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 unaffected 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.