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Old 6th January 2010, 04:58 AM   #1
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Default cabinet damping thoughts...

Ok to start there are a lot of smarter/experienced people on this site so these are just my thoughts contemplating my little project.

I read up on all the methods to reduce cabinet vibration and coloration of the signal in general. A lot of good reading on the web for sure.

I see that a lot of emphasis is on bracing and then what material to put on the walls. I also have seen some driver isolation techniques as well.

I started thinking (I may be the 1 trillionth person to do so...) that the main emphasis should be the "damping". That is while bracing shifts the frequency of the vibration it does not damp at all. It is possible to place a brace in the middle and then end up with a higher order vibration that is at least as bad as you started. It may also shift the primary node to a less obtrusive freq. range and be worth doing. I know this is not news to most of you (everyone).

So after bracing most everyone "stuffs" the cabinet. This reduces internal pressures (maybe a second order problem) from building and making the amplitude of the vibrations smaller. But we have not damped the walls of the cabinet yet. We have altered the freq. range and reduced, to some extent, the cause but we have not yet damped the panels.

Now adding mass to the panel via thicker panel or adding some form of pad will damp higher freq. but the added mass may make lower freq. nodes LESS damped as the mass impacts the ability of the cabinet wall to damp. So again we are shifting the problem to a lower freq. Not good if we add braces and them add mass and end up with a cabinet that sounds no better.

Soooooo, the I started thinking part is this. CLD seems to be a valid technique but I wonder if we can (maybe you guys have) do better.

One thought is that with CLD it would seem to make sense to have the base material (inner layer of our cabinet) promote some motion. Yes vibrate more.
So maybe choosing a material that flexes a bit more than the outer panel and also only fastened at the ends to promote motion. I am thinking of a spring here. The the viscoelastic material would be allowed to damp the motion and use up energy. If we fasten the base to the constraint layer "too well" then there is little relative motion and little damping. Thoughts?

The real thought I had was to just use a sort of shock absorber like a car. The panel wall vibrates like a spring so why don't we insert a "real damper"?

I am going on a hunt for some speaker shocks Once we admit that we have a problem (can't stop vibrations) we need stop them from ringing on for so long. Think a Porsche not an Buick.

Ok firing squad...
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Old 6th January 2010, 06:04 AM   #2
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Take aim...FIRE!

See post 13.

It seems like there is some sort of filling between the two wooden panels. This would stop virtually all resonances from reaching the outside walls, but some sound could still go through the back of the driver. The filling would help ease vibrations on the inside.

Is this idea what you're looking for?
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Old 6th January 2010, 08:12 AM   #3
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CLD is good... adding mass to a cabinet without increasing stiffness is taking the walls in the wrong direction.

If you brace you increase the resonance of the subpanels. These are not as bad because the energy to excite the resonces is inversly proportional to the square of the frequency.

Discussion on what materials to build speakers out of

dave
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Old 6th January 2010, 08:24 AM   #4
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I think Wharfedale did this with their new speakers.

"For Diamond 10 a new system of panel sandwich construction has been devised by our engineers. Sandwiches of our four core materials, including interleaved MDF, are first glued to a precise matrix.

Each panel is then coated with a special adhesive that can be cured to a high rigidity by using high intensity RF (Radio Frequency) energy."

Sandwiches, Matrices, and RF oh my I'd probably just slap a layer of glue or tar between two planks and call it a day.
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Old 6th January 2010, 07:50 PM   #5
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If you want damping, you need to think in terms of materials that will convert vibration/resonance kinetic energy into heat, same as your "speaker shock" analogy.

You have mass, stiffness (spring rate), and damping. Changing mass and/or stiffness does not remove energy from the system, only damping, typically by converting the energy into heat. Sorry in advance if this is basic/redundant.
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Old 7th January 2010, 02:35 AM   #6
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Thanks everyone. I looked at the posts linked. Wow that was some bracing
I thought of doing the box in the box as that link also shows. That could work very well but still needs to have some damping in between the to boxes.

As far as materials, at a casual glance it seems all the standard materials can work fine. That is MDF/HDF, BB plywood and solid woods. I think it is knowing the material properties so you can deal with the characteristics. Maybe Dave that MDF can work (for your ears) if approached differently than you do your incredible solid wood designs. I am sure you have tried everything you can however. I will do BB plywood myself since the wood store down the road carries it and I don't like how MDF is to work with as well.

As far as RF - I still believe in Santa I have been thinking about a magnetic damper however. I am not sure that there is enough velocity to make it work however. I also found some rubber shear mounts like you would find on a car transmission and that could work for a damper.

Boywonder, my looooong post was trying to articulate what you said very concisely Thank you.
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Old 7th January 2010, 03:05 AM   #7
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Quote:
Originally Posted by stephenmarklay View Post
Maybe Dave that MDF can work (for your ears) if approached differently than you do your incredible solid wood designs.
Been there done that. I haven't used solid wood (mandarin orange box donors) since my 1st project when i was 12.

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Old 7th January 2010, 03:10 AM   #8
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Originally Posted by enigmaticEntity. View Post
Yeah, every time I look at this aspect of speaker design I end up concluding tapered transmission lines are the best way of attenuating the backwave. That particular design has a relatively abrupt transition from chamber around the midrange to the tapered lines along the sides of the speakers. I suspect it might cause some reflection---be interesting to see measurements from it, but I've not run across any. At least on paper B&W's Nautilus is the best implementation of I've seen with respect to guiding the wave around woofer magnets and into the transmission line. For tweeters they use a vented pole piece and just attach the line directly to the back of the magnet. I have plans to try something similar with some buyout tweeters from Parts Express but haven't gotten around to milling out the center of the pole piece yet.

Tapered transmission lines and other nonuniform structures have the advantage they don't support resonances as well and hence spread coupling over a wider frequency band. As such vibration is less noticeable because the resonant peaks are fewer and smaller. My experience is simply curving the side walls is a major improvement over the typical rectangular internal volume. A couple commercial examples are Dali's Suites and Helicons (full disclosure: I've own a pair of Suites and have been making incremental upgrades to them for about five years). If you want to get formal about it, the shapes to avoid are the ones which support Fourier, Bessel, and Legendre polynomials (rectangular solids, cylinders, and spheres, respectively). I'd suspect some pretty good results could be had from applying the concepts from gedlee's oblate spheroid waveguides to back wave attenuation.

A somewhat different approach is the Linkwitz Pluto, which uses stuffed transmission lines to get about -40dB return loss on the backwave with very little panel resonance. Structurally the approach is quite elegant; not only is the pipe pretty rigid but a wave traveling down it pressurizes it more or less uniformly about its circumference. Net result is the pipe wall is fairly evenly loaded in tension and can't really go anywhere.

Quote:
Originally Posted by planet10 View Post
If you brace you increase the resonance of the subpanels. These are not as bad because the energy to excite the resonces is inversly proportional to the square of the frequency.
I agree, though the power spectral density of every track I've done a spectrogram on has been well approximated by 1/f and not 1/f^2. Also, the higher you can push the resonance the more effective damping is in attenuating the backwave before it gets to walls of the speaker, though all the material loss cuves I've looked at suggest this doesn't have much effect below 1 or 2kHz with typical speaker geometries and stuffing approaches.

It seems to me the most elegant method of reducing panel vibration due to backwaves is to using bracing to get enough rigidity (curves help) to push the resonant frequency above the crossover for the woofers which use the chamber. That way the crossover attenuation kicks in and you're not stuck relying on the 1/f PSD of music. Without a B&W type solution I don't know what to do with backwave off the pole piece of the tweeter though.

Quote:
Originally Posted by stephenmarklay View Post
The real thought I had was to just use a sort of shock absorber like a car. The panel wall vibrates like a spring so why don't we insert a "real damper"?
Cool idea, but you'd want to attach the damper at as many points as possible. I have visions of speakers filled with hooks and rubber bands but that's not very practical. Seems like it'd be easier to build a squishy wall which would be more in the direction of sorbothane lining.
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Old 7th January 2010, 03:53 AM   #9
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Quote:
Originally Posted by twest820 View Post
It seems to me the most elegant method of reducing panel vibration due to backwaves is to using bracing to get enough rigidity (curves help) to push the resonant frequency above the crossover for the woofers which use the chamber. That way the crossover attenuation kicks in and you're not stuck relying on the 1/f PSD of music. Without a B&W type solution I don't know what to do with backwave off the pole piece of the tweeter though.
That can be helpful. Most of the systems i design have no XO.

dave
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