Plans for Broadband absorbers, calculation right?

We have a big recording room for grand piano, 85m2. Because there are some fluttering echos between parallel walls, I decides to build some Helmholtz-resonators. I found very cheap mdf-panels with holes in it. The holes are 8mm in diameter and 14mm distance, 5102 hole per m2. The panels have 4mm thickness. I calculated, that starting at the small sid with 30cm thickness, the resonator has a frequency of 535HZ and coming down to the 5cm side ist 1312 HZ. If I need more absorption in the highs, I would stick kind of Illbruck or other foam absorbers. MY PROBLEM: Do I need kind of absorption material inside? How does it change the absorption and frequency?

yes, you do need absorbing material. What's your current acoustic design?

Right now, theres only a hanging ceiling, ca. 1m. Then there are 6 hangers, right over the drummers place. The sound is nice, to too long (could be a little shorter), but I thought to fight the flutters, I can lower the reverberation time in one step.

Flutter and helmholtz..... neh. Seems like your walls are all hard surfaces, start there by hanging broadband absorbers. While you're at it, you'd need basstrapping too. Place big absorbers in the corners if you will. There's plenty 'o info here on the forum and don't forget Ethan's website, there's excellent info on how to make your room sound better. here's a great starting point: http://www.johnlsayers.com/phpBB2/viewtopic.php?t=10304

I know all these things, not the first studio I build. Just Rockwool makes it all muddy. I just want to know, what effect absorption in a Helmholtz has for the frequency, cause some people say, it lowers it for ca. 25%

With that high a % open area the panel will act as a low pass filter in front of an an untuned absorbent material absorber. You will not get any resonator effect. Andre

Sorry, didn't want to offend you by any means :oops: I'm not THE expert on resonators, hope John or Andre will chime in on your question.

ah, Andre beat me to it by seconds :)

Ro wrote:

Sorry, didn't want to offend you by any means :oops:

No problem, dont fell offended. Why should these things just be a lowcut filter? Its nearly the same then Johns Sidewall Absorber ( http://johnlsayers.com/HR/index1.htm ), instead of slats, I use holes. And if I use his calculation-sheets, I get this frequencies.

g-slam wrote:

Why should these things just be a lowcut filter?

That is a seemingly simple question. I do not know of a simple answer. The most accurate answer is by Schultz, Acoustical Uses for Perforated Panels. I tried to attach it, but was told it is too big. Andre

Andre and others, check this PDF (it is in portuguese)...but you can clearly see several perforated panels tests with and without rockwool behind it.... they both have absorption at low mid frequencies (but not that much !). check page 5 "sem la mineral" means without mineral wool "com la mineral" means with mineral wool http://www.castelhano-ferreira.pt/pdf/AcusticaXXI.pdf

Thanks for the link Andre! From (who else,) Andre

Thats cool. I just got the idea to replace my existing hanging wall, with this wooden hole absorbers. Would look much better! :-)