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Old 22nd April 2007, 09:54 AM   #201
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Quote:
Usually cotteon or wool-felt. Sometimes polyfluff.
. . I thought you might have meant *just air . .

(With say a ‘typical’ vented midwoofer) how do you decide what amount to use/ start experimenting with?

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Old 22nd April 2007, 11:37 AM   #202
TNT is offline TNT  Sweden
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Quote:
Originally posted by planet10


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 very rapidly with frequency.

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

dave
Yepp - it really works, on these I can go flat out but not feel any vibration - stainless steel is nice ;-)

http://www.diyaudio.com/forums/showt...942#post989942

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Old 22nd April 2007, 01:02 PM   #203
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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
I'd like to broaden several points touched upon in describing the use of "thick veneer".

If success is measured by the veneer remaining crack-free and with tight joints, that success will depend upon several things.

Limit the changes in humidity the case experiences. These will drive the woods expansion or contraction.
Limit the width of panel. Width will determine the total amount of movement that might be experienced.
Limit the thickness of veneer. Thicker veneer will behave more like wood. It will contain greater forces of expansion & contraction and could overcome the adhesive's ability to keep it from moving.
Grain orientation in the veneer. Quartered or rift-sawn wood moves less than flat sawn.
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Old 22nd April 2007, 01:14 PM   #204
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Quote:
Originally posted by planet10


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

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.
Yes, higher frequencies have less energy to drive panel resonance, but that's not all you deal with in a speaker.
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.
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Old 22nd April 2007, 01:37 PM   #205
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Quote:
Originally posted by planet10


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.

Not hard to understand what you are trying to do Dave. What we have to assume is that there will be energy released into the box which has to be dealt with in an effective manner.
My repertoire of tools and methods to fix this problem does not include magic. I'm more of a hard science, show me how it works, type of guy.
I hear the term "energy storage" wrongly used to discribe decay time. I see "Q" used to add a little scientific sound to the arguement.
Energy, from whatever souce, has to be dissapated, it will not leave of it's own accord. Effective damping will do this.
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.
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Old 22nd April 2007, 02:09 PM   #206
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Quote:
Originally posted by peterbrorsson


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

Cheers
Peter

Peter! See earlier in this thread pics of finished baltic birch box. Also as mention earlier, I need to get a sheet of 1/2 MDF to build that one. I thought I had some, but it was 5/8". I will be getting busier with work over the summer and will not have as much time to devote to the "hobby", but will be getting the other two boxes together shortly.
Also, for me, I have another pair to finish - threeway ripoff design of the 3A Integral, a shape I admire. Started these after Xmas, still don't have the woofers.
BTW, I will be measuring nearfield frequency response of each material, to determine the audible qualities of it's resonance. I have(as yet) no means to measure actual resonance.
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Old 22nd April 2007, 03:22 PM   #207
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Quote:
Originally posted by peterbrorsson
Hej Svante!
Could it be Svante as in the Edge sim?

MVH
Peter

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Old 22nd April 2007, 03:30 PM   #208
Svante is offline Svante  Sweden
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Quote:
Originally posted by planet10

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)?
It must be an accelerometer. It is basically a piezoelectric crystal and an attached mass. The piezo produces a charge that is proportional to the applied force, and the force is, through F=ma, proportional to acceleration. The device should be connected to a charge amplifier, which produces a voltage that is proportional to charge.

Net result: a voltage proportional to acceleration.

I don't know where to get one, but a google for "accelerometer" would probably help. It is somewhat tricky to make accelerometers sensitive in one direction only, so there is a reason they can be expensive.
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Old 22nd April 2007, 04:14 PM   #209
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Quote:
Originally posted by rick57


I’m not suggesting you’re 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
Hi Rick,
Dave is right in my opinion. Some measurements about it were done by Moriyasu on Axpress 02/2002.

A sealed enclosure has more damping material inside than a reflex one, therefore the air trasmitted vibrations are lower in amplitude.
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Old 22nd April 2007, 04:19 PM   #210
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Quote:
Originally posted by Svante


It must be an accelerometer. It is basically a piezoelectric crystal and an attached mass. The piezo produces a charge that is proportional to the applied force, and the force is, through F=ma, proportional to acceleration. The device should be connected to a charge amplifier, which produces a voltage that is proportional to charge.

Net result: a voltage proportional to acceleration.

I don't know where to get one, but a google for "accelerometer" would probably help. It is somewhat tricky to make accelerometers sensitive in one direction only, so there is a reason they can be expensive.
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
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