[DrewWinters]

Well, after serving us well for many years, it's time for a little upgrade to our HT subwoofer. It's a large one, about 4'x2'x2.5' and weighs about 150lbs. While it sounds good, the box is vibrating too much. I'm not sure how much this is degrading the sound, but it would be nice if we could at least put something on top of the sub without it vibrating off within a few minutes (I'm exaggerating a little bit.) This, plus the fact that I think I made the box a tad too large (it's a .577 Q box, so I'm pushing the limits anyway, but I think it ended up closer to .55 somehow), leads me to believe the best thing to do is to thicken the walls of the box rather than build a new one (too much work, not enough time).

So, my idea was to use fiberglass resin to lay down a 3/4" layer to add enough mass to dampen the inside and to shrink the box by the few percent I need.

Is this a good idea? Can I use some play-sand mixed in with the resin to make the resin go further (it's kinda expensive stuff, I'm hoping I can get away with what I already have)?

If it matters, the box is 3/4" MDF (Front panel is 1.5") with fairly substantial bracing and at least 10-15lbs of polyurethane (if not more) on the outside. I'm rather surprised that it vibrates at all.

I hope to start on this very soon, so TIA for your thoughts!

 

[ooheadsoo]

How about some quick cement? One thing I did (for the hell of it) was to take (large) scraps of wood I had, cutting them to size and glueing them to the inside of the walls. You can also consider getting plumber's putty or rope caulk and damping the woofer basket.

 

[Sovkiller]

Do you have bracing on the enclosure, for such a large sub, you need a very good internal bracing, you could reinforce also the larger walls of the case with another piece of MDF attached with a construction glue to the inside, 3/4 for a 4' tall sub is not too much IMO...

 

[rsaavedra]

Quote:

Originally Posted by DrewWinters Well, after serving us well for many years, it's time for a little upgrade to our HT subwoofer. It's a large one, about 4'x2'x2.5' and weighs about 150lbs. While it sounds good, the box is vibrating too much.

Wow that's a large box! Does it have any bracing inside? Does it have ports or is it a sealed box? I think speaker builder DIYers can provide better suggestions on this that I could. But I can recommend however a general inexpensive tweak for subwoofers: placing concrete slabs on it. In fact I used to have bricks on all my speakers, check this pic, in particular the subwoofer on the left. Depending on how yours is built it might not be suitable to support such weights on top though.

I think you could probably get better suggestions posting some pics of your sub, pics of the outside and the inside maybe.

 

[ooheadsoo]

Since he mentions Q, I assume it is sealed. He claims it is well braced in the post. More braces would probably be impossible to add at this point...

Mass on the top was something I remember DarkAngel posting on maybe a few years back, but considering all the poly he put on there, it's probably nice looking and wouldn't want to put junk on them.

 

[DrewWinters]

It is very well braced (IMHO) - adding more bracing would be impossible, as would be adding anything that cannot be poured. The bracing is just 3/4" MDF with 3" holes cut in them, so even just pouring something inside can be difficult...

The outside is more or less piano-finish, so while I'm not totally against setting a slab of concrete on top of it, I'd rather have the mass inside the box. We put alot of work into this, so I'd rather not set a slab on top - but it would certainly hold it up. I've stood on top of the sub on more than one occasion without giving it a second thought.

Not bad idea, of course, but I'm what I'm really looking for is whether or not the resin will work and whether it's a good idea to mix sand in with it. I figure the sand will both allow a thicker layer of resin and add some mass (wouldn't the sand be denser?)

Thanks for the quick replies!

EDIT: Oh yeah, it is sealed. To be more specific, it's an active 15" Adire Audio Tempest w/ an HS200 amp. It's a wall-shaker, but it won't pull the air out of your lungs.

 

[rsaavedra]

Just in case, of course you can't see this in my pic, but between the concrete slabs and the sub there is a sheet of cork about 1/8 - 1/10th of an inch thick. So there is absolutely zero scratching on the sub surface. In fact, I also have a thin layer of cork between the concrete slabs in order to avoid any possible vibration/rocking between them. Similarly for all concrete slabs under the sub. The only exception is the bottom slab, which has three pipe-ending domed nuts as spikes into the carpeted floor.

 

[DrewWinters]

Ah, that does make more sense. I would rather not have to go buy something else though, we already have the fiberglass resin.

For curiosity's sake though, how much to those concrete slabs cost?

 

[rsaavedra]

Quote:

Originally Posted by DrewWinters Ah, that does make more sense. I would rather not have to go buy something else though, we already have the fiberglass resin. For curiosity's sake though, how much to those concrete slabs cost?

Very inexpensive, if I remember well they were like $3 or $4 each on Home Depot. Can't recall exactly, also I used two different sizes. Probably it was less than $3 actually. That's for the slabs, the small bricks on the speakers were much less, a few cents each, probably $0.40 or so each.

[Edit for completeness sake]: between all bricks and the speakers there is also padding. In that case was blue tack and little self-adhesive felt casters. There are felt pads also in between all side to side bricks, so that there is no chance for vibration anywhere in those speakers, except for the drivers and tweeters that is

There is a lot of padding all over that room to avoid all kinds of possible vibrations. The paintings on the wall have globs of polyester foam stuffed in the backs of the corners using dots of white caulk. The CD's are tightly packed on that shelf with padded metallic bookends. Components have gel feet (and also bricks -padded- on top of the amp if you look carefully). There is padding as well between the center channel and the TV.

I tell you I miss that room. The acoustics there were pretty awesome for the entry level cost my rig had. Goes to say the room is just another component in your system, I took lots of care on it. I used to turn the fridge and AC off to listen to music. That's why I started padding things to eliminate all little supurious vibrations I could detect, or that could potentially vibrate. Thanks to the quiet neighbordhood, after 9-10pm that ended up being a dead silent room to enjoy listening to music.

 

[ooheadsoo]

I see no reason why the resin wouldn't work. It's just a matter of density and rigidity and resistance to resonance vs. something more conventional like concrete. Making your own concrete like substance with the sand+resin sounds cool, if you already have the sand and resin, but if you need to buy the sand, you might consider concrete for the heck of it. I have a feeling that concrete is the more dense, rigid, and dampened material by far, due to the likely ratio of hard materials to cement vs. your own proprietary mix.

Standard concrete is roughly 2.3 metric tons per cubic meter, right?

Madisound's forum would be a great place to ask.

 

[DrewWinters]

Well I left out the sand because we decided to just try it the normal way first...

So we decided to just do the top to start with and we filled each quadrant with about 3/4" of resin. (Quadrant = the bracing cuts the top into four sections)

So far, the resin has not yet dried AT ALL in one half of one quadrant (didn't mix the hardener well enough I guess <smacks forehead>)... and in the other quadrants the "non-shrinking" resin has shrunk away from the edges. I'm thinking when we turn the sub over, the pieces will simply fall out - if not immeaditely, then after a while of thumping it probably will... I dunno...

The moral of the story is: I suck at fiberglass (yes, I even managed to get it all over my hands.) The end.

No really, what am I supposed to do now? How can I keep these things from falling out? What did I do wrong?

 

[rickcr42]

concrete pavers from home depot.Available in natural gray and adobe red and from round to square to rectangular in various sizes/weights.

If there is a downside it is that they are crumbly,always flaking,and will scratch your speakers finish.
Problem #1 is cured by scraping the surface with a trowel then sealing the block with fireplace sealer or spar varnish.Do may coats the material is porous.
Problem # would be to make sure the block does not move.If it "dances" with the music just add another block on top of the first one.You want direct wood-to-paver contact or you defeat what you are attempting and that is to take the box tone out of the overall sound of the music

 

[rsaavedra]

And watch out for "rocking" pairs of blocks. Some of the blocks might not be perfectly flat, placing one on top of another might not sit perfectly and the top block might rock slightly if you press hard enough on one of the corners. The thin slice of cork in between the blocks eliminates this problem. Actually (here goes a Martha Stewart kind of secret of the trade) even a few stacked Bounty paper sheets between the blocks might do the trick. Same for between the block and the sub, though i would rather suggest the cork slice there, or else quite a few bounty "pages".

The blocks are crumbly indeed, especially when manipulating the heavier larger ones, little crumbles fall off from the points where your hands where handling them. Once placed and left alone untouched from then on they should't crumble too much if anything. But the barnishing sounds like a good idea for sure, especially with the large blocks.

 

[rickcr42]

Quote:

And watch out for "rocking" pairs of blocks. Some of the blocks might not be perfectly flat, placing one on top of another might not sit perfectly and the top block might rock slightly if you press hard enough on one of the corners.

Exactly correct.The way pavers are made you are only truly guaranteed one side will be flat from the bottom of the mold though gravity usually makes the top level as well.The cork solution would only work with an already level surface since it will not compress and conform to the hills and/or valleys of the non-level block.the Bounty suggestion much better since it will but be aware you will have to replace it periodically as it compresses then over time loses and elasticity (recovery ability)

 

[DrewWinters]

Alright, so we are out of resin now...

If we start using cement, do we need to do anything other than just pour the cement in? Do we need to use something like hardware cloth to add structure to the cement? Or drill holes in the MDF to give it something to attach to? And finally, can we just pour some on top of the resin we have already poured? (To fill in the cracks). The cement I should certainly be able to thin out using sand.... this would not make it too weak would it?

Thanks again guys for the help.