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Old 22nd April 2007, 05:29 PM   #211
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Quote:
Originally posted by rick57
On another aspect, might the ratio of internal sound energy to panel resonance differ for sealed and vented?
Here i think we need to change from sealed & vented, to high pressure & low pressure boxes. I avoid both BR & sealed boxes in favour of aperiodic or TL boxes. They are low pressure.

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Old 22nd April 2007, 05:38 PM   #212
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Quote:
Originally posted by MJL21193
Long and short of this is that you use damping material - wool felt, cotton. You made it seem as though no damping was used at all, just air. To fill an air space(even partly) with damping material, you are damping panel resonance.
That is airspace damping, not panel damping... i was quite clear about that... none of those are effective into the low frequencies and do little to damp a panel (you only get an affect of less excitation of panel resonances if the panel resonances are high enuff to be above the effective minimum of the damping materials frequency range) ... when we are talking about panel damping we are talking things like the bituminum pads used by the BBC, tar-like substances applied to the inside of the panels, etc.

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Old 22nd April 2007, 05:40 PM   #213
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Quote:
Originally posted by MJL21193
Driving the resonance frequency up increases decay time. The higher the frequency, the longer it will resonate before decaying. Panel damping speeds up this process.
Why is that?

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Old 22nd April 2007, 05:43 PM   #214
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Quote:
Originally posted by MJL21193
Your push-push allignment, if wired in phase, cancels sound waves too, not just mechanical movement, so you really can use that as an example.
Energy from mechanical movement of the driver is SWALLOWED by the sound energy.
Any internal cancellation is minor in the scheme of things and only happens at very specific frequencies related to the distance the drivers are apart -- one hopes that these are high enuff that the airspace damping is dealing with them. At low frequencies the air only sees one compund driver.

Have you ever seen a subwoofer walk -- you know that isn't caused by the air moving around.

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Old 22nd April 2007, 05:47 PM   #215
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Quote:
Originally posted by MJL21193
My repertoire of tools and methods to fix this problem does not include magic.
And neither does mine... it is just as magic as the magicians card trick... no voodoo going on at all... only looking at the problem from a different perspective.

There is much less panel resonance because the panel resonances are not excited.

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Old 22nd April 2007, 05:47 PM   #216
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Quote:
Originally posted by MJL21193
Not hard to understand what you are trying to do Dave.
But from all you comments it is clear you still haven't grasped the concept.

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Old 22nd April 2007, 05:50 PM   #217
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Quote:
Originally posted by claudio


Cheap uncalibrated accelerometer, the same brand and model sold by Audiomatica: MSI ACH-01-03/10, available at Mouser P#824-ACH-01-03/10
Thanx... i'll look that up.

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Old 22nd April 2007, 06:05 PM   #218
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I use real wood because I build mostly OB's, so other than the mechanical operation of the drivers there aren't a lot of forces on the panels, at least with flat baffles, and leaks from a split wouldn't be the end of the world. Recently I've been using magnet mounts and decoupling the driver frame from the baffle, yet I still have a problem with vibration and sound coming from these solid wood baffles.

What I'm finding is that a lot of sonic energy is getting into the wood at the driver cutout. It doesn't seem to have anything to do with the resonant frequency of the panel, just how well the wood itself transmits sound. SPL's are extremely high right at the cutout because it is so close to the driver, so sound enters there and spreads throughout the baffle. It's not something I have the tools to measure, but I can easily feel the vibrations and hear a lot of sound coming through the wood with the screwdriver handle to the ear trick.

I've been chasing this baffle vibration thing for a while, and have gone to extremes to ensure they aren't the result of direct transmission from the driver frame. Plywood prototypes end up sounding much better than identical dimensioned solid wood, because of the internal damping of the ply. I thought I was going crazy at first, when the prototypes sounded much better.

My plan to counter this problem is to use an MDF ring right at the driver, then decouple that from the real wood baffle. It also give me the added benefit of being able to easily swap out different drivers in a finished baffle, even with flush mounting.
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Old 22nd April 2007, 09:45 PM   #219
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Quote:
Originally posted by planet10

Here i think we need to change from sealed & vented, to high pressure & low pressure boxes.
I could have sworn you said mechanical vibration from the drivers operation was the biggest sourse of energy in the box. If so, low pressure, high pressure - doesn't matter.


Quote:
Originally posted by planet10


That is airspace damping, not panel damping... i was quite clear about that... none of those are effective into the low frequencies and do little to damp a panel (you only get an affect of less excitation of panel resonances if the panel resonances are high enuff to be above the effective minimum of the damping materials frequency range) ... when we are talking about panel damping we are talking things like the bituminum pads used by the BBC, tar-like substances applied to the inside of the panels, etc.

dave
Damping is damping in any form. I provided a pretty accurate definition of damping earlier, didn't you read it?. Panel damping happens when a damping material contacts the panel (tuning fork I mentioned earlier). Or are you using "slight of hand" to make it float in the airspace without touching the panels?


Quote:
Originally posted by planet10


But from all you comments it is clear you still haven't grasped the concept.

dave
I have a good grasp of the concept. Anyone who reads (not skims over) what I have written here will see that.
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Old 22nd April 2007, 10:03 PM   #220
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Quote:
Originally posted by planet10


Any internal cancellation is minor in the scheme of things and only happens at very specific frequencies related to the distance the drivers are apart -- one hopes that these are high enuff that the airspace damping is dealing with them. At low frequencies the air only sees one compund driver.

Have you ever seen a subwoofer walk -- you know that isn't caused by the air moving around.

Two identical sound waves, of the same magnitude and phase approaching each other will cancel each other out, even at low frequencies. Are you disputing this?
My favourite panel damping is and always has been carpet, not asphalt impregnated roofing felt. It's effective for breakup of standing waves and also is a very effective panel damping solution It seems you have associated the use of damping with inferiior box construction. It is possible to build boxes with undersized structure and damp the resulting vibration problem, but I am not advocating that. I say build the box well, keeping in mind the amount of energy that needs to be disappated, then use damping to deal with it. Damping is not only effective, it crucial for vibration controll. Without it the boxes will ring like bells, especially if you drive resonance upwards.
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