Norman is still away from being able to post and he asked me to add this piece of useful info that helps to explain his diagram:
When it comes to absorption and diffusion products, it is important to understand their relationship to frequency wavelengths. Let’s look at a geometric diffuser and compare the physical frequency wavelengths to the physical device under test, and see how they interact in the world. Note how different wavelengths are affected by the size relationship of the device under test. In this instance we have a pyramidal diffuser that is about 12” wide. At 100Hz. the sound wave does not even see it and is totally unaffedcted because it is about 11.3 feet long. At 1kHz. (about 11.3 inches) it is affected by all faces of the pyramid. At 10kHz. (1.13”) it is only affected by the facing surface of the oncoming sound wave.