[mikeeastman]

I'm about to build a sub cabinet, it will be a double layered box with the out side Birch ply and the inside MDF.
I saw some ultra light MDF and as box will be heavy I thought I would use it to save some weight. The inside will be lined with 3/4" counter top material with No- Rez over that, anyone know why the Ultra light MDF would not work?

  Mike

[FullRangeMan]

I unknow ultra light MDF, but if it is airy inside/if it had air bubles inside I would no use it for enclousure at all, as it seems a fragile material.
Also I would suggest use the BB ply inside the enclousure and a solid MDF on outside, due BB ply be more prone to absorb wet/moisture.
A enclousure had to be heavy to minimize resonances.

[doorman]

FWIW, the dust produced by machining MDF is nasty stuff---
Best
Don

[mikeeastman]

I live in Az, so moisture not a problem. The ultra light isn't airy, they get the lighter weight by using a lighter weight wood to make it. I think with the 3/4" layer of  Synthetic Stone and No-Rez on the inside that resonances won't be a problem. I'm having a cabinet shop do all the cutting and routering so dust not a problem for me.

   Mike

[Bob in St. Louis]

Mass and weight are generally a very desirable trait in a speaker enclosure, even more so in a sub enclosure.
Personally, I'd look for the super-ultra-heavy MDF to build a subwoofer with.

Just my thoughts....

[PDR]

I'd consider sand, but pretty hard to get the lightweight stuff.....
Since I built my latest project with a sand layer, thats how I'll do any others.

[Bob in St. Louis]

Ditto that sand comment. 

[Vapor Audio]

Believe it or not, the ultra-light MDF isn't bad stuff.  Yes, the mass would be better.  But ultra-light typically is also "ultra-refined" MDF, which means it starts as 100% pine which is ground and recompomposed with formaldahide free glue.  And it's also, at least what I've used, water proof so it won't swell like typical MDF will. 

Even better are ultra-refined high-density MDF's like Medex, Medite, Ranger Board, Plum Creek, etc. 

[Bob in St. Louis]

And it's also, at least what I've used, water proof so it won't swell like typical MDF will. 

Well now....that's a game changer. That aspect right there is worth it, as far as I'm concerned.
Mass can be had by other means, but not having MDF changing its size based on the humidity levels is a concept I've never heard of.
That's pretty cool.

[PDR]

That is cool.....waterproof.

A funny story......sorry dont mean to derail.....was gluing together a set in the garage one hot summer day
and placed them outside on the ground to set up in the sun. I have 3 dogs......you can guess the rest.

[mikeeastman]

The last sub I built I did the sand design but used lead shot instead of sand, I thought I'd try something deferent and read about lining inside with synthetic stone material and thought I'd give it a shot. My other sub weighs over 200lbs.

  Thanks, Mike