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Discussion Starter #41
My suggestion on the bracing is if you are going to brace in the middle of the cabinet with 1"x4" or similar instead of placing the braces parallel to the walls of the cabinet put them at an angle.
What's the benefit of putting them at an angle?
 

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My thoughts are that if braces are needed to reinforce the cabinet and increase the frequency at which the cabinet resonates (as well as reduce the tendency to resonate) placing them as non-symmetrically located as possible and oriented so that they break the cabinet into a number of non rectilinear sections may be beneficial. I can see at least 3 benefits;

1) oddly shaped spaces will discourage standing waves.

2) panels whose braces are assymetrical and oriented "randomly" will tend to have fewer reinforced resonances. In other words if there are multiple panels with the same geometry and bracing they will tend to resonate at the same frequency, reinforcing (amplifying) the resonated sound.

3) reflecting high frequencies at an angle will increase the amount of travel before they will go through the speaker cone, (as opposed to hitting a flat piece of wood and bouncing right back) decreasing their amplitude, and increasing the amount of polyfill they will travel through, reducing their amplitude futher.

These are my ideas, I do not have test data, or other empirical evidence to substantiate them. Perhaps some heavyweights can weigh in on these concepts.

I would not expect a large difference, but with a little effort you may get some benefit.

Paul
 

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Discussion Starter #43
Paul,

Your thoughts seem very logical... And anyway I think that if the benefits are only minimal, there is no drawback!!
 

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Regarding standing waves, I have a set of Atlantic Technology speakers (System 8200) where the cabinets have sloped backs that is suppose to help with standing waves.

Would the same design benefit subwoofers?? Any guesses how much slope would be correct??
 

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Discussion Starter #45
Of course this would help for subs also. I applied such a shape for my car's, but for the monsters I will be building I have to keep the rectangular shape for better utilization of volume and save some space.

I do not think the slope is so critical....remember that all these are hepling factors but not basics IMO...

B Rgds
 

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for what it is worth is I like MDF
I on the other hand, would follow the advice of the cabinet maker, and build the box out of plywood. You want it to be void free (ie Baltic Birch or ApplePly).

Hopefully, without starting a holy-war, i will state my opinion, that MDF is one of the worst materials for building speaker enclosures. I'll put an exclaimation on that by saying that we get MDF for free, yet still go out and buy BB for making boxes.

For a subwoofer this is even more the case. Even staunch defenders of MDF will admit that the extra stiffness of plywood makes it better for subs. Probably the most important characteristic of a sub box is that it not act like a balloon. (another little realized problem with MDF is that it is not air-tight)

The box needs to be braced (and please do not be tempted to put them dead centre -- that is the worst place for them)

The need for stuffing in a sub is debatable, but the rational was covered pretty well in an earlier post. Do take into consideration that it will change the apparent volume of the box, and thus its impact on the box tuning needs to be considered.

dave
 

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Regarding standing waves, I have a set of Atlantic Technology speakers (System 8200) where the cabinets have sloped backs that is suppose to help with standing waves.
If you do the math, given the size of even a large sub box, and the frequencies at which it will operate, standing waves are not an issue.

dave
 

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Some of the cabinets that are built based on the LLT design defined elsewhere on this site can have dimensions exceeding 5'. Wouldn't this start causing problems around 113hz? Not out of the range of some subs.

Cost is one reason for some of us to be involved in this "hobby" and the difference between 13 ply, void-free BB and MDF can make or break a project.

Regarding MDF being airtight, I use what I believe to be a simple, effective, and low cost option to resolve this;

After the cabinet is built, but before finishing the outside I slop a thick coat or two of latex paint on the inside of the cabinet. To maximize my funds, I only buy "oops" paint from the local Home Depot/Lowes at $5.00 per gallon. I try to get semi-gloss, as this tends to be quite rubbery when dry, my current cabinets that I am building using a design from planet10's Frugal-horn.com web site, have a rather garish purple interior.:sick:

Paul
 

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but before finishing the outside I slop a thick coat or two of latex paint on the inside of the cabinet
Doesn't latex breath though? I wonder if an oil based varnish or the like would be better? Don't really know. :)

brucek
 

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Discussion Starter #50
The box needs to be braced (and please do not be tempted to put them dead centre -- that is the worst place for them)
Thank you for your post :T , I just need some clarifications pls
What do you mean by dead center?
The need for stuffing in a sub is debatable.
How? Pls elaborate, Do you mean that not stuffing would be acceptable?
Do take into consideration that it will change the apparent volume of the box, and thus its impact on the box tuning needs to be considered.
dave
According to the modelling softwares, lining the walls does not change the tune but slightly the FR...Anything wrong with it?
 

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Discussion Starter #51
I think that any porosity in MDF might be negligeable. But Plywood really seems stronger but at least double cost!!
 

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Actually, the amount of breathing that an untreated MDF cabinet with veneer, or some other nice finish on the outside does seems like it would be so minimal as to be meaningless compared to the dynamic changes of pressure induced by a subwoofer. I only paint the inside as a "belt & suspenders/chicken soup" type of approach, in that it doesn't cost much, and it may actually help seal any place I miss with glue & caulking.

I certainly wouldn't call it hermetically sealed, unlike Carnac the Magnificent's mayonaise jar kept on the front porch of Funk & Wagnall's house.

Paul
 

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Discussion Starter #53

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According to the modelling softwares, lining the walls does not change the tune but slightly the FR...Anything wrong with it?
I don't think it's bad, it's just that stuffing changes the effective size of the box. I think you can calculate that size change and alter your box size in the modelling software and see what happens.....

brucek
 

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Discussion Starter #55
I think you can calculate that size change and alter your box size in the modelling software and see what happens.....
brucek
Insulation is supposed to make the box appear bigger, not smaller....That's the issue: How larger??
 

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Discussion Starter #56 (Edited)
Cables now...What gauge is required to support 2000 W? Is 16 enough
 

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Thank you for your post :T , I just need some clarifications pls
What do you mean by dead center?
Do not put a brace exactly in the middle of a panel. A more general statement is that any brace should not divide a panel into 2 (or more) identical sub-panele.

How? Pls elaborate, Do you mean that not stuffing would be acceptable?
According to the modelling softwares, lining the walls does not change the tune but slightly the FR...Anything wrong with it?
As far as the frequencies a sub should be producing, damping has no effect. It will thou increase the appartent size of the box. I would not trust a modeler (except maybe MJK within linits) to account properly for the stuffing.

dave
 

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I think that any porosity in MDF might be negligeable. But Plywood really seems stronger but at least double cost!!
And what proportion of the total cost is that compared to the cost of the driver(s) and the effort you are putting into it?

Unless you are somewhere where the price of wood is out-of-site (ie BB in Western Australia), arguing about the difference in cost of panel material is penny-wise, pound foolish.

dave
 

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And what proportion of the total cost is that compared to the cost of the driver(s) and the effort you are putting into it?

Unless you are somewhere where the price of wood is out-of-site (ie BB in Western Australia), arguing about the difference in cost of panel material is penny-wise, pound foolish.
This is what I meant by a religous war...

Some people belive that the mass and uniformity of MDF makes for a better speaker, others believe the strength of BB is better and there are even a few lost souls that believe particle board is best. I have been unable to find a definitive answer by someone that has done empirical, objective testing, and until I do, I will use plywood and MDF interchangeably, as in the speakers I am currently building that have a bit of both.

I have thought a little about a test regime. If I can get around to it what I thought I might do is take a driver (I have some HiVi A5+ 5" drivers laying about) build two cabinets that WinISD says is an optimal for sealed with a 1-1/4" MDF baffle. One cabinet would be 13 ply BB and the other MDF with veneer, braced as similar as I can make it. For testing I would;

1) Do freq tests measuring from one meter on axis, and set amplifier for a reasonable level such as 85db.

2) place the mic face down on each of the panels coupling the mic to the panel, and run the same tests.

3) place the mic about 1-2mm away from each panel and re-reun the above tests.

What I would hope to find is if there are any differences.

Paul
 
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