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Wagner
09-29-2010, 11:39 AM
Hi,
I have two veneered boxes, well made and constructed of 3/4" plywood with MDF front and rear baffles
Origin uncertain, but fairly certain they are "factory", just don't know who's
They house 15" coaxials similar to "604"s

My problem is, well, they sound like boxes! :(

The knuckle test not so good. The 50+ years old plywood on the sides almost resonants on it's own with a sharp rap.

There are fiberglass panels stapled to the interior surfaces, but these boxes still sound hollow on certain material; again at the risk of being redundant, they sound like boxes! :D

I'd like to not butcher them. Is there a good solution at Home Depot or the like? Something I can spray or brush on to calm things down a bit without sacrificing interior volume too much?

Thanks,
Thomas

grumpy
09-29-2010, 12:18 PM
resonant walls?... very roughly speaking you can raise the frequency (stiffening/bracing),
lower it (mass loading), or change how narrow the resonance is (damping)... usually you
end up changing more than one.

Knowing where it's being excited helps to indicate where to go construction-wise.

Thicker walls (e.g., glue/screw mdf to the inside walls and/or bracing seem to be common
solutions in general). The fiberglass is in there to absorb in-cabinet reflections,
not dampen cabinet wall-flexing resonances.

my thoughts on it, anyway.

richluvsound
09-29-2010, 12:21 PM
www.plastidip.co.uk/eStore/index.cfm?type=Domestic_Solutions/Plastidip&stage=1

! sure this is a US company ....

http://www.therustshop.com/shop/index.php?act=viewProd&productId=123 another product .

for the later seal the box with polyester resin first .... make sure you get it into the corners . It stinks BTW ... this process will find any cracks and tighten the box up !

Good luck

Wagner
09-29-2010, 12:36 PM
Thanks to both of you.

To try and be a little more specific:
When I rap on 5 of the 6 panels (tops and sides, it just sounds hollow, more like a cheap 1/4" box rather than one constructed of 3/4" plywood)

When I strike one side in particular, I can induce a deep buzzy resonance, almost as if the plys aren't glued. I can hear this when I strike it hard, right in the middle. It's not evident with music, I just discovered it while investigating the boxy sound.

The boxes are low void, no void, assembled with lock miter joints where the panels join. Like I said, they sure look well made; plenty tight at the joints, just this resonance. Some sort of variation on an open baffle, as they have these long vertical slots on the rear section of all 4 vertical sides, with sort of a restrictor strip on the interior side of the slot to control what I don't know? All the bits making up this controlled port or slot are made of MDF and securely stapled and glued in place. I think it's just the plywood? :dont-know:

It's as if they've been baked in an oven and are dry or brittle, if that makes any sense.

I have 3/4" plywood JBL boxes, constructed in exactly the same fashion and the "rap test" yields a nice dead thud.

Does plywood "dry out" like this?

Thank you,
Thomas

Wagner
09-29-2010, 12:51 PM
resonant walls?... very roughly speaking you can raise the frequency (stiffening/bracing),
lower it (mass loading), or change how narrow the resonance is (damping)... usually you
end up changing more than one.

Knowing where it's being excited helps to indicate where to go construction-wise.

Thicker walls (e.g., glue/screw mdf to the inside walls and/or bracing seem to be common
solutions in general). The fiberglass is in there to absorb in-cabinet reflections,
not dampen cabinet wall-flexing resonances.

my thoughts on it, anyway.

Yeah, I've though about just "veneering" the four interior sides and back with maybe a 1/4" thickness of MDF :dont-know:

A brush or spray solution would be nice, with good endorsement.

Thomas

jerry_rig
09-29-2010, 02:36 PM
Modern speaker builders spend much effort on bracing internal panels. I can vouch for this as a means of improving the "rap" test. It is not too difficult to cut 3/4" thick by 4" wide boards to span the interior walls (particularly near their centers). You don't need too many. Cut the boards a tiny bit longer than necessary, glue the ends and tap them into place. It will tighten things up considerably. On the other hand, if your plywood is separating or the veneer is loose, this will not help too much.

richluvsound
09-29-2010, 03:22 PM
Wagner ,

what you describe sounds like the ply is de-laminating ... very rare ,but it can happen . If this is the case they are finished ..... I hope I'm wrong .

Some opinions would be helpful !

Rich

Wagner
09-29-2010, 04:00 PM
Wagner ,

what you describe sounds like the ply is de-laminating ... very rare ,but it can happen . If this is the case they are finished ..... I hope I'm wrong .

Some opinions would be helpful !

Rich

Thanks men for your contributions.

I do not think that is the case, just my feeble attempt to draw a picture of a sound with words.

It's just not the sound you would expect to hear hitting any piece of plywood.

It's very subdued, but audible, and as I said, the frequencies of music don't seem to excite it.

I stumbled into it contemplating the hollow, "boxy" nature of these cabs.

The sound very well could be coming from the area of the vent structure.

You know how hard it is to pin point the source of some sounds, especially when they are the ones you DON'T want.

Like rattles and squeaks in an old car.

Thomas

1audiohack
09-30-2010, 10:39 AM
If you think it may be the vent structure, stuff them with fiberglass and give a listen. Next load the whole box and listen.

If you think it's the panels, get three bricks and some cloth to wrap them in, a ratchet strap and four pieces of cardboard or cloth to fold around the cabinet edges so the strap does not damage the corners. Hold the bricks (assistant requred) on edge close to center of the side and rear panels, pull the strap around them all and strap them snuggly in place. This will strain the panels inward (be carefull) and add mass to the panels, listen again.

If it's the box that's offending and you don't have a ton of test gear, position the box so the offending panel faces up, pour a little sand (rice or dry coffee grounds) on the surface and sweep it with a signal generator (lap top sound card, test CD, what ever) connected to an amp and you will easily see if the panel takes off and at what frecuency.

In a half an hour you would know exactly what you are fighting with out doing anything irreversible. Nah, do the hardest thing first.

Robh3606
09-30-2010, 01:23 PM
I would just add braces with glue and screws. That should stiffen it up quite a bit. I also use a Dampening compound called MasCoat to help tighten the box up. It dries hard and fills in any and all voids along any glue lines.

Rob:)

Altec Best
09-30-2010, 05:21 PM
.

To try and be a little more specific:
When I rap on 5 of the 6 panels (tops and sides, it just sounds hollow, more like a cheap 1/4" box rather than one constructed of 3/4" plywood)


:useless:




Hey Thomas cabinets sounds interesting would you have some pictures you can post ? :)

Wagner
09-30-2010, 11:39 PM
I would just add braces with glue and screws. That should stiffen it up quite a bit. I also use a Dampening compound called MasCoat to help tighten the box up. It dries hard and fills in any and all voids along any glue lines.

Rob:)

You have a favorite technique?

Thanks,
Jim

Robh3606
10-02-2010, 07:20 AM
You have a favorite technique?


Not really I just use a fair amout of bracing, I use 2x3 and 1x2 for stiffenig the cabinets. Lots of glue and screws. Use 2x3 on edge and the 1x2 flat. I tend to break the panels up into odd sizes and will run a X Brace between the center on the panels in some boxes. I use the Masscoat to seal it all up and it does make a difference as it helps deaden the box when you do a knuckle rap test. It shouldn't because the added mass isn't significant but it seems to works just fine anyway.

Rob:)

http://www.audioheritage.org/vbulletin/showthread.php?18400-Cabinet-Dampening-Materials&highlight=cabinet+dampening