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
 
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) :D
 
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
 

CHRIS8

Disabled Account
2001-12-12 8:47 am
VA, USA
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
 
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
 

Digi

Member
2002-02-02 5:37 pm
USA
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
 
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.

-= SsZERO =-
 
Sand Cabs

The front baffle can be a double triple or whatever thickness of the material you are using for your exterior enclosure.
A note on the sand grain size. The smaller the grain the higher the mass that will be abble to be packed into the cavity. The speaker cavities should be tamped with a mallet or a hammer and a block of wood to promote the settling of the sand. Be 100% certain that your sand is dry. Wet sand will cause some serious problems.

Mark
 
Re: Sand Loaded Cabinets

Yes, I've thunk that. :)

-= SsZERO =-

mwmkravchenko said:
Has anyone thought of using a cardboard concrete pier tube for the inner enclosure of the sub and filling the voids with sand.

I used this technique on a line of small monitors that I designed in 89. They had an extremely dead cabinet and ease of construction. Easier than true double walls.
 
Sand Et Al

The card board tube method also has the added benefit of creating a internal barrier of a differning youngs modulus. This will provide a damping a band of unwanted noise that a MDF box alone cannot convert into lost vibration and huge amount of heat provided by the cesation of those little vibes.

Negative vibes it's getting kind of smokey in here. Time to open a window I think some brain cells are calling for help;) :eek:
 
has anyone used fiberglass to sitffen the inner walls?

I am thinking of a box using MDF/sand/MDF cabinet (18mm MDF and 12mm sand) with 3mm of resin bonded woven fiberglass on the inner wall.

The baffle will be 30mm MDF with fiber as making driver holes through aa MDF/sand/MDF combination might prove difficult.

can lead sheet also be used and how. it heavy but soft and while it can be used to damp panel resonances of Cd players (on top a CD players or glued to the underside of the CD player's cover) it is difficult to use it in speakers.
 
:cool:
about the fiberglass idea.

Not the greatest if you are thinking damping. Stiffness yes, here's the but. There is a good chance you are going to introduce resonances in the midrange that are going to be very audable. In short it is not worth the effort to combine all the MDF sand construction with the fiber glass idea. you might try a series of shelf braces perhaps crossed to form a septum in the vertical and horizontal planes within the cabinet. If it is a large one try a number of them.

Mark:rolleyes:
 
Stiff snad loaded cabinets.

:confused:

Ok the basics.
1. the cabinet isolates the rear pressure wave from the front pressure wave developed by your driver, when it is in motion.
Well duh eh?
2. The cabinet forms a compliant volume of air that aids the linear movement of your driver, when it is in motion.
Oh!
3. The cabinet does this and has a bad attitude. It likes to speak for itself.
Nuts!
Stiff cabinets perform all the above and unfortunately number three very well. The answer is the combination of the three possible boxes. Massive, lossy, und stiff. The stooges of the audio world upon whom so much genuine bovine renderings have been heaped upon that they could grow a genuine organic garden. Everyone has this golden opinion that is the end of all advice. I'm from canada PROVE IT!!!!
The truth is that only a combination of the three will give you the ultimate box that vill go forth un konquer ze audio vorld yah are you mit me????
ok.
I'm back. Your idea of a stiff outer shell is good. Multiple layers of say 1/8th" finnish birch will give you a stiff outer shell. Your baffle should be a composite of differing densities of manufactured wood products (research I can't give it all away). The interior cavity could be a cardboard forming tube that is shaped in the form of an elipse that is flattened on one of the smaller parts of the arch. The falt can be created by cutting angled curf cuts in your baffle and then glueing in the card board tube. the top and bottom of the tube could be sealed by routing a shallow groove to set and glue them into. The reamaing free "wasted space" that makes such a cabinet larger than a normal one will be filled with sand. Any dry sand. The sand should be vibrated to a decent compactness by tapping the enclosure with a hammer and block of wood (I'll leave the order up to you)(I'm evil I know). The result of all this extra work larger cabinet etc. is not going to be heard.

What????

Thats what those expensive drivers are for. You want to hear them your cabinet to do numbers on to two and thats it. If you are seriously going to try this let me know.

Mark ;)
 
Mr. Concrete

If you are in this string on damprd boxes it is beyond me why you are asking why not concrete. I don't want to be mean or imply that you are stupid. It's just that a monolithic structure like a concrete pipe is going to have a series of frequencies where it rings like a bell. The general flow of this thread has been to design enclosures that minimise those spurious noises.

Mark:) :)