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In Reply to: Re: Any/All similar sound absorbent material... posted by V on January 23, 2007 at 14:28:36:
'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.
Follow Ups:
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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