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Old 17th July 2006, 02:00 PM   #1
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Default Bracing GM MLTL 48"

Hello all. I'm currently building triangular GM MLTL 48"s. As I was glueing the first parts together (front baffle, top and bottom) I suddenly occurred to me that the design seems a bit, well, flimsy. I'm using 22mm MDF, and no, I will not go and buy plywood. I just need to get rid of this MDF and I have already cut all the parts. I'm a bit worried about resonances. So I came to a conclusion that I would make a few triangular braces out of plywood to give the cabinet some more support. OK, nothing too confusing there, but I started thinking that since installing braces will reduce the volume of the cabinet, and I have already glued the top and bottom in place 122cm apart, should I compensate the lost volume by moving the bottom down a few mm? IOW, which is more important, box volume or internal height?
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Old 17th July 2006, 02:30 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

If your triangular braces are to be placed horizontally they are a no-no.

Give a rough sketch of the cabinet, will advise sensible bracing.

To answer your question, in this case hieght, but its a small change.
You would not change height to compensate for too small a volume.

/sreten.
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Old 17th July 2006, 03:49 PM   #3
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Yes, I thought there might be a problem with horizontal bracing. I guess I'll try them without any internal reinforcement and If I can hear any resonances, I'll just add material on the outside.

I already have built the "normal" 31" version and I do like them. The problem is, that I built them very quickly as prototypes so the finishing is absolutely intolerable. What I noticed with this version was that it had a very specific resonance. I can't recall the frequency and I'm just too lazy to dig out my test CD . However, I listened to the resonance for a while and then started tapping the enclosure with my knuckle. I found roughly the same frequency as the resonance by tapping the top of the speaker. I know very little about these things but I would think that the top was/is actually the reason for that resonance instead of the unbraced sides.

However, all ideas for internal bracing are still welcome. Here roughly what I was thinking about. The red triangles are the braces and one side of the enclosure is "open".


Cheers!
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Old 17th July 2006, 06:12 PM   #4
Dumbass is offline Dumbass  British Antarctic Territory
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That bracing scheme seems reasonable, you just don't want them to be so big that they interfere with pipe resonance.

IIRC the greatest pressures exist at the ends, so some people double up the end pieces.
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Old 17th July 2006, 06:36 PM   #5
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Good advice. Bottom pannels in particular tend to sing, especially if spikes are fitted as you've got a hollow cavity right where you don't want it, under the enclousre, so doublingboth top and bottom is mandatory IMO. One other useful tip for any resonant cabinet: forget spikes. Apply Blu-Tak in sheets to the bottom (no gaps), then stick it to a slab of granite or marble as a plinth. Or even a concrete one at a pinch. You could always paint or veneer it. Kills any potential resonance stone dead, tightens up the LF like you wouldn't believe, and brings benefits to the mids too. Better still, it's cheap.
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Old 21st July 2006, 10:44 AM   #6
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Ok, I guess I should update. So I built them without any internal bracing and I'm now testing them to find the right angles, amount of damping material etc. So far, they seem to be pretty much as picky on damping as the 31" version. I struggled two days to come to a conclusion with those, and I guess it will take about the same with this larger version. I found that the 31" version had to have its top stuffed full and nothing under the driver or the highs would suffer. Vocals started to sound muffled and cymbals too. Particularly "s" sounded more like "sh" with too much damping. That seems to be the case with the 48" as well. Other than that, the 48" seems to have more "body", which was a very welcome surprise. Bass extension isn't really that incredible, but it is indeed very good for such a tiny driver. Not at all boomy but smooth and controlled. The 48" may not be quite as detailed as the 31" but I wouldn't call it smudged. Really the difference is barely noticeable. Actually, I'm having a difficult time choosing between the two. I guess it's too early to say anything yet, since I haven't quite finished tweaking the new ones.

Anyways, thanks for all the tips and they are still very much welcome.
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Old 21st July 2006, 12:09 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

Bracing that runs along the length of a panel is far more effective
then a number of cross braces. A 2" section of cabinet material
is the sort of thing you need. Do not place the brace in the exact
centre of the panel, offset it slightly so the two sides are different.

This sort of bracing will not interfere with the tuning of the MLTL.

/sreten.
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Old 21st July 2006, 02:09 PM   #8
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That's actually what I concidered after your first post. However, those braces could've only gone on the outside of the enclosure since I had already cut all the pieces. In my amateurish head I would actually think that these types of vertical braces could be beneficial in more than one way. They would stiffen the enclosure and probably break down some internal reflections too. Sort of like a diffuser. If that's the case, then triangular braces would be even better? I've let myself believe that the less parallel the sides of an enclosure are, the better. A cube is the worst form, and a ball or an egg is the best? Not really applicable to these TL designs I guess. Hmm... I can almost see myself buying a few concrete sewer pipes...

Cheers, literally.
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Old 21st July 2006, 02:31 PM   #9
sreten is offline sreten  United Kingdom
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Hi,

i don't think that the reduction in the cross sectional area caused
by putting the braces internally (i.e. the internal volume change)
would be particulary significant in an MLTL.

Or most designs for that matter, a good design should work well
with any of the parameters out by say 10 to 15 %, designs that
don't are too critical IMO.

(an example of critical design is a very extended bass shelf
alignment, the port tuning has to be exact for it to work well.)

/sreten.
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Old 21st July 2006, 03:23 PM   #10
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I use window braces in my MLTL's. I use a pair of braces immediately above and below the driver and another one dividing the bottom of the pipe by the golden ratio (or square root of 2 or whatever turns you on). This does two things. First, it does divide the panels into smaller section, particularly at the top of the pipe where internal pressures are greatest. Second, the top two braces form a solid structure around the driver so that the maximum amount of mass is concentrated around the driver frame.

The effect of window braces on the the pipe is acoustical resistance -- stuffing. As long as the cross pieces are reasonable sized -- 3/4" - 1", the braces will have virtually no effect on the pipe.

Bob
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