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Old 16th November 2012, 05:26 PM   #81
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Originally Posted by Pallas View Post
Interesting. That idea popped across my radar when I noticed KEF's cutaway of the LS50 in their white paper on that speaker, though they don't really elaborate.

What's the benefit of a lossy brace, compared to a fixed one?
You incur more losses in the energy transfer which in this case is a good thing.
Similar or same principle as that behind constraint layer boards.
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Old 16th November 2012, 05:28 PM   #82
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I've not heard that "against the grain" on ply is the weaker direction, it would seem intuitively to me to be stronger.
What is "against the grain" when it comes to ply anyway?
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Old 16th November 2012, 05:36 PM   #83
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What I'm describing is as follows. Imagine a 3/4 x 3/4 ply strip, fastened on either end to some assembly. When pressing against the finished face of the ply, I would expect it to be less stiff than when pressing against the laminations.
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Old 16th November 2012, 05:45 PM   #84
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when stressing that ply piece on the face there is tensile strength in each ply. When stressing the stripey edge, there is perhaps less.

Or I should say that its strength is influenced by the adhesives strength in shear stress, as opposed to tensile/compressive stress. I think I got that right.

Few glues are good at both. For the record im not slating ply for braces, merely stating that I doubt it matters much if the holey brace is MDF, ply or solid wood. I would use what i had in scrap, and save cash. Its not much to lose sleep over
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Old 16th November 2012, 06:36 PM   #85
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If I remember correctly, I read somewhere, some time ago, Jensen's recommendations for cabinet bracing. To clarify, this was from the days when Jensen made their big coax and triax drivers and had their complicated nomographs to design ports - pre Thiele and Small. Cabinets in those days were plywood, because MDF hadn't been created, and huge. Jensen recommended bracing to run parallel with the long sides, not perpendicular. IOW, divide the long sides into 2 narrow panels to increase resonant frequency, as opposed to 2 more typically 'square' panels.
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Old 16th November 2012, 06:47 PM   #86
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yes. I believe that is what is being discussed
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Old 16th November 2012, 07:26 PM   #87
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Panel excitation is mainly a linear process, so no matter how loud you play, the ratio between direct sound from the drivers to radiated sound by the panels is always the same.
I would disagree with this. To start a resonance you 1st have to pump sufficient energy into it at just the right frequency to get it going. Fighting this is the natural energy dissipation of the material which is related to the relative dimensional differences between the material thickness and the wavelength in question.

A high Q resonance needs more energy pumped into it and in a narrower bandwidth, making it hard for music to ever deliver that much targeted energy into the panel.

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Old 16th November 2012, 11:44 PM   #88
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The BBC (Harwood) reported resonances that were 20 dB below the driver.
For their own series of LS cabinets, yes.

Here's a quote from Alan Shaw, who now owns Harbeth and is the 'spiritual successor' to Dudley Harwood of the BBC:

"The interesting curves are the blue one, showing the cabinet output alone (ignoring the output of the bass unit) when the box is made from 18mm (3/4 inch) undamped panels. Note again that in the middle frequencies the output of the box is very nearly as strong as from the woofer. That implies that the cabinet is acoustically transparent and the sound normally constrained inside the box is setting the 18mm panels into sympathetic motion and they are acting as a huge radiating surface."


And to answer JRKO, I'd say that the typical 6" driver with a sensitivity of less than 90dB is unlikely to create serious panel resonances when mounted in a 3/4" thick cabinet when following 'standard' bracing and damping practices.

However, those going "pro", that is, using 12" or larger drivers with >95dB sensitivity and higher than 'sub' crossovers, are likely to find that the mass of the moving assembly coupled with requisite box volume does, indeed, create a situation where panel resonances in the midrange need special attention.


And speaking of the BBC papers, there were two notable, and perhaps apropos, discoveries. A box with panels of twice the thickness rang more than a 'normal' box. Also, a box which accidentally had loose battens rang less. IOW, while gluing all the panels together guarantees an air-tight enclosure, the resulting construct rings like a bell.
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Old 17th November 2012, 12:16 AM   #89
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Old 17th November 2012, 12:44 AM   #90
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Quick question guys.

Back to bracing:

I was thinking of using MDF for window bracing on my first DIY project that I am now working on (a pair of Paul Carmody's Amigas - https://sites.google.com/site/undefinition/diy/amiga ).

If I use MDF, or plywood, and create the windowpane braces, then should the holes in the braces be circular? It is my understanding that parallel surfaces within the enclosure are conducive to standing waves. Is this true? Are there any advantages to circular holes in the braces?

Last edited by Frankie Carbone; 17th November 2012 at 12:46 AM.
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