His daily YouTube video yesterday he proclaimed diffusion was superior to absorption. I'm no expert but I believe he way overstated this. I'm my opinion a combo is best in general but its room dependent in many aspects. Am I wrong?
I don't agree with approximately 60% of what he says in those videos. Often he makes questionable claims without any evidence or proof or even support from the others in the industry and is only speaking from his own personal opinions - often stated as fact. However, sometimes I agree 100% with him.
That said, every room is different and even the speaker being used matters when considering the necessity of acoustical treatments. In general, the "ideal" is for 50% of all flat reflective surfaces should be treated in some way, but not more than 25% should be absorbers - and absorbers should be as broadband (usually meaning thick) as possible. So, the other 75% of the "treated" area should be diffusion OR angled reflectors or sunken cavities. So, things like soffits, sunken "art nooks", exposed beams or prominent pillars count as additions to the room treatments.
In some ways, I agree with the advice to not rely solely on absorbers. I've been in rooms where the owner went overboard on absorbers, covering the majority of the walls and ceiling with foam or fabric wrapped fiberglass or mineral wool, and those rooms truly suffered from those poor decisions. However, I've also been in rooms which were almost entirely treated with diffusion treatments, and the results were VERY pleasing. So, if one were to overkill anything, it is better to go crazy for diffusion than for absorption. Personally, I find absorption applied solely for early reflection points (where reflections arrive within approximately 20mS behind the initial source arrival time) and the rest of the large reflective surfaces being treated with angled reflectors and diffusors to be the perfect solution.