MDF/Sand/MDF cabinet details - diyAudio
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Old 10th March 2002, 05:25 PM   #1
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Default MDF/Sand/MDF cabinet details

I recently checked out an interesting article in AudioXpress on material choices for cabinets.

Cutting to the bone, the conclusion from that article was that MDF/sand/MDF + bracing was just about as good as you could make a cabinet.

Now, what do we know about:

Sand and wood thickness (I am thinking 6+mm MDF + as much sand as practical (perhaps 12mm) might be a good solution)

Size of sand particles (I am thinking that too fine particles will do a worse job of damping than relatively course grain particles)

Can anyone shed light on these parameters?

Petter
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Old 10th March 2002, 07:32 PM   #2
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Hi Petter.

Wood-sand-wood is an almost ideal buildingmaterial for loudspeakers, and only almost, because it's not as easy as just thick wood.

Idealy it should be MDF-sand-'another kind of wood'. The 'another kind of wood' is in order for it to have another acoustic profile than the first layer of wood. It should also, for the same reason, have a diffrent thickness.

A layer of sand has the delightfull quality of transforming motion = resonance into heat, and therefor lowering the acoustic output comming from the sides of the enclosure. Just remember to use oven-dried sand

I don't know if various grainsizes has different acoustic profiles or not, or a mix will be of any good, but why don't You try it out. I'd shure like to know.

About thickness in general: I'd go for as thick as practically possible, but don't fill up Your livingroom => LOW WAF(wife acceptance factor)
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Old 11th March 2002, 01:52 AM   #3
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Along those same damping lines, if one was using just a single layer of MDF, would spraying a coating of automobile rubber undercoating on all surfaces inside the cabinet prove to be an effective dampner?

Grant
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Old 11th March 2002, 02:21 AM   #4
Super is offline Super  United States
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A good, dry silica sand should do the trick.

Yes, the damping spray does do a good job, though I wouldnt rely on it entirely. Just give it a good thick coat, and use some sort of acoustic foam as well.
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Old 11th March 2002, 09:15 AM   #5
Nisbeth is offline Nisbeth  Denmark
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Petter, you might be right about larger particles damping resonances more efficiently, but smaller particles fills the space out more efficiently and so gives the highest weight, which would then provide for better damping or....?

Just a thought....

/Uffe
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Old 11th March 2002, 10:20 AM   #6
djss is offline djss  United States
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Default Sand = heavy

I was considering building a similar box for a set of 8 12 inch subs. In the end, we use 3 layers of 5/6 inch MDF for the sub box, coming to nearly 2 inches thick, plus internal bracing. At 2000W, a glass of water sitting on top won't have ripples in it: dead rock solid.

The equivalent design with MDF/sand/MDF would have weighed (calculated approx.) 150+ lbs. more. They already weigh in at 200 lbs. each (x2).

As far as I can tell, the addition of sand would make little difference next to thick MDF+bracing, and would pay a large hit to any thought of moving the speaker.

DJSS
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Old 11th March 2002, 10:01 PM   #7
CHRIS8 is offline CHRIS8  United States
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Petter...

A few ideas came to mind, this might be a good or bad thing depending on how you look at it

Do you intend on decoupling the inner enclosure from the outer shell? Also, do you intend to decouple the driver from the front baffle? If you are going to go through the trouble of building this, you may as well go all out IMO.

Doing the above things will reduce cabinet resoances even further. At least, decouple the drivers from the outer cabinet.

I recommend using 1" or so thick blocks of a soft rubber like compound(Shore A, 30 or lower softeness perhaps?) to space and support the inner cabinet inside the outer shell, then filling with sand. Actually, isolating with just empty space(air) and EFFECTIVE decoupling of the surfaces and air tight seals should be very effective, without need for sand. It would be interesting for you to measure relative vibration of the outer wall before AND after filling with sand and posting the results here(if you used an effective decoupling method).

To isolate the drivers from the outer enclosure, you could cut holes in the outer enclosre, and extend the drivers from the inner enclosure by making a raised platform, that extends partially to the outer shell so that they flush mount. To seal this joint between shells, you could use soft rubber sheets and adhesive to make gaskets, or just use a soft sealilng/caulking compound. Alternatively, you could also attach and decouple directly to the outer enclosure but you would not benefit from the damping effects of the mass coupling the drivers with the inner shell, and would still have to make a decoupled connection path for the drivers.

-Chris
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Old 11th March 2002, 10:27 PM   #8
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Default Thanks for all the great responses!

How about this construction technique:

1. Place oversize panel on work bench
2. Glue material to build the required distance to suit required final shape all around (+ a solid oversized piece where driver should be), and also a spacer piece where binding posts go.
3. Top up with sand
4. Glue top panel.
5. Cut to correct size and miter
6. Cut out spekar hole as required.

Build speaker as normal and pretend there is no sand.

Petter
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Old 12th March 2002, 02:02 AM   #9
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I have built a few cabinets with MDF/sand/MDF walls. I wanted the outside of the cabinet to look nice so I opted to put cleats around perimeter and a cleat in the middle of the first layer of wood. Then I attached the second layer. I used 12 3/4" steel rods for bracing, but I only attached the braces to the first layer. The overall thickness side walls is 2.25", the front is over 3" with asphalt between the 4 layers. The internal rear baffle is slanted about 40 degrees and the rear panel is parallel with the front creating a triagular chamber filled with sand. The base is 3" thick same for the top. I compared this to the 1.5" thick enclosure prototype, I must say the differences are astounding. I don't have an accelerometer to test the measureable differences, but I can cleary hear the difference. BTW, I used a SS 18W/8545 for this enclosure. If you can get a hold of 1" or 1.5" MDF it can help reduce resonances even further. The added mass helps, but the thicker the MDF is, the greater the density variation through the depth of the panel.

Kind Regards,

Digi
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Old 12th March 2002, 10:48 PM   #10
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Why not use something like dynamat to line the inner walls of the speaker enclosure? Dynamat is specifically designed to dampen sound vibrations...and it is available in various grades depending on what you want to do with it. Maybe sandwiching a layer or two of dynamat between two sheets of wood would work a better than sand, if not, it would be easier to work with.

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