# Perforated metal (tuned) absorbers



## dannut (Oct 20, 2011)

Hello everyone!

Lets talk about perforated metals and their usage for sound absorption.

There are two options - as a transparent cover for another absorbing material (rockwool, fiberglass etc) and as a tuned resonant absorber.

Now to a real-world issue I'm having - specular ceiling reflection, with a twist. You see, usually we have speakers mostly with board dispersion, illuminating the ceiling with almost fullrange sound. So commonly a ceiling 'cloud' is needed with 4" material and airspace also. I am looking for alternatives.
As the roof insulation is 30cm (about 12") rockwool, and the ceiling material is gypsum board, how about leaving a hole in the ceiling about 2*2? feet at the first reflection points and using perforated metal instead? How pretty the ceiling would be with white paint? How to deal with flanking sounds to other rooms through the roof?
And the twist part - using a narrow, 45deg dispersion speaker, so no issues with reflections above 1kHz region from the ceiling. BUT in practical implementation the directivity will widen below 1000Hz, so I would still get problematic specular reflection, but band limited below 1kHz. Perfect application for a 'tuned' resonant absorber with a 200/300-800/1000Hz bandwidth. Is it possible? How would the construction look like?

You see, I'm not too keen hanging a 8" deep (visual) obstruction in the ceiling, so looking for some clever alternatives dealing with ceiling reflections.

Daniel


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## bpape (Sep 14, 2006)

First remember that it's really more of a zone rather than a point. Second, if you're worried about the midrange moreso that the highs due to controlled directivity, you're going to need to look at the wavelengths vs the size of opening.

If purely for reflections, 4" with a 4" gap is more than you need. We're not dealing with direct angle here, we're dealing with more random angle. That said, 4" with a 2" gap or 3 and 3 would be nice to have.

If the primary issue with leaving it open is isolation, you could always use box in that area so that at least you're minimizing how much energy gets out into the rest of the ceiling system and also making the layer above the ceiling double the mass/thickness.

Bryan


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## dannut (Oct 20, 2011)

Thanks for the feedback.
Good points about the 'size' of the wavelenghts involved. About 1m*1m opening covered with perorated metal (with the right perforation, need to look up from Dr. Schultz) should be adequate to 200-300Hz

4" material (+airgap) hanging in the ceiling is not a pretty sight. Thats why the inquiry about perforated metal, could be made nicely flush with the ceiling and can be painted like the rest of the ceiling.
Is there a nice way to hide the holes also? With open-cell foam and somekind of paint?

What about the tuned absorber? Any experience with them?


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## bpape (Sep 14, 2006)

Tuned will be narrower in nature. My thought was if you did a 4" panel and boxed in the ceiling cavity to isolate it better and leave the gap in there.

Bryan


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