Odd cuts and left over's are often free or very low cost. A couple of those back to back will absorb a lot of sound - but can also leave things sounding very "dead".
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Old 26th May 2011, 07:27 PM   #28
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Nothing beats fiberglass for absorption vs. thickness.
Gary Pimm has had superb results using UltraTouch (recycled cotton wall insulation), better than fiberglass in his boffle inspired boxes. We use the denser UltraTouch 1/2" felt for most of our builds.

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Old 26th May 2011, 07:32 PM   #29
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Works "like" just not to the same degree.
Some date here: Bradbury's Fiber Equations

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Old 26th May 2011, 08:54 PM   #30
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Dave, the problem is that a porous low frequency absorber needs to be very thick. One meter or even more. Common fiberglass simply becomes reflective when made thicker. Sound waves need to penetrate the whole absorber. Only fluffy material has these properties. Of course the amount of dissipated energy decreases with lower density. That's the trade off.
Besides speaker design I've spent some time in the architectural acoustics field, so I've been bored to tears looking at material acoustic absorption tables.

Most of the specified absorbing materials (materials specifically intended to soak up sound) plot like a high pass filter with some turn on frequency and near 100% absorption above that (and sometimes an alpha even greater than 1 i.e. more than 100% absorption). When a material is specified at a variety of thicknesses the data always seems to shift left one Octave for every doubling of thickness. In other words, twice as thick means it starts absorbing an Octave lower. I have never seen a phenomonon where a material became too thick and then became reflective.

Here is an example Coefficient Chart look down to the section of "absorptive wall material" and you will see fiberglass board in 1" 2" 3" and 4" thickness. compare the numbers for 1,2 and 4 inches and you will see that each time the thickness doubles the table shifts a column to the left, more or less. Note that 4" thick is specified as 99% absorptive for every frequency from 125 up, (except .97 at 4kHz) the highest overall absorption on this particular table.

I looked around for polyester batting but haven't found any numbers on it, primarily because it isn't considered a useful absorbing material nor an architectural material. (Again, your polyester foam is a different matter.)

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David S.
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