[jhbrandt]

This region is known as the transition region where the wave model changes to the ray model.

As you know, in an enclosure, sound behaves like waves from VLF up to a point that is dictated by the volume of the enclosure and then proceeds to change to the ray model which begins approximately 4 times the upper frequency of the wave region. Is that confusing enough? Maybe SAC could chime in here and completely baffle the rest. (I understand you SAC...)

All joking aside, this region is best treated with broadband absorption and diffusion. Absorption because in this transition region are usually many near incidences (same frequencies) of axial, tangential, and oblique modes or standing wave reflections - even in the best of rooms. Diffusion because these frequencies can also take on characteristics of the ray region and can cause specular distortion in the form of comb filtering and the Precedence Effect or Haas Effect and can mess up the imaging.