Perf Acoustical Panel Design Question

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

Mike - 20 Jan 2004 18:43 GMT

I am designing a metal perf panel to absorb an octave centered at 250
Hz.

I was checking the formula for perf panels which I use. It is as
follows:

Res freq=200*square root(perf %/(air space*hole length)). I have also
added the correction factor for hole length which is: effective hole
length=.8*hole diameter.

With this information, it seems that a steel material of .02 thickness
and ½" holes providing 55% perf and 69" air space would be an
effective resonator for about 275Hz. Yet, a panel of the same
thickness with 1/8" holes yielding 10% perf gives resonance at 219Hz
for the same air space.

This seems odd to me that such a large variation in hole size and
perf% can yield such close results. Does this seem right?

I need to use this thickness metal and I would like to use the 69" air
space. I can change the perforation percentage and any material used
in the air space.

Thanks for any input.

Reply to this Message

Tony - 20 Jan 2004 20:55 GMT

> I am designing a metal perf panel to absorb an octave centered at 250
> Hz.
[quoted text clipped - 3 lines]
> added the correction factor for hole length which is: effective hole
> length=.8*hole diameter. .........

It isn't wise to apply a formula blindly, it can turn round and bite you.

This looks like a version of the Helmholtz absorber (or resonator) formula.
The correct term outside the square root is c/(2*PI) where c is the velocity
of sound. Of course all the units must be the same, so if c is in feet per
second all the dimensions must be in feet. Not quite sure where the 200
comes from. Maybe it is meant to be 2000, that would be somewhere near
right with all the dimensions in inches.

That's the second problem. The first problem is that a thin panel over an
air space will not behave anything like a Helmholtz absorber. If it is
damped it will behave as a panel absorber somewhat modified by the holes so
you need a different formula altogether.

Tony Woolf

Reply to this Message

Eric Desart - 20 Jan 2004 21:44 GMT

Mike:

A link to an organisation originally referred to by Noral
http://www.iperf.org/perfhand.htm
Look at the different extremely valuable documents.
Formulas and tests are provided.

Maybe you find the European sister organisation too (ones found it).
They also have lots of info (work together with the US organisation)

Best regards
Eric

> > I am designing a metal perf panel to absorb an octave centered at 250
> > Hz.
[quoted text clipped - 19 lines]
>
> Tony Woolf

Reply to this Message

Natural Science Forum / Physics / Acoustics / January 2004

Perf Acoustical Panel Design Question