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Old 16th November 2012, 01:34 PM   #71
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Originally Posted by Baseballbat View Post
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.
Panels have a damping factor, so at low levels, it's not so linear. Saturate the damping - then it becomes linear until the panel goes into breakup mode. That's when it sounds really nasty.

Mind you, I know these things because I've built studio monitors. Real ones. They're big, and they do 110dB without strain. Currently I'm working on guitar boxes. Resonances in the ~500Hz midrange are a real problem. Of course, the pressure levels are stupid loud.
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Old 16th November 2012, 01:35 PM   #72
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+1 a monocoque or rigid box with ring more than one with seperate compliantly glued panels. Surely.
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Old 16th November 2012, 02:08 PM   #73
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How much will a midrange unit be excited at 500 Hz? Have you hit a panel resonance at 500 Hz?
Would be interested to know what volume is required to make an unbraced/undamped sheet of 18mm baltic birch ply that is 95cm x 40cm start resonating at 500hz so that it is audible with music (not test tones)?

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Originally Posted by Keriwena View Post
Given the size of the typical speaker you guys build, no. But if you have panels of more than a couple square feet, it can be easily done with some rockin' roll.
At some volume i'd guess thou? So with P10 size boxes not an issue. With the Emken, maybe (panel size above) and real big JBL/Atec style its an issue unless you don't brace/damp or brace/damp poorly. Other than that its a pure engineering issue (how do we damp & brace this military supersub that starts earthquakes?), not real world in your lounge playing your fav choons at normal listening levels type issue?
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Old 16th November 2012, 03:19 PM   #74
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Originally Posted by Keriwena View Post
Panels have a damping factor, so at low levels, it's not so linear.
Yes, of course, but that's at VERY low levels. It is not damping, but the cause is the same, friction, which is not linear at low stress.

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Saturate the damping - then it becomes linear until the panel goes into breakup mode. That's when it sounds really nasty.
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Originally Posted by JRKO
That's the usual working mode of speakers.
Would be interested to know what volume is required to make an unbraced/undamped sheet of 18mm baltic birch ply that is 95cm x 40cm start resonating at 500hz so that it is audible with music (not test tones)?
It is only interesting when the panel is resonating at this frequency, and there are tests made on this topic, unfortunately not very exhaustive, at least to my knowledge. The results hint to a minimum level of -30 dB below direct sound, under best circumstances, and -20 dB with normal music and the usual living environment.
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Old 16th November 2012, 03:57 PM   #75
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Originally Posted by Baseballbat View Post
It is only interesting when the panel is resonating at this frequency, and there are tests made on this topic, unfortunately not very exhaustive, at least to my knowledge. The results hint to a minimum level of -30 dB below direct sound, under best circumstances, and -20 dB with normal music and the usual living environment.
Well, the BBC reported resonances that were louder than the source (driver), and I'll concur with that.

But....

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Originally Posted by JRKO View Post
At some volume i'd guess thou? So with P10 size boxes not an issue. With the Emken, maybe (panel size above) and real big JBL/Atec style its an issue unless you don't brace/damp or brace/damp poorly. Other than that its a pure engineering issue (how do we damp & brace this military supersub that starts earthquakes?), not real world in your lounge playing your fav choons at normal listening levels type issue?
Yes, at some volume. Say, approaching 100dB. Not your typical, 'I'm not worried about the neighbors at this level' sort of thing. But, rather than have someone extrapolate from common practice, where it doesn't happen, that it can't happen, I'd rather mention it can, because you never know who'll be reading through these threads in the future.

Maybe... someone that might start a thread, "And Now for Something REALLY BIG"?
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Old 16th November 2012, 04:13 PM   #76
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Well, the BBC reported resonances that were louder than the source (driver), and I'll concur with that.
The BBC (Harwood) reported resonances that were 20 dB below the driver.
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Old 16th November 2012, 05:13 PM   #77
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Originally Posted by Charles Darwin View Post
Generally it is a good idea to make the bracing from a different material than the cab itself and use a lossy adhesive ie one that does not set solid.
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?
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Old 16th November 2012, 05:53 PM   #78
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Originally Posted by mondogenerator View Post


Ive heard 'translam' designs criticised for using ply in its weakest dimension, yet window braces in ply or MDF both use the material in the weakest dimension. In this method of use, neither are as effective.
I've not heard that "against the grain" on ply is the weaker direction, it would seem intuitively to me to be stronger.
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Old 16th November 2012, 05:55 PM   #79
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Originally Posted by Keriwena View Post
But, rather than have someone extrapolate from common practice, where it doesn't happen, that it can't happen, I'd rather mention it can, because you never know who'll be reading through these threads in the future.

Maybe... someone that might start a thread, "And Now for Something REALLY BIG"?
Now there is a thread around here called something like that I'll have to remember that my thread is for posterity!!! Didn't actually get round to bracing or even cabinet design. The only thing I want moving is the driver.

It can happen, but would be most likely in a big cabinet that is played very loud, built by someone who ignores all common sense and help offered, rather than most readers/posters here?

An average builder following basic design concepts & instructions shouldn't have an issue
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Last edited by JRKO; 16th November 2012 at 05:57 PM.
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Old 16th November 2012, 06:02 PM   #80
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Originally Posted by badman View Post
I've not heard that "against the grain" on ply is the weaker direction, it would seem intuitively to me to be stronger.
I should correct myself, and clarify. There is no way to show that ply is stronger than MDF when it is stressed perpendicular to the edge of the board. No such measures exist, to my knowledge. M.O.E. is quoted for stress on the face of the panel. It may well be stronger, and by that token so is MDF. My point being that many materials are comparable in strength, when used as a holey brace. I could answer conjecture with more conjecture, but im staying objective in my point. Ive been kicked enough for making a valid point thanks very much.
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