Spkr Box Panel Thickness

[richie00boy]

For a sub, but applicable to any box I guess.

Thin (12mm) MDF with several braces, or thick (18-24mm) MDF with a single/no brace?

Is well-braced thin panels better in some way than heavy thick panels with little or no bracing?

I have read that some people claim that thick panels is not good because they just store the energy and release it later. I think this may be snake oil to some extent as doesn't the energy get converted to heat, not movement?

[marsupialx]

I'm fond of a couple of medium walls with deadening material 'floating' the interior walls. A mix of sand, groung up cork and ground up rubber, or something along those lines.

[Frazzled]

When I was in high school, oh so long ago, one of our class projects was to be in the design and building of a set of speakers that would sit on stage in our auditorium.

The final design called for a box within a box, 5/8 or 3/4 can't remember which, MDF.

The front panel of each box was the same piece of wood, the inner box helt the drivers and electronics while the outer box was filled to capacity with sand.

Upon completion measurements were taken 10hz to 30khz and the response curve was REALLY flat. When all was said and done our professor told us that the basic design was actually not one of our own but had been previously published by some other designer and that we had basically been given enough clues and information to complete the design without being given the design, it was kind of a let down really we had thought we really discovered something when in fact our prof was basically holding our hand the whole way.

It makes me wonder if those speakers are still sitting there on that stage and if they still work? If they don't work I doubt anyone has tried with and success to move them

[johnnyx]

I built a 120l sub out of 24mm MDF, with 2" x 2" timber braces. The rear panel still vibrated. I concluded that small boxes are stiffer, and that a shelf brace with large holes drilled (using a 3" or so hole cutter) beats timber for stiffness.

[markp]

There are several schools of thought in box design. Thinner rigid walls that have a high resonant freq that is out of the woofers power band so it is not excited. Thicker heavy walls that are too massive for the woofer to excite with enough energy for them to make noise. An absorbative fill to suck up the excess energy in the box. There are many others.

[richie00boy]

I did mean a shelf brace with holes in when I wrote about bracing

In the interests of being able to actually move the thing, I think I will try 12mm wood with plenty of bracing. The only hard part is bracing the front and back. Maybe a shelf that intersects with the ones in the other plane? Is it bad to connect braces together?

[leadbelly]

Originally posted by richie00boy
In the interests of being able to actually move the thing, I think I will try 12mm wood with plenty of bracing. The only hard part is bracing the front and back. Maybe a shelf that intersects with the ones in the other plane? Is it bad to connect braces together?

You are not mentioning the rest of your design, so it hard to answer in detail. Required box stiffness is not the same for an 8" sub and a high excursion 15"

If you are talking about a big sub here, I would say that you can only use 12mm MDF if you keep the maximum unbraced span at something like 5 to 6 inches to be safe.

Yes, the braces can be attached to each other, as I have done here:

[Paradise_Ice]

I understand what you mean Rich.

I used to make subwoofer boxes from MDF and i found that 25mm, 1 inch MDF gave the most solid and better sounding box but had a weight problem! that means they got to heavy to lift safely! once your using a whole sheet for an average floor standing speaker, it can be 50Kg without drive units!

I use the rib technic for 16 mm MDF now, its so easy cheap and effective compared to precision cut braces that have to be clamped down.

I cut 45mm by 16 mm slats or Ribs as i call them and glue them to every inner walls in the box, you can used as many or as few as you like.
You can change the resonance of a panel by adding a rib and this has a massive effect the sound quality of the box.
Its kind of like submarines, there only 2 inches of steel but with massively strong ribs or rings running along the length give it the ability to go to great depths.

A submarine would need to be made from much thicker steel to give it the strength equal the ribs stability.

Some of my friends even have steel rods with threaded ends protruding from the large side panels to tension the panels
in there speakers! they look ugly but it does works.

The strange part is there are so many grades of MDF, some are terrible and fall apart, then some have a better density and are very hard to cut, what is my point? i forgot.LOL

[Jonathan Bright]

With this question the work of Bowers & Wilkins is really useful. Many years ago they published a short paper on cabinet wall construction. Their research led to their "Matrix" enclosures. From memory they said that the stiffness, mass and damping of the walls were the items to evaluate. When it came to the colouration that enclosures produced the key parameter to reduce was the length of the reverberation time. They then explored the effects of each of the three factors to see how it effected reveb' time. I think the conclusion they came to was that the best results were achieved with fairly low mass and high stiffness and damping. It's a long time since I've read the paper so don't take my word for it. But to my mind it was a very convincing work with detailed theory and practise. I'm sure they will have the paper on the web site. There is a lot of v.good articiles there. Google wil find it.

[David Gatti]

You can still see lots of great technical articles on this and other subjects at :
http://www.bwspeakers.com/index.cfm/...s.techfeatures