dynamat or rubberized spray for HT subwoofer

[justinc]

would the use of either dynamat or rubberized under carriage spray on the inside of a subwoofer box improve the sound by helping to deaden vibrations?
The plan is to make the boxes 3/4" mdf and brace them both front to back and side to side. I was then considering spraying a couple good layers of one of these compounds on all internal surfaces, followed by using egg crate on all sides. Would this help with the sound of the subwoofer. or is this step just a waste of time? thanks alot

[tiroth]

You really don't need damping, so I'd skip the rubberized sealer. (I have used it sucessfully in full range boxes, but it isn't needed in a sub) If you want to go nuts, add more braces or increase the wall thickness. I like to see a brace every 6". These should be placed asymmetrically or even on angles if your woodworking skills are up to it.

I think the foam is a great idea because it can help kill some of the unwanted noises that can creep in even in sub cabinets. All walls is probably overkill though.

[justinc]

does it matter if the braces are out of 1/2 or 3/4 mdf, also I would assume the best method for cutting out room for airflow in the braces would be to use circles, to take advantage of the arch?

[tiroth]

All things being equal, 3/4" is obviously a lot (3x) stiffer, but if it comes down to having a limited amount of volume, it is probably better to have 3 1/2" braces than 2 3/4" ones, because you end up with smaller unbraced panel widths. It's even better, but more expensive, to use hardwood or quality plywood for bracing.

I don't know what the best method is for cutting openings, but I do use circles myself. Fast and easy with a hole saw or router.

[trev]

justinc. When it comes to the sound, the arrangement of the holes isn't important.
The wave is so much larger than the box that you don't actually have any soundwaves bouncing around inside the cab with a sub, your'e just pressurising the box off and on and so as long as the holes aren't really small, the driver will have no trouble vibrating the air in the box. Just be sure not to create any little sub-enclosures but that doesn't mean going mad and putting holes in everthing.
The arrangement of the holes does become important when your working out what will be the best place to put them in terms of rigidity. I use straight sided holes with rounded corners so that it's easier to place them in such a way so as not to have any too close together, meaning the gap between them is just a thin peice of wood, or having them too close to the sides so that the remaining peice that's joined to the side is doing little to support it. As a general rule, make sure there's at least an inch of wood between one hole and another or a hole and another panel.

As for eggboxes, their shapes is good for absorbing treble but bass doesn't even notice it. It's easier not too think of bass as sound because it doesn't behave the way we picture sound in our imagination. Because bass is so large, it pretty much ignores anything smaller than it is and will just diffract around it or go straight through it. Think of bass as a big wall of pressure (17.2m at 20Hz!) and it will be closer to the truth. Unless your'e dealing with very large objects (houses etc), bass ignores what shape something is and just cares how hard it is. With subs, your'e not dealing with a normal speaker so much as a pressure chamber and there is no such thing as too rigid which is why I make all my subs from solid diamond. No really, honestly...

[simon5]

It's always better to do circle holes for more stiffness. You have no stress point around a hole, in a square you got four corners which support much of the stress. If they are rounded, it's less important, but it's still better to do circle holes.