Daft thought

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Ant
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#1 Daft thought

Unread post by Ant »

Just had a daft thought.
We know the panels of a speaker each have their own resonance, and that controlling that resonance to a degree can minimise the influence of the box.
We can control that to a degree by damping them or adding mass to them
The xerxes turntable controls resonance in the top plate with a cutout, so i thought occurred. as an example, my speakers are barely moveable so adding mass isnt an option, but routing slots in the panels and then filling them with rubber to seal the panel back up could be an option? And as they are aperiodic loaded with a great big vent, the big vent could be removed and substituted with a load of small slots filled with compressed wool, enough to make up the area of the original slot. That would break up panel resonances and provide the same loading as the big slot does.

Or is it actually a daft thought
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Scottmoose
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#2 Re: Daft thought

Unread post by Scottmoose »

Not a daft thought. That, or variations thereof, can work depending on exactly how implemented (& providing you don't kill the box rigidity in the proces! ;) ). It's like putting a crack in the structure of a bell. In cases where additional mass is not practical, or you can't increase rigidity to raise panel resonance above any point they're likely to be excited under operating conditions, then breaking up a homogenous structure with slots, grooves etc. to change its resonant profile can work surprisingly well.

I wouldn't rule out some mass-damping by the way. It's not my favourite approach (I always prefer raising panel resonance for bass enclosures, but that's usually not practical for an existing box) but even what might seem relatively small localised additions of mass-damping can have quite a significant effect. The old knuckle-rap test is more or less worthless as it really isn't very representative of how the box behaves under actual operating conditions. But if you play some bass heavy music & go over the panels to find the points where resonances are centred, you can add a small pad of something like bitumen to those specific locations -doesn't have to be huge, something like a 3in square is often enough, even with fairly large boxes. Bit less 'brute force' than lagging out the entire box with the stuff, and a whole lot easier on the back / floor-joists / sack-barrow / sanity etc. as relatively speaking the total amount of mass added usually isn't all that great.
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#3 Re: Daft thought

Unread post by simon »

Scottmoose wrote: Mon Oct 03, 2022 8:21 am I always prefer raising panel resonance for bass enclosures
What would be the ways of doing this Scott? Reducing panel area by reducing the space between bracing? Making the bracing more rigid? Something else?
vinylnvalves
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#4 Re: Daft thought

Unread post by vinylnvalves »

Tying the panels together across the cabinet, with lengths of broom handles etc is effective, as the panels want to move in opposite directions. Halving the unsupported panel or membrane, lifts its frequency by a power term, so powerful. Look up Timoshenko plate vibration theory, for a better explanation.
I would not lower the panel modes unless you want to use them as an acoustic source, like 1960’s speakers with thin ply panels.
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#5 Re: Daft thought

Unread post by Scottmoose »

simon wrote: Mon Oct 03, 2022 9:20 am
Scottmoose wrote: Mon Oct 03, 2022 8:21 am I always prefer raising panel resonance for bass enclosures
What would be the ways of doing this Scott? Reducing panel area by reducing the space between bracing? Making the bracing more rigid? Something else?
Right; lots of bracing, preferably longitudinal a la the kind Dave Dlugos likes for medium - larger enclosures if you want to go to town as technically it's more effective than purely lateral, with dissimilar spacing & using the most rigid material you can. The other advantage of raising the panel Fs is that you also raise its Q so the BW is narrower, further reducing any possibility of it being triggered as it tends to need a fairly powerful resonance centred on that point to set it off -unlikely if well above the box operating band.
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#6 Re: Daft thought

Unread post by simon »

Cor, Timoshenko's a name I've not heard for probably 3 decades...
Scottmoose wrote: Mon Oct 03, 2022 12:58 pm Right; lots of bracing, preferably longitudinal a la the kind Dave Dlugos likes for medium - larger enclosures if you want to go to town as technically it's more effective than purely lateral, with dissimilar spacing & using the most rigid material you can.
Holey braces, like Dave's Mar-Ken12?
http://frugal-phile.com/boxlib/P10free/ ... 310817.pdf
Are they much better than say window-type bracing?

(Sorry for the thread-jack Ant)
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#7 Re: Daft thought

Unread post by Scottmoose »

Right.

Significantly, although this does vary with context. For smaller boxes, it's arguably overkill. For larger, it's usually the most effective solution (albeit a right pain in the neck to make, as Colin will be the first to testify ;) ) assuming the cabinet doesn't have other internal panels that help brace it by default. Window braces are okay, and a whole lot better than nothing of course; the big boxes Colin's making at the moment use three of them for e.g. We did look at using Dave's holey brace configuration, but since the panels are already quite rigid as they're braced on the longitudial by the tall midrange sub-enclosure, we found we could get more or less all the rigidity needed without needing to go to the extra complexity. Same for some very tall boxes I'm working on at the moment -I looked at the longitudinal holey brace configuration, but half a dozen near-solid panels like a rather restrictive window-brace (the restriction is just a tweak specific to this particular enclosure; it's not something I'd recommend in general) do pretty much everything I need on that score, as the width & depth of the box is quite small.
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#8 Re: Daft thought

Unread post by simon »

Thanks Scott
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