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Old 22nd April 2007, 06:27 AM   #191
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Quote:
Originally posted by MJL21193
The Q factor or quality factor compares the time constant for decay of an oscillating physical system's amplitude to its oscillation period.
True, but it is not the Q i am talking about. By high Q i am talking about the energy required to stimulate a panel resonance. This implies that a large amount of energy in a very narrow bandwidth needs to be injected into the panel to make it vibrate.

The energy is quickly released/dissipated because with music we will not have a continuos input of energy, it will be bery transient. As soon as the energy in the panel falls below a certain amount, the resistance of the panel to vibrate at any but that very narrow bandwidth will quickly damp the resonance.

dave
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Old 22nd April 2007, 06:53 AM   #192
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Quote:
Originally posted by rick57
fwiw some math on the relationship between size, mass, thickness and MOE, and panel resonant frequency
Thanx for digging that out... i would have read that when Hu 1st posted it some 5 years ago, and it would have played a role in the evolution of my box building philosophy...

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Old 22nd April 2007, 08:14 AM   #193
Ricky is offline Ricky  United Kingdom
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I should like to add my tuppence worth to this.

I have read with interest that people think that MDF "sucks the life" out of a loudspeaker. I think that's a bit extreme but it certainly can add a cold/clinical edge to the proceedings. Hardwoods, Oak and Beech in particular seem to give a more natural sound. The only problem is that a hardwood will move according to humidity.

At Arcaydis, I designed a cabinet in solid American White Oak and sold 20 pairs to a Canadian distributor. Every single pair came back. Why? Not because there was anything wrong with the speakers. The average humidity in Canada is 6%. The Oak literally "dried out". I found a remedy by laminating 8mm Oak to 12mm MDF in a flat press. Then I got the cold clinical edge.

So here's my tuppence worth. Decide what thickness cabinet you want. Let's say 30mm. That's 12mm of hardwood flat pressed to, wait for it, 18mm double density chipboard. One tip. Put a layer of veneer on one side of the chipboard. This will remove the "bendy spoon" element.

Right. That's me done. Wish you all well.


Ricky.
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Old 22nd April 2007, 08:25 AM   #194
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Quote:
Originally posted by Ricky
The average humidity in Canada is 6%.
Maybe in the winter in the flatlands... in the winter here it is well above 50% and a lot less in the summer...

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Old 22nd April 2007, 08:28 AM   #195
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Quote:
Originally posted by rick57
. . Iíve seen figures that the energy of a large orchestra live by octave, eg over 125-250 hz is about 4 dB more than 1-2 kHz, loosely suggesting a modest benefit in pushing panel resonances up 1khz plus.
Quote:
Originally posted by planet10
I don't know the exact math, but IIRC the amount of energy available to excite a resonance is inversly proportional to the square of the frequency...
The part of excitation that originates from the sound pressure inside the box is largely proportional to 1/f≤ if the radiated sound is flat. Besides the direct mechanical excitation from the driver, the sound pressure inside the box is the dominating source of excitation. The pressure outside the box is typically neglectible.

So pushing the a resonance upwards by two octaves, using the numbers above should decrease excitation by 24+4 =28 dB.

In the simulation below it can be seen that the pressure inside the box (dashed) is tilted by approximately -12 dB/oct (=1/f≤) relative to sound pressure at 1m (solid).

Click the image to open in full size.
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Old 22nd April 2007, 08:32 AM   #196
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Quote:
So here's my tuppence worth. Decide what thickness cabinet you want. Let's say 30mm. That's 12mm of hardwood flat pressed to, wait for it, 18mm double density chipboard. One tip. Put a layer of veneer on one side of the chipboard. This will remove the "bendy spoon" element.
Hi!
Agree, but had some good result with with low density also!
Easier will be a layer of lacquer (and cheaper) instead of veneer on the back. The furniture industry use glued paper.
There exists a ratio of how thick layer of glue and paper it has to be to avoid warping in long term. Have to ask one of our customers about that.

John,
did you finish the two boxes, mdf/bb and measured them?

Cheers
Peter
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Old 22nd April 2007, 08:36 AM   #197
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Hej Svante!
Could it be Svante as in the Edge sim?

MVH
Peter
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Old 22nd April 2007, 08:37 AM   #198
Ricky is offline Ricky  United Kingdom
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Quote:
Originally posted by planet10


Maybe in the winter in the flatlands... in the winter here it is well above 50% and a lot less in the summer...

dave

Hi Dave.

You have just proven the point against using just hardwood. I found that the composite route faired well. Has to be flat pressed with PVA tho.

Ricky
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Old 22nd April 2007, 09:12 AM   #199
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Quote:
Originally posted by Svante
o pushing the a resonance upwards by two octaves, using the numbers above should decrease excitation by 24+4 =28 dB.

In the simulation below
Thanx for that, it puts a real "face" on things... as well one can expect a bit more attenuation if the panel resonances get up into the region where the air-space damping is effective.

Another thing it twigged is that one wants to avoid putting the panel resoance into the same place/frequency as a standing wave.

dave

PS: when John Atkinsom measures panel rsonances with his little device... what kind of device is that and where would one get one (or get something to repurpose)?
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Old 22nd April 2007, 09:50 AM   #200
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Quote:
The actual air moving inside the box actually has much, much less energy to drive a panel resonance than the mechanical mechanism that attaches the driver to the box...
Iím not suggesting youíre wrong, but has this been measured?

On another aspect, might the ratio of internal sound energy to panel resonance differ for sealed and vented? Or not because were so far above the vent's range
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