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Old 8th December 2005, 06:23 PM   #1
mazurek is offline mazurek  United States
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Default damping ringing box - more bracing and/or stuffing?

I need to kill some vibrations.

I have a subwoofer box that a local craftsman built from my plans for some dayton reference 12hf drivers (if in Columbus, OH, I highly recommend Edwin Daniel). It it designed for Q=0.707, volume-parts is about equal to 2.1 cubic feet.

I am now testing it out. Subjectively, it rings a lot when I give it a knock. The back, top, and bottom walls vibrate a little during test tone playing. I am not sure if the top and bottom only vibrate because the room is vibrating, tell me what you think of the bracing.

I tried adding stuffing, which definitely helps to minimize the ringing. Do you think I should add more bracing to the back top and bottom. The attached picture shows the current state of the bracing. I don't want to recklessly add bracing and make the box too heavy, any better recommendations than just nailing in a lot of wood?

Also, I am going to use one pound of stuffing, as I don't want to drastically change Q (new Q=0.65). Can I just have it all lying on the bottom of the box. Would you recommend stuffing even more?

Thank for your help.
Lee
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Old 8th December 2005, 06:25 PM   #2
mazurek is offline mazurek  United States
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Default picture of bottom inside

This is the bottom half of the subwoofer, top half is symetrical. No bracing is on the back, but the braces on the sides extend to the back.
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Old 8th December 2005, 06:50 PM   #3
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First of all, rapping it is not realistic, as that puts in freq much higher than its operating range.

The most effective bracing is to connect pairs of opposite panels.

They become essentially infinitely stiff at that point, as the equal and opposite air pressure on each side cancels.
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Old 8th December 2005, 07:20 PM   #4
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Quote:
Originally posted by noah katz
The most effective bracing is to connect pairs of opposite panels.
And making sure you fasten them off centre and making sure you don't place the woofer in the centre of the baffle and where possible, attach the woofer to the back panel.
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Old 8th December 2005, 07:26 PM   #5
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I don't think off center is necessary or desirable for sub boxes, assuming the first wall resonance is well above the sub operating freq range.

Since resonances won't be excited, the name of the game is minimum flexure, meaning minimum distances between braces and corners, and that is achieved by centered braces.
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Old 8th December 2005, 08:02 PM   #6
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Ringing isn't a problem since it's out of the sub's passband.

You might add a couple of side to side braces (glued in). Other than that it looks like a good job...
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Old 8th December 2005, 09:56 PM   #7
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Quote:
Originally posted by noah katz
I don't think off center is necessary or desirable for sub boxes, assuming the first wall resonance is well above the sub operating freq range.
Just to be safe i'd still put them off-centre so as to distribute resonances -- you never know, it could get excited as a harmonic*, and off-centre pushes will actually kill the lowest panel resonance, where as a centred brace can actually encourage it.

But pretty much, flexure from air pressure (*which is itself enuff to cause the panel to resonate) is the biggest issue. A brace will help -- more intersting might be a pice of ready rod thru the cabinet cranked up enuff to pull the box walls in slightly.

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Old 8th December 2005, 10:33 PM   #8
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This is one example of a sub box with dowels used for bracing. The dowels were cut slightly longer than the dimensions and were sanded flush with the sides of the enclosure after the glue dried.

The holes in the dowels were going to be used to mount the woofer but they proved to be too fragile so I rotated the woofer about 15° and used T-nuts for mounting.

If the image doesn't load here, the following link will load it from the server.

http://www.bcae1.com/images\jpegs\sp...etherwpoly.jpg
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Old 8th December 2005, 11:59 PM   #9
simon5 is offline simon5  Canada
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I like to spam my work everywhere...

Here is an example of connecting opposite panels together...
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Old 9th December 2005, 12:01 AM   #10
simon5 is offline simon5  Canada
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