perforated sheet

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pedro - 13 Jul 2006 17:46 GMT

As a university work assignment I'm making a preliminary design of an
anechoic chamber.
I receive a suggestion from a technician to install the anechoic
materials (foam wedges) in a rigid perforated metal sheet backed by a
porous material such as mineral wool.
As I understand, the absorption performance of the perforate will
depend only on the geometry (the hole radius and number of holes) and
not on the material.
Why use more expensive sheet metal perforated instead of hardboard or
plywood?
Thanks for help.

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Tony - 13 Jul 2006 22:12 GMT

> As a university work assignment I'm making a preliminary design of an
> anechoic chamber.
[quoted text clipped - 6 lines]
> Why use more expensive sheet metal perforated instead of hardboard or
> plywood?

Not quite sure how you intend to mount the wedges, but in answer to your
question, the acoustic performance of perforated sheet depends also on sheet
thickness. The high frequency absorption decreases as sheet thickness
increases, particularly with small holes.

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Chris Whealy - 14 Jul 2006 08:19 GMT

> As a university work assignment I'm making a preliminary design of an anechoic chamber.
> I receive a suggestion from a technician to install the anechoic materials (foam wedges) in a rigid perforated metal sheet backed by a porous material such as mineral wool.
> As I understand, the absorption performance of the perforate will depend only on the geometry (the hole radius and number of holes) and not on the material.
> Why use more expensive sheet metal perforated instead of hardboard or plywood?

The theories that predict the impedance and resonant frequency of a
perforated sheet assume that the sheet itself is perfectly rigid and
therefore has no influence on acoustic behaviour. If you used a metal
sheet, then it would be far more rigid than a plywood or hardboard
sheet, and therefore, the performance would be closer to the predicted
values.

The factors affecting the impedance of a perforated sheet are the hole
radius, the repeat distance between holes and the panel thickness.

As either the panel thickness, or the hole spacing increases, the high
end absorption drops off. The high end absorption drops also off as the
hole radius decreases.

Have a look at my Porous Absorber Calculator spreadsheet that might help
you in this area. http://www.whealy.com/acoustics/Porous.html

Chris W

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The voice of ignorance speaks loud and long,
But the words of the wise are quiet and few.
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Natural Science Forum / Physics / Acoustics / July 2006

perforated sheet