In Reply to: dampening material posted by grssplst on January 7, 2002 at 14:41:30:
Yoav's approach is probably the best overall. Trying to calculate your way into the answer probably is an exercise in frustration (or futility)But I do have something to add, and that you need to keep in mind. A small, sealed, and filled enclosure is properly called an acoustic suspension enclosure, and the filling is not there primarily for Q control or reflection control. It's there for thermodynamic reasons. A closed volume of air if compressed or expanded will not only change pressure, it will heat or cool. (adiabatic) This because no heat transfer is taking place. So the compression obeys a power law relationship and is not linear. Therefore, it introduces harmonic distortion. In Vilchur's Acoustic Suspension design he used the stuffing to transfer heat into and out of the air during the compression and expansion at an audio frequency rate. Since the mass of the filler, and it's thermal capacity is many thimes that of the air, the temperature of the air remains nearly constant.(isothermal)If the temperature remains constant, then the pressure/volume relationship is linear. This reduces the pressure rise (making the box look bigger), but it also reduces the harmonic distortion.
So if you leave out the filling so that your math works more precisely, you will lose the major benefit of the acoustic suspension principal. Further fiberglass has a lot more thermal capacity and better thermal properties than polyester fiber or Acousti Stuff. I have used 6" unbacked home insulation succesfully. JUst be sure you place the sheets edge on to the woofer. The surfaces of the insulation is pretty dense and closed, and will impede expansion and compression. Edge on the glass fiber is porous to the pressure waves.
Vilchur's original experiments used very fine brass wool (like 000 steel wool),but the production speakers used glass fiber. There is a measureable difference in harmonic distortion between glass and brass fiber, but supposedly the difference was inaudible. So the production AR's used glass since they din't need to worry about electrical insulation problems that way.
The fact that the filler is a good absorber, and prevents internal reflections at higher frequencies is just a happy side effect of it's primary purpose. The approach I take is to completely fill the box, and shift the Q by changing the packing density.
Typically, you can expect a 10-12% decrease in Q, and a 6% decrease in the in box resonance from the basic calculations. (that's from a paper by P. J. Snyder)
Hope this helps. I'm really an acoustic suspension buff, and I'd like to see better understanding of its non-obvious, but rather elegant principals. Hope you don't mind a thermo lession.
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Follow Ups
- Re: dampening material - Bold Eagle 18:17:09 01/09/02 (1)
- Re: dampening material - grssplst 19:18:43 01/09/02 (0)