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Old 1st April 2003, 05:20 AM   #11
Kanga is offline Kanga  Australia
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There are two possible sources of flexing of the walls.

1. Sound waves from the back of the woofer. Because of the low frequencies involved, this will result in the same pressure applied on all walls of the enclosure. The sound would need to be incredibly directional for more sound pressure to be applied on the rear wall than the sides.

2. Vibration resulting from the motion of the woofer cone. This will be transmitted throughout the cabinet from the front baffle, which will experience the worst vibration, around the side, top and bottom to the rear. If anything I would have thought that the rear wall may have somewhat less vibration than the other walls.

So my conclusion is that there won't be a big difference in excitation energy from wall to wall, with the rear wall probably having less excitation.

At the moment the speaker has a shelf brace running horizontally and connected to all 4 walls, and positioned just below the bottom of the woofer (which is at the top of the enclosure). This should stop most flexing of the front baffle resulting from the cone motion.

I have done some more tests with a longer wider brace (300mm x 25mm wide x 50mm) and this seems to work much better. I think that the reason is that the brace is stiffening the wall it is in contact with, rather working by being connected to the other wall via the threaded rod.

The next step I will try is to get rid of the threaded rods, and construct a solid piece joining one wall with the other. This will make the two side walls like the flange of a beam, and the brace like the web. This should stiffen the whole thing up a lot.

Mick
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Old 1st April 2003, 11:17 AM   #12
OMNIFEX is offline OMNIFEX  Jamaica
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Hey. I can't be judgemental on what works for you, due to using different size woofers, and, cabinets.

The only thing I can say, you will have better results using a brace mounted right under the woofer, with the other side of the brace attached to the rear wall. It will stiffen the front board (Where the woofer is mounted) considering it is weaken due to the cutout hole in which the woofer sits in. It helps the rear wall as well.

This is why you generally find every major speaker manufacter brace front to back, oppose side to side.


Did you ever consider making the a t brace in which all four walls are reinforced by one another?

Best Regards,
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Old 2nd April 2003, 06:34 AM   #13
Kanga is offline Kanga  Australia
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Default Problem solved

The third generation brace seems to be the way to go. With sore knuckles after lots of tapping I now have only a faint hint of the 200 Hz resonance, and a much duller resonance on tapping.

Pictures tell the story much better, so here we go. The photos are taken looking down from the base of the speaker, with the bottom access panel removed. You can see the shelf brace that sits just below the driver (driver not installed yet). In the photo you can see the first generation brace towards the bottom of the photo, which is just a 150mm length of 25mm MDF on its side with a couple of holes in it. Its really just a support for the all-thread. This didn't make much difference to the resonance of the side wall. The second generation brace is at the other end of the all thread, near the top of the photo. It is designed to actually stiffen the wall, rather than just be a support for the all-thread. It worked quite a lot better.

The second photo (next thread) shows the 3rd generation, which is a solid piece of 25mm MDF that spans the two walls, and this worked very well, as detailed above.

So the conclusion is that just having a post-type brace that connects two walls doesn't seem to do much for reducing wall resonances. Best to have something that stiffens the wall.

Thanks for your comments Omnifex. It is probably too late now, but what is a t-brace?

Mick
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Old 2nd April 2003, 06:36 AM   #14
Kanga is offline Kanga  Australia
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Second photo
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Old 2nd April 2003, 12:20 PM   #15
OMNIFEX is offline OMNIFEX  Jamaica
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Here's a t brace.......

Click the image to open in full size.



Although they made a frame around the "t", you can
achieve the same results just adding the "t" to the
walls of the enclousure.


Due to so many different styles called Cross Bracing, I call
it a "t" brace.

For the record, this particular style, is the true meaning
of Cross Bracing.

You use 2 by 2's (Or in my case 2 by 4's, or 4 by 4's)
in the center of the box.

The good thing about this, is it takes less internal volume,
than tons of bracing in the box. Wrap some Fiberglass
around it, and, its good as gone, as far as the speakers
concern.
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Old 2nd April 2003, 01:43 PM   #16
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If you brace between two opposite walls, the stiffening effect might be much better, than a simple knocking test make you believe.

Remember that you excite both walls in the same direction (i.e. both from left to right or vice versa) with your mechanical "pulse". Simply because the panels are mechanically COUPLED by the brace! Though the whole thing is stiffer than one wall only.

Your driver however excites both panels inward or outward at the same time and therefore the brace works much better for this excitation mode than it does for your knocking test !

Regards

Charles
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Old 2nd April 2003, 03:01 PM   #17
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if you want to get tricky, you could try something like the following, where all the unbraced panel spans have different dimensions. this allows for multiple resonant frequencies in the cabinet walls, so the excitement response is spread out instead of focused at one frequency.

/andrew
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Old 2nd April 2003, 09:20 PM   #18
qwad is offline qwad  Australia
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I HATE TO SPOIL ANYBODIES FUN AND GAMES, BUT WHERE DO YOU GET THE 200HZ FROM?, ALSO AT RISK OF INJURING YOUR DIGITS EVEN MORE DID YOU HOLD YOUR EAR AGAINST THE ENCLOSURE WALL WHILE JOYFULLY RAPPING IT MAY I SUGGEST YOU TRY THIS AND SEE IF IT IS STILL AT THE FREQUENCY YOU DETERMINED INITIALLY I VENTURE TO SAY THE RINGING IN YOUR EARS WILL BE MUCH HIGHER AND FULL OF HARMONICS THAT YOU SHOULD TRY TO SUPPRESS ALA BBC METHOD AS THESE RINGING TONES WHEN EXITED BY THE WOOFERS WILL BE MUCH MORE AUDIBLE THAN FROM YOUR POOR FINGERS ALONE HAVE FUN WITHOUT PREJUDICE, BEST REGARDS, TJB
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Old 2nd April 2003, 10:16 PM   #19
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Old 3rd April 2003, 12:32 AM   #20
Kanga is offline Kanga  Australia
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Quote:
Originally posted by phase_accurate
Remember that you excite both walls in the same direction (i.e. both from left to right or vice versa) with your mechanical "pulse".
Phase accurate - you are quite right, and this is something that I had thought of. Unfortunately until I get the drivers mounted and can run some tests I can't really verify effectiveness of the simple brace between the walls.

QWAD!! - until I got to the third version of the brace it was quite easy to hear the frequency of the resonance after tapping. Its not a pure sine tone, but pure enough to hear that it was a G an octave below A 440Hz, which would be approximately 200 Hz. Once the bracing became more effective it became harder to hear the frequency, as it became a higher frequency sounding rap.
Once I have the drivers mounted I will be able to confirm this better by running some sine tones through them and listening with a stethescope for vibration in each panel. I'll give you an update then. As I've said before, all I'm aiming to do is to push resonant frequencies up past 400 Hz or so which is well past the 200 Hz XO frequency.

Faust - I have seen suggestions of not having the brace in the centre of the panel so that the excitation frequencies of each divided section are different, and I tried this. However it seemed that having it in the middle worked best in terms of giving the dullest thud. Unfortunately the 3rd generation brace is not glued in yet, so the tests may not be truely representative.

Mick
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