resonating panel absorber at an angle

[claesbjo]

Hi, I was thinking of mount a panel absorber (wooden box mounted against wall covered with a plywood sheet). In order to break up the rectangular shape in my room, I was thinking that i could make it angeled against the wall, starting at maybe 5-6" and ending at 1".

Will this make the absorber work at the whole frequency span affected by the airspace under the plywood sheet as indicated in the formula for the panel trap , f=170/(sqrt(md))(master handbook of acoustics) or does it actually work on the airspace volume that has to be compressed by the vibrating air, so that it doesnt matter how far it is between the panel and the wall, as long as there is a certain volume of air that the panel will react with?

Hope my question makes sense

best regards, claes

[Ethan Winer]

Originally Posted by claesbjo

In order to break up the rectangular shape in my room, I was thinking that i could make it angeled against the wall, starting at maybe 5-6" and ending at 1".

The air serves as a spring, so what matters is the depth of the panel rather than the total volume inside the cavity. I've done what you suggest, but with less of an angle. If the panel trap is two feet wide and you figure the flat depth should be 4 inches (as an example), then make one side 3 inches deep and the other 5 inches deep. This way the average depth is still 4 inches, and you get a useful amount of angle to avoid flutter echo.

--Ethan

[claesbjo]

Thankyou for that Ethan.

Claes
edit:
By the way, does it matter how large the area of the box is? any specific minimum and maximum?

cheers

[Ethan Winer]

Originally Posted by claesbjo

does it matter how large the area of the box is? any specific minimum and maximum?

If it's too narrow, the secured edges will prevent the panel from vibrating much. And if it's too wide the panel may vibrate in more than one mode. See this:

Vibrational Modes of a Rectangular Membrane

What you want is the 1,1 mode vibration in the upper left, not the other modes along the top row. At least I'm pretty sure this is correct.

Two feet wide is known to work well, and makes efficient use of standard 4x8 plywood, so I suggest that.

--Ethan

[claesbjo]

Ok thank you again, it was a quite interesting page that you linked to, found this there aswell http://www.kettering.edu/~drussell/Demos/RoomModes/driving.html , and i found the first chapter about pressure nodes, and how placement of the soundsource(the speakers, I presume) affected the room response for resonating freq:s quite interesting.

cheers

Claes