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Subwoofer Cabinets - material and dampening
I would like to verify a couple of things sub cabinets.
My subs will be doing freqs below 50-60Hz(haven't decided yet).The box size is roughly 350mm cube.
Fact or fiction ?
1) at those wavelengths, padding/stuffing the box does not achieve anything, may as well have hard surfaces .
2) Bracing - normally it reduces wall flex(resonance & loss) and joints opening up. But in my case I will be using a Passive radiator, will my box still have resonances ie need bracing ?
3) Similarly the need for better material, say ply over mdf, is less with a braced box and for those freqs.
Anything else I should consider ? are feet important ?
Comments
FWIW my isobaric subs are about that size and I've never really noticed a problem with flexing. However they are birch ply, I think MDF will flex more. The upcoming ones will have bracing though. The "shelf" style of bracing seems to me like a waste of material, also a PITA to cut holes out of it, I'll be using 50mm struts e.g. imagine two glued along opposite sides, and then one between the two sides.
I've only used ply, I think I'll stick to it, at least with Birch ply it's not too much work to finish it so it looks OK and apparently MDF dust is toxic.
[Edit (I should think before I type!)] With a PR system stuffing will reduce the output level slightly, depending on how much. So some may be OK but not too much. I'll model it later on to illustrate.
For feet, yes they're very important, I think some like this would look good in your living room
Oops
Honestly, if I were building a sub then I'd give serious consideration to using Structaflor which is the wax impregnated particle board used for flooring - cheap and stable. The blue tongue version is 25mm thick. But the Red (22mm) and Yellow (19 mm) are probably easier to source.
With a sub we want to achieve maximum stiffness (for best performance) without making it too heavy (pragmatic). Box fill serves to lower the Q of a sealed box, which this is not as John pointed out. It's a way to put a driver into a slightly undersized box without high Q. Fill is detrimental here.
Bracing serves to increase stiffness, but it does not prevent flex of the walls, or resonances. It simply pushes them higher up, in the case of a subwoofer, placing them further away from the passband. A good design for a sub box is a bad design for a full range speaker, because the cabinet would tend to ring and the bracing design being optimised for maximum stiffness would likely also have many internal resonances. A bad design of bracing in a full range speaker can stuff up the frequency response - the holes in the bracing can act as little helmholz resonators.
Ply is more expensive and has more problems (voids, delamination) but it's stronger and lighter. The strength makes it better for subs without making it very heavy, so performance wise it's a better material. However, 18mm MDF with some bracing is perfectly fine for a sub and I tend to not use ply unless it's really necessary - say with a bass horn or a really big box. Yes, ply is better but it's advantages often become moot in a well braced design with a big arse bloody heavy sub driver!
Seano, I like the idea of 22mm over 18mm mdf, so might take your advice.
Paul, thanks for the theory, I have never really understood Q. You missed the point that my box will not be sealed. It will have a passive radiator, so I expect resonances etc to be much less as the walls will surely flex less. Even so, I will still brace the box, because I can, and one day I may need to stand on it to see the parade go passed.
Birch Formply :-) not expensive and virtually void free . After cutting up 5 sheets of the stuff only found a couple of voids 1mm x 1mm in size .
http://www.plyco.com.au/index.php?main_page=product_info&cPath=59_76&products_id=193
Or you could go with one of the best Form ply's ( Austraslian made) but your looking just over $100 a sheet .
The 25mm sheets weigh 49Kgs a sheet .
http://www.plyco.com.au/index.php?main_page=index&cPath=48_52
Cheers
Hey Mal
What do you reckon about some of that bamboo ply of Plyco's as a sub or speaker box material?
As an aside, I just bought 21 sheets of 9mm CD non-structural radiata ply to use as ceiling lining on our verandah. $30 per sheet.
Their Bamboo is really nice and would look great as a speaker cab but can be $$$$$$$$.
Their Birch ply is suited for CNCing being very flat across the board .
Doc on the forum has used their Bamboo for his kitchen bench tops .
Under the veranda are you going to use a moister barrier ( sarking foil ) between the ply and roofing material .
Cheers
Yep...Sisalation Tuff-Stuff has been up there a while. Just have to re-tape some joins.
Is the bamboo ply made like regular ply or more like a chopping block?
Like regular ply...except in smaller strips. There are a couple of different strip orientations too.
here's their line up
http://www.plyco.com.au/index.php?main_page=product_info&cPath=77&products_id=198
Cheers
Any idea of the cost Mal?
That reminds me... I have a couple of bamboos that needs to be dug up, should try and get to it while the soil's soft... (from all the rain)
I think for 30mm thick $300+ a sheet .I could find out if you give a sheet thickness you want .
Cheers