I recently moved to a rent appartment and my listening space will be in a
(L)3.7m (W)4.5m (H)3m rectangular room with all concrete walls/floor ceiling.
No need to express how terrible it sounds.
I am planning to use a lof of foam in all corners to control the bass but with such small room, trebble becomes dead very quickly. So I need to build a wood or plexy pannel that will transmit LF and reflect Highs. I will insert this pannel in front of a 10" thick foam(2x4 Feet) and try to curve it in order to diffuse the reflected HF.
Now I found this formula taken from ASC patent #4,548,292 that calculate the High Pass frequency of perforated pannel.
Fx = 40 p/d
p=to the percent ratio of open area to closed area.
d=hole diameter in inches

For example of a pannel is 2% perforated with .25inch diameter halls. I get a cross over frequency of F= 40 * 2/.25 = 320Hz.

Anyone have experience with such formula? I am concerned this might create some undesirable helmholtz resonance.

Tkx,Sam.

I would highly recommend using (or starting with) broad band bass traps. There are problems through out the bass/mid range and trying to make tuned traps for that is not recommended in a small room (or should I say starting with that). Broad band bass traps can be bought or built. If you want to build them and keep the high end in the room then use rigid fiberglass with FRK on the fronts. The FRK will reflect some of the highs in the room and also act as a membrane to help absorb more low end. These panels should be no less then 4" thick and straddling the corners.

Glenn

I would highly recommend using (or starting with) broad band bass traps. There are problems through out the bass/mid range and trying to make tuned traps for that is not recommended in a small room (or should I say starting with that). Broad band bass traps can be bought or built. If you want to build them and keep the high end in the room then use rigid fiberglass with FRK on the fronts. The FRK will reflect some of the highs in the room and also act as a membrane to help absorb more low end. These panels should be no less then 4" thick and straddling the corners.

Glenn

There are no sources of foiled faced FG in my area(I hate to work with FG as well ;). I can cover the foam with plastic wrapper but lack of acoustical data is a concern.
Anyways, I will need to diffuse the reflected waves and curving foil/plastic sheets is not possible. FRK will not solve it all.
Am really stressing out the need of difusion as in such a small rooms, HF reflections are dense within the 20ms Haas time frame and comb filtering is terrible. In normal room, you can absorb first reflection locations that are closest to the speaker only as other reflection points are delayed in time and lower in amplitude. In a small reflective room like mine, there is a lot of discrete high signal coming right after the direct one and its practicly impossible to predict and treat each location individually. Not even factoring more then 1 seating location....
I am thinking about absorbing first 8 reflection points on the front/ceiling/floor with foam only. And place diffusors all over the back and side-Back walls. The diffusion panel should also act as LF absorbers at all corners. Hence my original enquiry to build combos...

Sam.

The formula is incomplete. You need the volume of air or depth of the airspace for such a calculation. Regardless, perforated absorbers with this degree of hole area typically result in Helmholtz resonators. And as Glenn said, such tuned absorbers are not generally a good thing in lieu of broadband treatment.

Regards,
Terry

formula is incomplete. You need the volume of air or depth of the airspace for such a calculation. Regardless, perforated absorbers with this degree of hole area typically result in Helmholtz resonators.


Ouch, then the forumla is not for me as am not looking for passband narrow resonating absorbers. I am surprised that ASC tube traps integrated helmholtz resonators in its device to absorb bass.

Any pointer on where to look next? All I need is a rigid panel that passes lows and reflects highs in a predictable manners. By the way I wouldn't call this a tuned absorber per say as I wanted tuning to be very shallow.

Tkx, Sam.

By the way, If I roll an FRK panel and form a tube 10" in diameter with foil facing outside. Would that absorb bass and start diffusing from around 400Hz?

Ouch, then the forumla is not for me as am not looking for passband narrow resonating absorbers. I am surprised that ASC tube traps integrated helmholtz resonators in its device to absorb bass.

I wouldn't put much stock in the accuracy and detail of the "Background of the Invention" text in a US patent. A patent examiner is mostly concerned with whether or not you have referenced all the prior art, and about how broad/narrow your patent claims are.

Art Noxon once sent me some papers on his tube trap. It is based on a design from the 1940's. I can't find the file right now, but as I remember, the early work by this other guy (maybe Harry Olson?) was pretty light on theory.

- Terry

Bought a package of slightly damaged fg ceiling tiles, 2 x 4 1/2 thick with a plastic face.
My plan is a corner trap. 2 wide, 8 tall.
I'll go 8 layers leaving the plastic on the front. Any recommendation if I should leave the plastic in any of the inner layers?
Should I add whatever I have left over after trimming at the 45 to fill up the corner? It would probably give me another inch.

Remove the plastic on all inner layers.

--Ethan