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Bracing an enclosure

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Silver - Posts: 433
Silver spacespace
Joined: September 19, 2004
Location: United States
Posted: April 05, 2006 at 1:32 AM / IP Logged  
I just read a few articles on bracing (bout to build a new box for my sub) and was curious on everone's oppinon on this. How much bracing does everyone have in their custom made boxes? I've read that you should brace the crap out fo the box and also just a few 3" triangle corner braces are plenty. My current box just has 1 devider but is mostly fiberglass and was able to hold up to my 200 lbs and that of my friend (160 lb) at the same time so the fg was of no concern but the mdf was what I am wondering about. Just curious as to everyones oppinion.
Copper - Posts: 85
Copper spacespace
Joined: July 18, 2004
Location: United States
Posted: April 05, 2006 at 10:34 AM / IP Logged  
i just finished a box i was working on with a friend of mine, and being as it fit into a storage well in the car i didnt do much added bracing for the MDF, but i found that when we put in the dowels to hold the speaker rings in place, using the extra dowels around the rings helped not only to brace the rings,  but to add some reasonably useful curves to the fiberglass as it was laid down as it is stronger in curves, and also helped to shape the box (the fleece wrapped around the dowels beautifully). so my take on it is that a good structurally sound box may not need so much bracing at all, especially if it fits somehwere(cargo hold, etc.)  
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Copper - Posts: 204
Copper spacespace
Joined: March 15, 2006
Location: United States
Posted: April 06, 2006 at 6:28 PM / IP Logged  
Just because you can stand 400 lbs on an enclosure doesn't mean it is going to sound good. Feet are big and it spreads out well over an enclosure. It is the walls of the enclosure vibrating however much they do that causes cancellation and sloppy sounding bass. I bet over the same surface area your fiberglass section could hold up to the same weight. And if the fiberglass section is curved, it probably vibrates/resonates even less than the mdf.
fiberglass reminds me of peanut brittle... but fiberglass tastes better!
Copper - Posts: 185
Copper spacespace
Joined: March 03, 2006
Location: Canada
Posted: April 07, 2006 at 2:03 PM / IP Logged  

it all depends on how large the MDF panels are...and if the corners are screwed and glued..and the shape of the box...and what size MDF you are using...and what kinda sub amp combo you safe and use 3/4" or greater...and provided your panels arent larger then i'd say 2 square feet youwont need much bracing..remember bracing also eats up air if you need to cut down on internal box volume then add some braces..if your already pusshing the limits on internal volume then either cut down on the braces or pack the snot out of her with polyfill..

shorty and curlies of it...if it feels sturdy and you feel the 3" triangle will work then go with it...if you can bend the panel over your knee and make it flex then your gaonna have to brace it...chances are you wont be able to bend a piece of 3/4" MDF though...christ i can hardly bend a full sheet of

if your unning a 13W7 with a couple 1oo1.1's then your gonna need some seriously thick wood..double up the MDf and brace her well....

Fiberglass Guru.
Member - Posts: 3
Member spacespace
Joined: April 22, 2006
Posted: April 22, 2006 at 1:35 AM / IP Logged  
I performed some vibration calculations on the resonant frequency of an MDF board. Using available material properties, assuming nominal thickness of 3/4in, and a dimension of 9in ... the resonant frequency is roughly 140Hz. This is nearly an octave above the range required of a subwoofer so I'd say something along the lines of 9in is the largest dimension you'll want to allow without bracing. Logically, a longer dimension will lower its resonant frequency, which will cross into the operating range of the subwoofer and will cause distortion.
Caveat, the calculation I used included an assumption which did not take into account the second dimension of the panel. So, considering this effect I would say a dimension of no more than 12 inches can be used without bracing.
(For those knowledgeable and interested, the calculation I used is that for an undamped harmonic system with a harmonic forcing function, the panel is assumed to be a rectangular beam with two clamped ends.)
Member - Posts: 3
Member spacespace
Joined: April 22, 2006
Posted: April 22, 2006 at 1:39 AM / IP Logged  
One thing I forgot to mention, which you may or may not be aware of: While putting more power to a subwoofer will increase the pressure level inside of the box, thus increasing the amplitude of the vibration of the panels... the actual frequency of the resonance is determined completely by the dimensions of the panel (as well as the material properties...). So, despite how loud you plan on playing the subwoofer it is reasonable to conclude that the amount of noise from resonating panels will always be proportional to the output level of the box. In other words, even a "quiet" box should be appropriately braced ... as a quiet box will still produce quiet noise.

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