Panel absorber construction question

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Dr X

New member
When you're making a panel absorber, how critical is the way you attach the ply to the box?...I want to make some bass absorbers using 1/8" ply over an 8" gap. Can I just glue the ply to the front of the box directly onto the mdf (probably 3/4" or 1") for a flush edge? Or do I need to have the ply sitting "inside" the mdf mounted on beading or something. Could I nail the ply to the mdf and seal it afterwards? Hope you see what I'm getting at. Thanks in advance.

Simon
 
When you're dealing with sound, especially low frequency, the main thing is nothing loose enough to vibrate (except what's supposed to, like the panel)

MDF is kind of porous unless the edge is sanded, and not the strongest thing to sink a screw into. If I were doing this, I think I would use cleats (beading) made of at least 3/4" square solid wood, glued AND screwed both to the panel AND the frame, THEN caulked. That way the only vibrations you hear are (hopefully) "Good Vibrations" (as the Beach Boys would have said)

Hope this helped... Steve
 
Thanks

So the actual attachment technique isn't too critical, though? So you could glue and nail or screw as long as the materials can cope and it's airtight. I was concerned about whether the attaching technique was critical to the ply's ability to flex in the appropriate manner. Thanks for that.
 
I'd screw the panels to the frame. You can then attach a contact mike to the panel and tune it by tightening, or loosening the screws. If you run a frequency sweep through a speaker close by the panel will resonate at it's tuned frequency which can then be viewed on an analyser. :)

cheers
John
 
Thanks John

I'm building the trap as per your original thoughts, John. I had the room checked out and it needs to be around 100 hz. Thanks for your input again.
 
From and efficiency perspective a loose, yet still air tight, connection to the frame is best - something sort of like a "resilient channel gasket". This allows the diaphragm to work more like a piston rather than a drum head. In fact, that F=60/sqrt(M*D) formula you see bandied about is actually only true for a piston like diaphragm.

And another note about panel absorbers that never seems to get mentioned. Yes, it's clear from the formula the one can decrease the resonance frequency by increasing the mass per unit area of the panel. But what this equation doesn't tell you is that increasing the mass also decreases the efficiency of absorption. In order for the sound to be absorbed it must actually move the panel. The more inertia the panel has the more difficult it to move and therefore the less efficient it becomes. For a given target frequency efficiency increases by decreasing the panel mass and increasing the cavity depth.

barefoot
 
Here's how I would build a panel absorber:

piston.gif


barefoot
 
I think so. I believe KEF used to use passive radiators - as does the Mackie HR824. My design is essentially a passive radiator.

barefoot
 
I was just thinking it might be better use fabric instead of plastic. It would probably be more robust if someone bumps or leans on the panel, and you avoid the problem of bubbles when laminating. It could then be made airtight by painting it with a flexible latex paint. Sort of an all-in-one functional and aesthetic solution.

barefoot
 
Hmm fascinating stuff

If you used the plastic film approach, though, wouldn't the film/rubber stretch'flex rather than the ply it's attached too? ie wouldn't the ply sort of stay flat and all the movement graduate to the film? I suppose, thinking about it, this is why you talk about a piston motion. But wouldn't the stretchiness of the film be difficult to predict and tune?
 
Actually..thinking about it even more...

I was thinking about gluing the panel to the front with No Nails (don't know what it's called in Oz or US!) which is a stretchy , flexible rubbery glue. Barefoot's theory would suggest that this would be better than nails or screws, yes? As the ply will be allowed to move a little with the glue?
 
Simon,

Yes, the whole point of the flexible suspension is so the panel can move as unit. This exaggerated drawing illustrates the difference in deflections between the two cases:

deflect.gif


The piston panel moves a greater volume of air and the effective absorption area is basically the whole panel rather than about half the panel area for the rigid mount case.

In my design and for normal panel absorber aspect ratios, say like 2'x4'x8", the vast majority of springiness (compliance) comes from the air inside the cavity rather than the suspension. This springiness along with the mass of the panel, and the amount of damping, determines the resonance frequency. The springiness of the suspension becomes more significant as the box volume increases - or if you happen to stretch it very taut. Note that I changed my mind and said a sealed fabric would probably be better than a plastic film. I'll change that in my drawing.

Personally I would use a good deal of damping and aim for a more broadband absorber. If you need to tune it to a specific frequency, then design it for a slightly higher frequency than you want and add mass by gluing scrap pieces of plywood or whatever to the back of the panel until you reach your target. If you just do one and record the weight or dimensions of the pieces as you go, then you wont have to repeat the process on the following ones.

Liquid Nails is the common brand in the US for the glue you're referring to. It's generically called construction adhesive. I would both glue and nail the ring to the frame in my design. Just enough 3d finish nails to hold it in place while the glue dries. About one nail every 6". I would use screws on the back panel for access. You can even forgo the back panel and use the wall you're mounting it to as a seal. Just make sure you use some kind of good airtight gasket.

barefoot

http://barefootsound.com
 
One final question...

Does a bass trap have to be attached to the wall for most effective results or can it free stand just in front of the wall ie just sitting on the floor a couple of inches away from the wall?
 
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