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In Reply to: Any/All similar sound absorbent material... posted by Thomas Martens on January 18, 2007 at 08:53:20:
One might call it that, but standing waves are low frequency waves, and they go right through damping material unimpeded, so it wouln't work. Standing waves also tend to be non-existent in most modern speakers that are simply too small inside for them to develop. C-V might have put that there to do what damping is supposed to do, absorb midrange reflections, but if it was done in lieu of lining the box it's bogus.
Follow Ups:
The box is insulated on each parallel side. The woofer crossovers over at 300 Hz to the mid. The insulating material is 1/2" thick and is almost like the padding used under wall to wall carpet. With the tweeter removed I can feel the air movement from the woofer so it is getting through the padding. The box is basically 32" x 17" x 15" with 2 4" ports 10" long.
Thanks Bill!
Assuming a 30.5" internal measurement a quarter-wave resonance could arise at about 110 Hz, taming that could have been the intent, though it would be woefully inadequate. Adding a very dense layer, such as felt carpet underlayment, to the top and bottom would help, while a ventilated shelf brace across the cabinet midsection, padded on both sides, would be a worthwile addition as well.
That's not true. Insulation that lines the walls does nothing for midbass absorbtion because the energy node isn't lumped up against the wall. Best place to put an absorber is at the 1/4 wave node, which is spaced out from the wall. You can line the box to remove upper midrange but to get the lower midrange/midbass, you MUST space out away from the wall. Cerwin Vega isn't the only one to do this.
'Best place to put an absorber is at the 1/4 wave node'
That's all well and good, but an inch thick layer isn't going to do the job. Even a 500 Hz wave will pass though as if it isn't there. Breaking up the interior into sections too small to allow midbass resonances to arise, and gaining additional panel bracing in the process, is a far more logical course.
Not so. In fact, an effective damper could be made as a very thin membrane that vibrated with resistance to remove energy from the node. The insulation does exactly that. Its position is what is important.
Hmm...V's theory requires the right membrane in the right place to defeat a specific resonance. There would be lots of trial and error.
Bill's idea (of breaking the box up with shelf braces so that each section is too small to allow a midbass resonance), is always a good idea - no trial and error is needed, more bracing is generally useful for box (outer wall) rigidity, and it nukes all resonances, rather than a specific one.
Life is short. My vote is for the method that always works :-)
Shelf braces in themselves have no effect on standing waves. They are there to make the panels more rigid, nothing more. A damper is something used that presents a resistance to motion, like the shock absorber does for a car. Shock absorbers prevenet the car's springs from bouncing violently with the weight of the car, and they damp the vibration to one or two cycles.This is exactly what sound absorbent material does inside a speaker cabinet too. It damps unwanted resonances that setup inside the cabinet. If the absorbent material isn't put in the right place, it doesn't do any good. It would be like putting the shock absorber in the trunk. Lining the sides of the cabinet with thick insulation does a good job of absorbing sound in the upper midrange, but it doesn't do anythung at lower midrange and midbass frequencies. Basically, the thickness sets the lowest frequency where damping is effective. If damping is required at a lower frequency, space the insulation material away from the cabinet walls, where it can absorb energy effectively in the frequency range required.
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