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PostPosted: Wed Feb 28, 2018 10:16 pm 
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Joined: Fri Feb 02, 2018 9:26 pm
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Location: London, UK
Hi,

I'm trying to get my head around panels absorbers in particular application which implies a bit of theoretical acoustics reasoning and some practical advise/experience by someone who found himself in a similar situation.

What is the approximate performance of a panel absorber made of 19mm plywood 2500x1200mm size hung on wires with a cavity of 300mm behind? (Sketch attached of layout and panel absorber and picture of 40mm fibreglass panel (different panel) showing the fixing mechanism used similarly for the 19mm plywood panel)
The issue is that the panel is open at the perimeter (no closed volume). Does this mean that the air does not really act like a spring in the mass-spring system? At least at the perimeter? The panel has a frame, around 100mm deep, made of battens to attach it to steel wires. Would increasing the frame depth so to enclose more in a confined volume the air behind, increase its performance?
What would be the size for such a panel to be effective in general (i.e. it could be considered a panel absorber) and in the range 125-250Hz?
I imagine putting two panels next to each other would increase their performance, is it right?

Has anyone ever worked with this kind of DIY panel absorber? Would insulation 50mm help making the absorber a bit more broadband?


Apologies for the numerous questions but I am just trying to find ways to improve this panel absorber which seems to do little at the moment...
Thanks!


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PostPosted: Thu Mar 01, 2018 5:19 am 
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Joined: Thu Aug 21, 2008 10:17 am
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Location: Santiago, Chile
Quote:
What is the approximate performance of a panel absorber made of 19mm plywood 2500x1200mm size hung on wires with a cavity of 300mm behind?
If you are asking about the resonant frequency of the thing in the photo, then t is basically nothing at all. Panel traps don't work like that. Panel traps require s SEALED, AIR-TIGHT cavity behind them, not open air. The air trapped in the cavity acts as a spring, which is what creates the "Mass-Spring" resonant system. If there is no sealed cavity, then there is no spring, so there is no resonance.
Quote:
The issue is that the panel is open at the perimeter (no closed volume).
Then all you have is a piece of wood hanging in the air! :)

Yes, that piece of wood by itself does have a natural resonant frequency, like all other objects in the universe, but it's no use as a tuned trap like that.

Quote:
Does this mean that the air does not really act like a spring in the mass-spring system?
Right.

Quote:
At least at the perimeter?
No, not even at the perimeter.

Think of it this way: A piston inside the engine of your car is really great at compressing the fuel/air mixture, so the spark can ignite it. But how well do you think that piston would work of you took away the rest of the engine, and just have the piston all by itself? Can it still compress the air? Nope.

Quote:
Would increasing the frame depth so to enclose more in a confined volume the air behind, increase its performance?
Only if you make the frame deep enough, and rigid enough, and put a thick, heavy rigid back on it, and seal it all air-tight.... In that case, it would increase the performance from the current level of "zero" ro something useful....

Quote:
What would be the size for such a panel to be effective in general (i.e. it could be considered a panel absorber) and in the range 125-250Hz?
The SIZE of the panel is not relevant for a tuned panel resonator trap. It's the DENSITY that matters. The size is only relevant in the sense of "it needs to be big enough in order to be effective", but it does not change the tuning. The equation is simple:

D = 28900 / (M * f^2)
d = depth of airspace in inches
M = surface density of panel, lb / ft^2
f = peak absorbing frequency

So weigh your panel accurately, divide by area, and you have the density.

Quote:
I imagine putting two panels next to each other would increase their performance, is it right?
Only if you build them both correctly.

Quote:
Would insulation 50mm help making the absorber a bit more broadband?
You do need some damping inside the air cavity, yes.

Quote:
I am just trying to find ways to improve this panel absorber which seems to do little at the moment...
It "does little" right now, because it is just a piece of wood hanging on a wire! All it does is to reflect some higher frequency sounds back to the room, while diffracting other sounds, and doing nothing at all to lower frequencies. But the effect is very, very small, since it is just a piece of wood hanging on a wire.

Question: Why do you think you need a tuned trap in your room?


- Stuart -

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