@grunge1000
When a speaker emits sound waves, they travel to your listening position but also to the ceiling, sides walls and floor where they are reflected toward the listening position. While these first reflections arrive later and are measurably distinct from the initial signal, they reach our ears too soon for our psychoacoustic system to distinguish them from the initial signal and they become one in our head. What can be lost or overwhelmed by this wave timing are the subtler sounds that can help depict hall size, performer placement, soundstage width and depth, and the music’s overall tonal signature. This is the "psycho" part of psychoacoustics. First reflections also cause comb filtering when they arrive at the ear just milliseconds after the direct sound. At some frequencies the two waves interfere constructively yielding peaks in the frequency response and at other frequencies they destructively cancel each other out, creating nulls.
First reflections are distinct from the constructive and destructive interference that occurs later when waves continue to bounce around the room causing other frequency peaks and nulls. This flutter/slap echo can be ameliorated by further acoustic treatment throughout the room.
Absorption simply captures these interfering waves before they can reflect and converts them to heat. Diffusion breaks the waves up and reflects them in different directions instead of directly back at you. To better understand diffusion’s role, imagine being in a room and shooting a shotgun directly at a flat wall (no diffusion) versus against a rock wall (diffusion). The return path of the former is predictable and most are flying directly back at you. The latter will have much of the shot going elsewhere and taking a more circuitous route, bouncing off the surfaces of the room and even bumping into other shot before they might return to you. Because they are fewer, have been delayed, and have a lower energy, their return will not bother you as much as if they came flying straight back at you. Similarly, diffusion provides a time delay along with a weaker signal, allowing your psychoacoustic system to ignore or at least differentiate the reflections from the initial signal. Scattering panels work similarly and often are combined with absorption. I treat first reflections first because they are the second loudest signals in the room and are the largest source of interference with the original speaker signal. So at a minimum, you want to avoid "first reflections" and instead have either "first absorption" or “first diffusion”. But as you ask, which one?
GIK says, “For most accurate sound, thick absorption in these early reflection zones generally works best. However, some prefer to use other types of treatment (like diffusers or hybrid devices) to achieve different sounds. When deployed successfully, these treatments significantly tighten the stereo image, reduce comb filtering, and make long listening sessions more comfortable and accurate.” (https://www.gikacoustics.com/blogs/knowledge-base/acoustic-primer)
Arqen states, “Absorption on the side walls focuses the soundstage and improves imaging, while reflective or diffusive side walls give a wider sound, with less precise imaging.” (https://arqen.com/acoustics-101/reflection-free-zone/)
Further, if you have a small room, absorption is de rigeuer because there is simply not enough distance between you and the diffuser for it to effectively/smoothly work. The dispersion characteristics of the speaker are also a factor, with point source speakers likely sounding better with absorption than diffusion.
So absorption may be more frequently recommended not because it is inherently superior but because most speakers are wide dispersion and most rooms are relatively small.
As with all things audio, I invoke the disclaimer that you do whatever sounds best to you, but at least there is a bit of science that can increase your odds of success.
My virtual system has pics: https://www.audiogon.com/systems/11207
