OK... I am trying to absorb two room modes -- 120 Hz, and its harmonic, 240 Hz, by means of a slot absorber. Following the forumlas in F. Alton Everest's books, come up with the specifications.
I build two absorbers, both tuned to 120 Hz. Both traps are roughly 6' tall by 3' wide by 1' deep. The slats are "2x4"s, which are actually 1 3/4 by 3 3/4. The slot (space between the slats) is .26" (or approximately .25 inch).
To make the trap a little more wide-band, I placed 1" of the most porous (I could find) styrofoam "insulator" board directly behind the slats (touching them). This board was then run the entire height and width of the trap. Then the boxes are tightly sealed at the sides and rear.
Today while testing, not only were 120 and 240 not absorbed (no matter where I put the traps), other frequencies, such as 30 and 60 Hz were now reinforced, whereas before they'd been no problem.
I tried the traps in the corners, vertically and horizontally. I found the peaks of the wavelengths in the room and put the traps there with no improvement. I laid the trap flat on its back in a location in the room where there was a mode peak. Absolutely nothing improved.
What on Earth have I done that this should fail so miserably?
With thanks,
Barry