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i was just browsing through the high efficiency forum, and someone suggested "mass loading" speakers by putting bricks on top of them more or less. i could see that helping a little, but most of the box resonances in a typical speaker would occur in their larger front, sides and back baffles than tops though i could see doing so not only mass loading everything, but also adding tension to all the side walls.
i've owned mission and especially energy minimonitors that had superior drivers to my NHT superzeros, but were almost useless as far as i'm concerned when their ports were plugged because of lousy 5/8" (or less?) MDF adding lousy box resonances that were just as bad as the hideous port resonances unplugged, so i can relate to inadequate box issues.
i could also relate to "classic" speaker owners who want to keep their original factory cabinets and finishes. so, why not borrow some "car audio" tech and line your cabinets with fiberglass to improve them?
it would be a lot easier to just plaster fiberglass inside a cabinet than to try and build a full glass cab from scratch, and besides adding rigidity to a cabinet, the different density of glass would help dampen resonances wood can't without severely altering internal cabinet volume.
i once bought an empty 12" sub cab advertised as 3/4". but found out that it was only that thick on the front baffle and everything else was 5/8" or 1/2"! i dampened the cabinet by gluing 1/2" thick pieces of MDF throughout the box which was fairly labor intensive after having to cut all of the pieces down to fit through the woofer cutout and work only one side at a time with internal weights to improve bonding.
the cabinet ended up a lot better damped, but also weighed a ton after that besides the loss of internal volume from both the MDF i added as well as 2" & 3" acoustic foam lining leaving MAYBE 1 cubic foot of internal volume.
if i had to do it again, i'd try using fiberglass or hempglass instead as it would be so easy to roll it out against flat MDF compared to hand layering a contoured car sub.
actually, this is the tech i'm considering for a system i'm building from scratch with minimal tools. i was thinking of building an external shell out of laminate and building it up from the inside instead of fumbling around with glass over flannel tech because i need straight edges and corners for my application as well as wanting to keep weight down.
another idea for adding rigidity to cabinets without much weight would be to add internal bracing "ribs" out of balsa, or even cardboard that one glasses over.
here's to hoping this idea proves to be the perfect solution or inspiration for someone here
Edits: 02/03/17Follow Ups:
Practicing Curmudgeon & Audio Snob
......just an 'ON' switch, Please!
I've fixed a small sailboat with fiberglass and epoxy when I was a teenager. I've often wondered in my later years how would the stuff behave lining the inside of a loudspeaker cabinet. Something to try out for sure - but it's quite nasty to work with!
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