After years of deliberating, I finally want to try and build some panel absorbers (largish boxes, 10 to 20 cm deep, with a big panel glued airtight to the front and some wadding inside). Advantage of the panel absorber is that it has one main resonant frequency which can be used to address room modes.

1. I know the main resonant frequency is determined by the strength (let's say "s") of the panel/membrane as well as the depth ("x") of the box. Does the general size also come into play, i.e. if I build an absorber with a given s and depth x, will the length y and width z of the box have any effect on resonant frequency, or does a larger volume (with fixed depth) just result in higher efficiency? I figured I might as well just build two floor-to-ceiling absorbers and give that bugger 80Hz boom of mine something to chew on.

2. Does anyone know whether it's better to seal the box airtight at the back too, or should I just seal the front membrane (panel) to the box and leave the back open (it'll rest against the wall, but that won't be airtight, will it). Martin (Clark), I think you once said it doesn’t need to be sealed at the back, but if anyone has experimented I’d appreciate your input.

Cheers

Thomas

Hi, Thomas,
I've built three (identical) aborbers, sized 106mm/95mm, each, closed at all six sides. 5 built with 19mm MDF, the front side is the membrane, 3mm thick. Filled with rock wool, which is attached to the back side, and - important - NOT touching the membrane.
If I remember right, I've tuned them to 110 Hz (using a graphical table), a frequency where I had a cumulation of room modes, both theoretically and practically. The tuning frequency is determiend by the wood weight per area (Flächengewicht), which means, by the thickness of the membrane.
When I built them (9 months ago), I did use the mixed (and partially controversy) results of various web sites I've consulted. They're still in my listening room - which means they do what they've been built for: Take out bass energy in a pre-determined, not too narrow frequency band.
I've taken them out once - to see if they have become "obsolete" after putting in some more and different furniture into the listening room, and going through some h/w upgrades. Boy, they were back in after seconds...

Good (German) reading: Uni Essen

Best regards, freundliche Grüße

Stefan

[This message was edited by sjust on Wed 01 September 2004 at 21:20.]

mixed (and partially controversy) results of various web sites

Stefan, I've also found contradiciting information on various sites. This image on the site you referred me to:



again seems to contradict your advice of not having the wadding touch the membrane.

Never mind, the fact that you're happy with yours is encouraging! Also, by the looks of the graph displaying the efficiency with and without wadding, I might just try and build different types.

Please check your PTs, Stefan.

BTW, does anyone know of a test disc with test tones going from 30 to approx. 200 Hz in more increments than the ten or twelve I have on my XLO disc?

Thomas

quote:

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Originally posted by Thomas K:
BTW, does anyone know of a test disc with test tones going from 30 to approx. 200 Hz in more increments than the ten or twelve I have on my XLO disc?

Thomas

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Best solution: PC output + some basic signal generator Freeware.