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Old 1st October 2010, 06:50 PM   #1
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Default Do open baffles shake?

Anybody out there with good mechanical intuition?

I built a very large OB sub. Please don't laugh. It is built into a table. Below about 35 Hz, the whole assembly starts shaking. The 60 inch wide bottom of the board is "anchored" on a deep-pile carpet and doesn't shake - only the top end (and table top) shake.

I plan to attach it to the foundation wall right behind the baffle board with before and after mic measurements.

Link

But I'd like to understand why it shakes. (I don't think it is resonance.)

For boxes, the air inside acts like a balloon on all surfaces and there shouldn't be any forward and back force. I think. But does the moving mass of the big-magnet 15 inch (with 20 Hz resonance) driver in an OP have a urge to shake the baffle board forward and aft?

Thanks.
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Old 1st October 2010, 07:02 PM   #2
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The acceleration of the moving mass - Sir Newton's third law. Whichever way the motor is pushing the diaphragm, the basket tends to move the other way, and it's going to take the baffle with it if it isn't rigidly secured.
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Old 1st October 2010, 07:05 PM   #3
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Quote:
Originally Posted by 454Casull View Post
The acceleration of the moving mass - Sir Newton's third law. Whichever way the motor is pushing the diaphragm, the basket tends to move the other way, and it's going to take the baffle with it if it isn't rigidly secured.
Yep!

That's why you shouldn't measure speaker parameters by hanging them, you can get a kind of mechanical resonance (anti-resonance?) which can create interference. And a raw open baffle needs to be REALLY solid.
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Old 1st October 2010, 07:07 PM   #4
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My (test) open baffle with a 15 inch driver at the bottom caused the top to rock VERY visibly on low bass notes. These panels do need stiffening.
As a bass only unit I think they might need sand filled baffles WITH additional stiffening !
Concrete panels might be best ............
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Old 1st October 2010, 07:07 PM   #5
defect9 is offline defect9  Ireland
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consider that the electromotive force to move the driver also pushes the entire speaker in the opposing direction. this is slightly compounded by the air's resistance to the driver's movement.

motor pushes driver in one direction, air only moves so fast and causes resistance, therefore motor is pushed away, along with your table. the unanchored end has the least movement resistance, thus it vibrates.

below 35hz the excursion of the driver is probably enough to cause significant air resistance
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Old 1st October 2010, 07:08 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

The force reaction due to the moving mass of the cone has to be dealt
with irrespective of it being open baffle or a box louspeaker. Your issue
is almost certainly resonance related even though you say its not.

rgds, sreten.
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Old 1st October 2010, 07:13 PM   #7
preiter is offline preiter  United States
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Boxes have forward and back movement as well, but the ratio of the box's weight to the moving mass is high, so they don't move much.

Also, a box is much more rigid than your table. Imagine if the sides of your table were sheets of wood instead of legs. The table wouldn't shake then.
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Old 1st October 2010, 07:29 PM   #8
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Hold on, for an OB, shouldn't the forward and backward forces cancel (unless the shaking is at the driven frequency... which it doesn't seem to be)? Should that "analysis" also apply to boxes?

Maybe I should try to measure the table resonance?

Thanks for replies, so far.
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Old 1st October 2010, 07:51 PM   #9
Pano is offline Pano  United States
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The open baffle has nothing to keep it from shaking. No box bracing, anyway. I've seen open baffles over 2" thick and with side wings. They don't shake much.
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Old 1st October 2010, 08:06 PM   #10
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My open baffle speakers sounded better when braced from behind.

Zilla
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