Wayne Parham Messages: 18786 Registered: January 2001
Illuminati (33rd Degree)
Look for response blips in the midbass to midrange, from about 60Hz to 300Hz or so. Above that the insulation will work. You will want to use something that can measure amplitude response, although you can see evidence of the standing waves in an impedance sweep. That's a simpler measurement, however, it won't tell you how severe the peaks might be.
One thing that will help is to use a sheet or two of insulation spanning the cross-section of the cabinet, splitting the top and bottom of the cabinet (and maybe even in more than one dimension) in addition to the insulation that lines the walls.