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?
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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).
Markus, didn't you use that pillow stuff in your designs some time ago?
http://www.diyaudio.com/forums/multi-way/153199-enclosure-stuffing-10.html#post1952407
http://www.diyaudio.com/forums/multi-way/153199-enclosure-stuffing-10.html#post1952407
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/documents/light-panels/Light-Panels_Flyer.pdf
Nothing like it I know of here.
Dan
Too bad it's not American. They have some other cool materials available as well: http://www.freundgmbh.com/en/documents/light-panels/Light-Panels_Flyer.pdf
Nothing like it I know of here.
Dan
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.
dave
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.)
Regards,
David S.
Still - getting things to behave below 200Hz is very tricky.
Above 200Hz - not so hard.
Still - getting things to behave below 200Hz is very tricky.
Above 200Hz - not so hard.
What is tricky in molding a concrete woofer box?
What is tricky in molding a concrete woofer box?
I ain't falling for dat one!!!!
Hey Honey - what do ya think about digging out the basement so's I can pour a few hundred sq. ft. of concrete for an I.B. woofer?????
Dave, you can't make common fiberglass work at very low frequencies just by increasing its thickness. Flow resitivity would be too high. It's all explained in Fuchs "Schallabsorber und Schalldaempfer". Don't know if it's available in English. You'll might find a detailed discussion in one of D'Antonio's texts.
Hey Honey - what do ya think about digging out the basement so's I can pour a few hundred sq. ft. of concrete for an I.B. woofer?????
Mine asked to remove stairs and door from living room to workshop, so I put concrete horn subwoofer under the floor where that stairs were.
Yes, you keep repeating that but the data disagrees with you. Anechoic chamber wedges (almost always in fiberglass) are very effective and scale to any size. Don't they know better?
David
el 'Ol, thanks.
re: post #9,
Oh my! That is very instructive.
I have done something similar behind the wideband driver in a closed box (B2) and it works as a deflection panel very well indeed - I added a smooth "tube" around the driver's magnet/frame structure with a half sphere on back of magnet that successfully reduced peaks/dips across mids - "borrowed" the ideas from the Rethym speakers.
I'm also a bit in the dark as to why it works at all as it doesn't seem to agree with any of the conventional speaker box theories ......
A few questions if I may - What driver are you using? What diameter green pipe at the rear? What happens without the vertical metal (?) strips? And where does the pink baffle plastic extrusion come from?
Rather interesting exercise with amazing results.
re: post #9,
Oh my! That is very instructive.
I have done something similar behind the wideband driver in a closed box (B2) and it works as a deflection panel very well indeed - I added a smooth "tube" around the driver's magnet/frame structure with a half sphere on back of magnet that successfully reduced peaks/dips across mids - "borrowed" the ideas from the Rethym speakers.
I'm also a bit in the dark as to why it works at all as it doesn't seem to agree with any of the conventional speaker box theories ......
A few questions if I may - What driver are you using? What diameter green pipe at the rear? What happens without the vertical metal (?) strips? And where does the pink baffle plastic extrusion come from?
Rather interesting exercise with amazing results.
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