QuickTime 7mb>. Here's a still image with better details on the spectragraphs < gif 500K)
Note that I used a large diaphragm Rode NT2000 mic placed against the boundaries with the capsules perpendicular to the boundaries. In each test, the clip starts with the mic capsule pointed directly at the pink noise sound source (a speaker 4' away in a deadened studio) so the impending sound waves are parallel to the flat boundaries. While recording, I rotated the mic array 90 degrees to simulate off-axis sources, paused, and then reversed the rotation back to 0 degrees, the starting point. The spectragrams show quite a few differences that can also be heard by ear. Based on what I heard in this test and being quite familiar with the sound qualities and imaging of parallel boundary arrays, I made arrays using 7" hard wood spheres.
After recording with these "perp2sphere" arrays for a year, I have formed some impressions about them which Mike Rooke has helped me flesh out. Here's a sample recording with two "perp2sphere: arrays used in surround.
I'm happy to discuss what others find/hear in this test of boundary comparisons. Rob D.