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Old 14th August 2006, 05:01 AM   #1
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Default interesting damping material

Just finished spending a day experimenting with box cavity damping. The speakers in question are ATC SCM10's which are factory damped with long fiber wool. This ia a small cavity sealed box with an impressive 5.5 inch bass/mid driver that has a very long linear excursion.
A friend of mine attended the Hi Fi Show in Germany this spring and had an invite to spend the nite at a private showing of the new KEF Reference loudspeaker. One of the cool things about the KEF is that it is internally damped with activated charcoal. This is a neat idea as activated charcoal is very porous. Kef claim to have reached a virtually theoretical cavity volume increase of 28 out of 30 % using the charcoal. This got me thinking. Charcoal is messy and expensive. Pearlite is a heat expanded form of rock used in the gardening business usually white but comes in various colours. Pearlite is basically rock heated up like popcoarn or Rice Crispy's and ends up like small kernals of porous soft rock which is soft enough that you can crush it to powder with your fingers. Pearlite is more porus than activated charcoal and is clean and cheap to buy. I replaced all the internal wool damping with the pearlite in the ATC SCM10's. This resulted in a significant improvement across the band with especially improved midrange and more extended bass. You do need to take some precautions however. Pearlite must be sifted first to extract all the fine powder as you only want pieces the size of rice crispy's and larger. Further you need to insure that the pearlite is kept out of the driver/s. This done I would have to say that this is by far the best damping material that I have ever used. This is I think as a result of the fact that pearlite is so very porus and has high resistivity to passage of air and also in that it has tremendous frictional dissapation due to the partical vibrating against one another. Bug screen and fiber batting like polyester or acrylic will keep the pearlite where you want it and permit free air flow about the back side of the driver. For ported boxes you would want to keep a fair size area (should think a minimum of a 1/2 cu. ft.) around the reflex vent free of any damping material to insure correct vent resonance. There you have it cheap and SOTA damping material that works like a charm. You read it hear first. For those who cannot readilly get thier hands on pearlite or who disbelieve a good second choice would be rice crispy's (dont laugh) but they will cost more than pearlite however if you are not fussy you can at least eat the rice crispy's after. You can expect the pearlite to be more effective than the rice crispy's as it is far more porous in structure. BE FOREWARNED you must properly prepare for using pearlite to insure that it only goes where you want it to. This is a real gem of a damping material especially for ultra small cabinet speakers and I should think the absolute bomb for TL's. I am sure that some will laugh till they hurt but do give it a try and for those to busy laughing well that's your loss. Very best regards Moray James.
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Old 14th August 2006, 10:07 AM   #2
sreten is online now sreten  United Kingdom
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Hi,

I'm not one for knocking some-one who's prepared to
try things based on a series of logical presumptions.

Quote:
KEF's research shows that the apparent increase in volume
achievable in practice can be as large as 3x. Still greater
enhancements are feasible but rendered impractical because
the activated carbon then adds too much internal damping.

Increasing the apparent cabinet volume by a factor of three equates to:

1) a 30 per cent (over a third of an octave) reduction in the bass
corner frequency while keeping sensitivity and box volume unchanged;
2) a 4.8dB improvement in sensitivity while keeping bass corner
frequency and box volume unchanged;
3) a downsizing of the cabinet internal volume by two-thirds
while keeping bass corner frequency and sensitivity unchanged;

or a spectrum of trade-offs between these extremes.
If your fill is effective as KEFs, then the bass driver parameters
may not suit particularly the new effective cabinet size meaning
"improvements" in bass extension may be modest.

(edit : pearlite = vermiculite in the UK, not keen on the rice krispies)

/sreten.
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Old 14th August 2006, 04:43 PM   #3
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Here is KEF's white paper on the matter from 2001 in pdf form

http://www.kef.com/technology/pdfs/acewp.pdf

Interesting reading indeed.

J.D.
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Old 14th August 2006, 04:58 PM   #4
ScottG is offline ScottG  United States
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Very Cool! Thanks moray james!
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Old 14th August 2006, 05:09 PM   #5
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Default Thanks for running and not laughing!

Good to see people run with the ball! Walton thank you for the reference to the KEF paper. The simple solution to the dust and particle issue (driver contamination) is to place the perlite into sealed plastic bags and then install the filled bags into your speaker box cavity. Regards Moray James
here is a link to other comon garden materials that may also be of use http://www.gchydro.com/information_Growmed.asp
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Old 14th August 2006, 05:46 PM   #6
sreten is online now sreten  United Kingdom
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Default Re: Thanks for running and not laughing!

Quote:
Originally posted by moray james
......The simple solution to the dust and particle issue (driver contamination) is to place
the perlite into sealed plastic bags and then install the filled bags into your speaker box cavity.........

Huh ? I dont think so. Would completely defeat the point.
The vermiculite would need baking and then quickly put
into completely sealed boxes adding hygroscopic crystals
(the little bags that come with cameras / videos etc).

The drivers would also need to be fully sealed, no air path.

the hygroscopic issue is major, rice krispies don't stay
crispy for very long if you leave them out of the packet.

I've also visions of opening up your speakers after a
good pounding to find a pile of dust at the bottom
of the speaker.......

/sreten.
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Old 14th August 2006, 06:18 PM   #7
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Default Installing Perlite

I will have to disagree with you on this one with all due respect. The perlite will work just as effectively packaged into plastic bags and sealed. In fact it may well be more effective that way. Have a look at Ted Jordan's work on membrane absorbers. Most all damping materials will hold moisture and I don't see a problem with Perlite being any worse than fiberglass or any other synthetic fiber material even wool. As for the break down of Perlite I guess that the thing to do is to damp a sub woofer box with Perlite and then check it out after a few weeks of good use. If fresh large size particles of Perlite are only used and the contents are placed inside of a sealed bag then it will be eady to determine how much of an issue breakdown will be. Issues of longevity are secondary to the fact that this material works extreemly well as is. Contamination is not an issue if the Perlite is installed inside of a sealed plastic bag. Moisture does not damage Perlite can hold more moisture than do conventional damping materials but the plastic bag method would als deal with that issue as well. Lets try to look on the positive side of things here. I am sure that your valid concers can be dealt with with a little thought. Regards Moray james.
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Old 14th August 2006, 07:55 PM   #8
Tachyon is offline Tachyon  Sweden
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WARNING:

When working with perlite in dry form a face-mask MUST be used. Since perlite is a form of natural glass those tiny paricles is hazardous to lungs.

Perlite when used in horticulture is mixed with water before mixing it with soil to avoid the dust.

Be safe!
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Old 14th August 2006, 11:21 PM   #9
maxro is offline maxro  Canada
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Default Re: Installing Perlite

Quote:
Originally posted by moray james
.... The perlite will work just as effectively packaged into plastic bags and sealed. In fact it may well be more effective that way. Have a look at Ted Jordan's work on membrane absorbers....
I'm afraid I don't understand something here. I tried googling for Edward Jordan Membrane Absorbers. No dice. Maybe you can provide a link. How is the air supposed to penetrate the sealed plastic bag? Or does it not have to? I must say I don't like the idea of rustling rice crispies inside my speakers. Maybe some bricks of porous lava rock or similar would work, or does this method rely on the frictional movement of small grains?

Max
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Old 15th August 2006, 01:08 AM   #10
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Default vermiculite.

yeah i agree... be careful with this stuff.

do not breath it in. I think a manufacturer was actually sued here in the USA by ppl getting lung cancer from breathing in this stuff.

but good idea anyways
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