Remove or permanently relocate all major objects, such as furniture, in the room for which you are designing diffusers.
Set the speaker on a stand 5 feet off the ground in an open grass field. Set the microphone at the same height on a stand, 39.5 inches from the speaker. Run pink noise through the speaker, and graph the speaker's energy output at every 1/3 or 1/10 octave; this will be your reference.
Set the RTA outside the room, with a microphone cable that reaches anywhere in the room. Set the speaker up in center of the room. Generate pink noise. Move the microphone around along a virtual 1-foot grid between the four opposing wall surfaces. Compare your RTA readings with the reference graph, noting the volume bumps at what frequency.
Divide 1,100 by the frequency at each bump to calculate wavelength in feet. Each figure in feet correlates with a physical room feature, such as distance between floor and ceiling, two walls, a room corner or composition and surface texture of walls, floors and ceiling.
Lay out a floor grid spaced at 6 inches on a draw program. Use the wavelength figure in feet from Step 4 and divide by four for quarter wavelengths to break up a resonance caused by parallel room surfaces. Make panels out of plywood covered with felt, then corrugated roof paneling. Panels should be quarter-wavelength wide, reach floor to ceiling or wall to wall along the ceiling and placed at about a 45-degree angle to the adjacent surface, with 5-degree random variations to prevent resonance between panels. One edge of each panel should touch the adjacent surface. The array should look like a giant half-open venetian blind.
Build semicircular traps 3-feet wide in corners between adjacent walls. Use thin slats to form the curve. Place these from floor to ceiling.