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Old 29th December 2010, 10:25 PM   #1
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Default Do drivers shake their mounting boards?

Do drivers, esp. subs, shake their mounting boards? Why?

Cones react against the driver frame and, I guess, the mounting board or whole box is rigidly attached. But if the signal is close to symmetrical, why should there be shaking? An audio signal would feel like a buzz at audiio frequencies, not a shake (maybe 2 Hz?). Granted there is an initial pulse from, say, drums. But the DC (or 1 Hz) aspect of the drum output can't make it into (let alone right through) any audio-frequency system.

My OB giant sub shakes its mounting very profoundly when playing loud very low notes - and the wall to which the mounting board is attached. Just OBs?

Speakers don't shake except for panels that are containing pressure.
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Last edited by bentoronto; 29th December 2010 at 10:30 PM.
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Old 30th December 2010, 02:24 AM   #2
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Hi bentoronto - well the short version is - yes, drivers shake their mounting boards. Here is a link to one thread on the subject: Open Baffle Vibrations

There is a lot of other stuff around including NOT using any baffle at all (Stig Erik) to using CLD (Constrained Layer Damping) stone, glass etc. http://www.perfect8.com/nav/pdf/Encl...fleStudies.pdf

Good luck with your search about this problem - it has been, and still is, a work in progress by many others.
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Old 30th December 2010, 03:18 AM   #3
sreten is offline sreten  United Kingdom
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Hi,

Newtons third law "For every action, there is an equal and opposite reaction".

Your wrong about the frequencies, it must happen at all frequencies, and does.

After that it gets rather complicated when it comes to loudspeakers.

FWIW for OB's driver assymetry can cause DC forces related to the programs
level envelope that could cause rocking modes in large driver open baffles.

rgds, sreten.
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Old 30th December 2010, 03:38 AM   #4
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A little tricky, but not immpossible, to due on an OB, mounting woofers push-push is a very effective way to actively cancel reactive forces.

dave
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Old 30th December 2010, 04:59 AM   #5
anatech is offline anatech  Canada
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Hi Dave,
I thought they did that to cancel out the non-linearity between the cone moving outwards and the cone moving inwards. Push-push tends to give you the same non-linearities in each direction.

As for the OB, it's still going to be excited by the drivers and tend to support vibration modes. At least they should now be equal in both directions. Sounds like fun, but what about the W.A.F. ?

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Old 30th December 2010, 09:21 AM   #6
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Interesting reading c2cthomas's links to methods of addressing vibration. At different times, I've had bunches of concrete blocks or a wine rack sitting on top of my Klipschorn - which has a very convenient large top surface. If I were a golden-eared listener, I'd tell you how much my ears were opened by using one kind of concrete block versus another kind.

But no one in the first link addresses my question: why?

Comforting of sreten to document that Newton thought about these things. But more seriously, the issue of asymmetry of the driving signal has a few aspects. First, it relates to the ancient issue of absolute polarity in reproduction. Big can of worms. Second, wouldn't you find the cone on the other side of the room if there were a DC component somehow able to propagate itself through my AC-coupled amp?

Likewise push-pull mounting of a pair of drivers. Several benefits but again, if there were a polarity or DC aspect to the signal, it wouldn't reproduce the sound right. Finally, push-pull or single-driver, where are those shaking forces coming from?

I can't say if my bolted-to-the-drywall giant OB sub is shaking at its own personal resonance (incidentally stimulated by low notes) or is being driven more directly by the sub. And I guess it is important to understand which it is.

For sure, when driver energy is going into making stuff shake and when shaking stuff is making noise (not to mention the bunch of loose glass picture frames my wife has sitting on my giant OB sub), it has to be very harmful to sound quality.
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Last edited by bentoronto; 30th December 2010 at 09:29 AM.
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Old 30th December 2010, 06:50 PM   #7
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Quote:
Originally Posted by anatech View Post
I thought they did that to cancel out the non-linearity between the cone moving outwards and the cone moving inwards. Push-push tends to give you the same non-linearities in each direction.
Push-pull id=s aimed at canceling some non-linearities. If you have good woofers not much benefit. Push-push on the other hand actively cancels most of the reactive forces, dramatically reducing energy machanically passed to the box.

dave
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Old 30th December 2010, 07:12 PM   #8
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by bentoronto View Post

But no one in the first link addresses my question: why?

Comforting of sreten to document that Newton thought about these things. But more seriously, the issue of asymmetry of the driving signal has a few aspects. First, it relates to the ancient issue of absolute polarity in reproduction. Big can of worms. Second, wouldn't you find the cone on the other side of the room if there were a DC component somehow able to propagate itself through my AC-coupled amp?
Hi,

No. It has nothing to do with drive signal assymetry. The assymetry in the driver,
not the signal, can cause the "DC" position of the driver to vary with bass drive.
(Very generally, for cheap drivers, they tend to move backwards with drive.)

This means you will get a low frequency AC force (according to Newton) related
to the level of the bass envelope, this would cause the low frequency AC forces.

I think this does address the original question.

rgds, sreten.

Push - Push open baffle ? that is a new one on me ......
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Last edited by sreten; 30th December 2010 at 07:29 PM.
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Old 30th December 2010, 07:26 PM   #9
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Quote:
Originally Posted by sreten View Post

Push - Push open baffle ? that is a new one on me ......
Push-Push Dipole Concept

Click the image to open in full size.

dave
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Old 30th December 2010, 07:30 PM   #10
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Yes, it's down to Newton's laws. But more subtly, one you have a driver (mass on a couple of different springs) mounted in a baffle (another mass, different stiffness) possibly all mounted in a subframe/wall (esp. drywall, which is hardly stiff) - you've got a nice system of coupled oscillators.

Then the maths gets hair-raising very quickly (think of a pendulum hanging from a pendulum...) and yes it's entirely possible to see large modes not apparently present in the driving function.
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