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Old 20th April 2007, 11:24 PM   #181
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Quote:
Originally posted by planet10


I have been following that thread... the midrange problem is likely a reflection issue ....

As to why the build sounds as good as it does i would suspect that all things are contributing.

dave

Reflection from the divider back through the cone? The carpet improved the situation immensely, but not entirely. What I'm saying is are the panel resonances in the trouble range, and are they enough to contribute? I found that giving the woofer a more rapid cut on the crossover cleaned up the rest. Perhaps by doing so, the frequency that excited the panels in the first place, was missed. Just a thought.
By the way, for what it's worth, I didn't get the reflection issue with the original version, which had the divider at the same depth. A 1st order series filter gave "good" results.
Also, I haven't abandoned the resonance test preposed earlier in this thread, I just don't have the 1/2 inch MDF yet (thought I did, but iy was 5/8). And now that I have been converted to BB, I need an excuse to buy it.
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Old 22nd April 2007, 04:24 AM   #182
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Dave, most of what you said in Post #177 makes sense to me, except for two things.

I though Q described only the width of the resonances, but you’re also saying correlates with the speed with which energy is released?

“Airspace” damping helps kill high-frequencies, but what is it?

Thanks
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Old 22nd April 2007, 04:28 AM   #183
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fwiw some math on the relationship between size, mass, thickness and MOE, and panel resonant frequency

http://www.snippets.org/pipermail/di...er/003539.html
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Old 22nd April 2007, 04:44 AM   #184
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Quote:
Originally posted by rick57
Dave
Isn’t most of the benefit from pushing resonances up, they are then more easily effectively tamed by damping or stuffing eg with fiberglass, ie this might attenuate a further eg 4-6 dB?
That would be my thinking too. If the idea is to produce a silent box, every method to reduce panel resonace should be used.
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Old 22nd April 2007, 05:00 AM   #185
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Quote:
Originally posted by rick57

“Airspace” damping helps kill high-frequencies, but what is it?

Damping: Is any effect, either deliberately engendered or inherent to a system, that tends to reduce the amplitude of oscillations of an oscillatory system.

Take a tuning fork as an example of an oscillatory system. What is the more effective means of damping it's vibration? Allow it be damped by the air that surrounds it, or wrap it with a cloth?
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Old 22nd April 2007, 05:16 AM   #186
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Quote:
Originally posted by MJL21193


Damping: Is any effect, either deliberately engendered or inherent to a system, that tends to reduce the amplitude of oscillations of an oscillatory system.


In a sealed speaker box, the energy released from the driver into the box has to be dealt with. This energy takes the form of mechanical vibrations, as the sound wave act on the interior of the box. If a panel is free to vibrate, it will do so, dissipating the energy through movement. If it moves freely, energy is dissipated quickly. If it is constrained, energy dissipates slowly (so called energy storage). To release this energy (remember the tuning fork?) damping is needed.
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Old 22nd April 2007, 05:58 AM   #187
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Quote:
Originally posted by rick57
Dave, most of what you said in Post #177 makes sense to me, except for two things.

I though Q described only the width of the resonances, but you’re also saying correlates with the speed with which energy is released?

The Q factor or quality factor compares the time constant for decay of an oscillating physical system's amplitude to its oscillation period. In other words: does amplitude decay quickly or slowly during the oscillation period. Compare to tuning fork again, air damping - high Q, cloth damping - low Q.
Since it's the target to reduce vibrations quickly, low Q damping works the best.
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Old 22nd April 2007, 07:12 AM   #188
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Quote:
Originally posted by rick57
I though Q described only the width of the resonances, but you’re also saying correlates with the speed with which energy is released?
yes

Quote:
“Airspace” damping helps kill high-frequencies, but what is it?
You mean what am i using for airspace damping? Usually cotteon or wool-felt. Sometimes polyfluff.

dave
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Old 22nd April 2007, 07:18 AM   #189
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Quote:
Originally posted by MJL21193
If the idea is to produce a silent box, every method to reduce panel resonace should be used. ]
No. You haven't grooked what i'm trying to do.

If we assume that a panel will resonant no matter what we do (true), then what i am trying to do is move the resoances to a place where they will never be excited. If excited by an external source (ie a whack with a steel rod) it is not silent ... but that is not something that happens when you are listening to music.

Think of it as magic, sleight of hand.

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Old 22nd April 2007, 07:23 AM   #190
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Quote:
Originally posted by MJL21193
[In a sealed speaker box, the energy released from the driver into the box has to be dealt with. This energy takes the form of mechanical vibrations, as the sound wave act on the interior of the box. If a panel is free to vibrate, it will do so, dissipating the energy through movement. If it moves freely, energy is dissipated quickly. If it is constrained, energy dissipates slowly (so called energy storage). To release this energy (remember the tuning fork?) damping is needed.
The actual air moving inside the box actually has much, much less energy to drive a panel resonace than the mecanhcal mechanism that attahes the driver to the box... this is why push-push is so effective, The majority of that energy is actively cancelled. And airspace energy to drive a panel resonance declines vwey rapidly with frequency.

And further, a lot of the stuff used in practice to damp panels, stores energy.

dave
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