Do open baffles shake?

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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.


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?

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.


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.
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 ............;)
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
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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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.
My open baffle speakers sounded better when braced from behind.



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Thanks for ideas and good jokes (incl. Panomaniac).

Seems to move at very low frequencies, not like being excited at one resonant note.

Might be in synchrony with note, hard to tell.

Weighs like 60 lbs, 3/4 inch MDF with various braces... the whole assembly shakes. Maybe the cone is "recruiting" the shaking at a sub-frequency... but not quite like resonance... but I don't know what I am talking about.

Soon I'll anchor it to wall and do before and after measurements.

I posted my quite favorable sentiments about this speakers in full range forum recently. A truly major issue is room resonance but almost nobody tries to wrestle with it. This is one good strategy.
Hi bentoronto, disclaimer: I am a newb.

Yes, it moves much, much more at lower frequencies. I got the chance to observe this first-hand recently.

The baffle vibrates more at low frequencies because the woofer has greater travel (excursion) and thus more momentum. It's the same principle as swinging on a playground swing. If you swing high enough, the frame of the playground swing begins to rock back and forth.

Others have developed a good way to isolate the driver from the baffle via two separate assemblies. One part holds the driver by clamping to the magnet (so from behind). The other part is a free-standing baffle with driver cutout. Push the first assembly toward the second one, so that the driver lines up with the cutout, and seal the gap with closed-cell foam tape.
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