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Best Way to Brace a Box


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soundnsecurity 
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Posted: June 20, 2012 at 3:27 PM / IP Logged  
im about to build a box that is about 9Ft^3 internal. the front baffle will be doubled but i dont want to double the rest of the box because i dont have the space. i have about a 1/2 cubic foot to spare for bracing and i want to make the most of it.
my question is what is the best way to brace the walls of a box without obstructing the air flow inside the box?
box dimensions are 49 wide X 22 tall X 19 deep external. the box will also have a port on one side of the box that is 4 X 20.5 X 14.2.
the subwoofer going in is an 18" SX from RE audio. im designing this box more for quality and will be turned down 90% of the time but i need this box to be strong for the other 10% of the time that i have it shaking the block.
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: June 20, 2012 at 5:57 PM / IP Logged  
There are many approaches.  The key is to connect the front and back baffles, the top and bottom, and the side to side with something rigid.  Many people cut holes and patterns into sheets of MDF and glue and screw to connect the baffles (one piece can connect front, back, and two sides.)  Another approach if you don't have enough available airspace (or time and money) to insert two or three extra sheets of MDF is to use 1" dowels (or a reasonable facsimile like 1X1 pine.)  Cut the dowels to length and wedge between opposite baffles and glue and screw in place.  6 dowels (2 each for each direction) will work.
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soundnsecurity 
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Posted: June 20, 2012 at 10:26 PM / IP Logged  
thanks for the response man. i never thought about using wooden dowels before but i have heard of using threaded rods and bolts, which is almost the same thing. im not looking to be quite that extreme but id rather use MDF. its not a money thing thats stopping me from double layering the whole box i would just rather not have anything in the middle of the box obstructing the air flow. dowels would be ok because they are round.
my idea was to cut a few sections about 5 X 20.5 to go up against the walls and glue and screw them to the inner walls of the box. each piece would be .044 cubic feet which would allow me about 11 of these pieces to make up the 1/2Ft^3 i need to get down to about 8 cubic feet.
i have used pine 1X1"s in a few boxes before and im not sure that it is the best solution to have a good sound. in fact thats what i have in the edges and corners of the box i have my current sub in now.
if you say that using dowels will be better than lining the walls with MDF strips then ill try it because whats the worst that can happen. im willing to try anything as long as its a serious solution.
DYohn 
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Posted: June 21, 2012 at 10:53 AM / IP Logged  
If you want to prevent flex, doubling the walls can work.  If you want to prevent losses due to vibrations, connecting opposite walls is usually the better, easier and lighter solution.  Whoever told you that the shape of the construction inside the enclosure has any impact on the output is full of baloney.  As long as there are pathways that do not act like ports, square edges inside the enclosure do not impact a subwoofer system's performance.
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soundnsecurity 
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Posted: June 21, 2012 at 3:54 PM / IP Logged  
alright thanks that helps a little bit. nobody told me that square edges hurt the sound, its more about me not having anybody telling me anything. i have been mostly left alone to figure things out by myself so there is still a lot that im just not totally sure about and im always willing to learn something new from someone who knows better than me.
you say anything is fine as long as it does not act as a port, can you give me an example of what you mean? i understand what you are saying but im having a hard time picturing it in my mind.
DYohn 
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Posted: June 21, 2012 at 7:20 PM / IP Logged  

Here's an example using dowels.  These could also be square 1X1's

http://www.subwoofer-building.com/images/pics/my%20diy%20subwoofer%20bracing.jpg

Here's an example using MDF with opening cut into it:

Best Way to Brace a Box -- posted image.

If you have a divider with a single hole or with several small openings, it can cause the enclosure to function like a tuned chanber - a quasi-fourth order bandbass, which you do not want.

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soundnsecurity 
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Posted: June 22, 2012 at 10:03 AM / IP Logged  
ok so in the second picture, wouldnt every hole act as a port because you are pushing air through them? even though its still just the thickness of the wood, it still forms a cylinder inside the box? probably a very stupid question but i gotta know.
DYohn 
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Posted: June 22, 2012 at 10:33 AM / IP Logged  

soundnsecurity wrote:
ok so in the second picture, wouldnt every hole act as a port because you are pushing air through them? even though its still just the thickness of the wood, it still forms a cylinder inside the box? probably a very stupid question but i gotta know.

No because the area of the openings is large compared to the area of the "enclosures."  The relative air pressure across the openings will be equal (or nearly enough so that no Helmholtz effect occurs.)  Ports work because of a difference in air pressure.

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soundnsecurity 
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Posted: June 22, 2012 at 10:51 AM / IP Logged  
alright cool. i guess ill leave you alone now and do some research so that i can try to understand this a little better. do you have any links to any good reading material on the subject?
DYohn 
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Posted: June 22, 2012 at 11:05 AM / IP Logged  

soundnsecurity wrote:
alright cool. i guess ill leave you alone now and do some research so that i can try to understand this a little better. do you have any links to any good reading material on the subject?

You don't need to leave me alone, that's why I'm here!  Best Way to Brace a Box -- posted image.  Reading material on enclosure design?  Hmm, most of the web-stuff that I've seen is focused more on different alignments rather than the esoterica of how to design bracing, but let me look and see what I can find.

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