| interesting damping material [message #21661] |
Sun, 13 August 2006 23:20  |
moray james
Messages: 51
Registered: May 2009
|
Baron |
|
|
interesting damping material Post #1
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. 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.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| Re: interesting damping material [message #21670 is a reply to message #21669] |
Tue, 15 August 2006 05:01  |
wunhuanglo
Messages: 912
Registered: May 2009
|
Illuminati (2nd Degree) |
|
|
|
This discussion sounds like an apple-and-oranges thing to me. KEF's point was effectivey to substitute activated charcoal for fiberglass. I'm not sure I buy into that myself - the hydrodynamics of the situation have to be entirely different since the open area of the charcoal has characteric dimensions on the molecular scale as opposed to the open area of the fiberglass which is of a far different character. But once the charcoal (or pearlite) is in a bag it's now supporting a continuous reflective surface. That surface may be malleable, it may break up wavefronts, it may dissipate energy due to its lack of rigidity, but the mode of operation, if you will, is entirely different with respect to the air moving in the speaker cabinet. In the case of the loose fill situation it's more of velocity dissipation by hydraulic resistance (losing air pressure down a long hose) where in the baggie case it's analogous to using any other sort of non-rigid medium to absorb energy. I believe one of the earliest uses of the pearlite-in-a-bag concept was in Jim Lansing's 1937 Iconic loudspeaker where he used mineral wool held to the back wall of the speaker cabinet with a sheet of cheese cloth-like material. This way air was free to move through the medium while dissipating energy through frictional losses.
|
|
|
|
Posting Rules
Members
Search
Help
Register
Login
Get a T-Shirt!
