Diffusion vs. Sound Absorption views

Essentially 
Diffusion everywhere, particularly front and back walls and ceiling in your case. 
Absorption only where benefits are obvious through experimentation 

How do you know when panels are needed? I'm interested in the topic, but don't notice a problem with Golden Ear Triton 3+ about a foot from front wall and 6" from side walls. If it already sounds good don't mess with it? Tinker for better sound? Or is there some measuring equipment for this? 

Conventionally, diffusion improves perceived ambiance while absorption improves imaging.


I suspect that the answer is somewhat dependent on the distance from the rear of the OB loudspeaker to the rear wall.  Why, if the speaker is too close, then there is much more of a chance of the out of phase wave interfering with the sound coming off the front...so absorption should help.Here is a brief but interesting article:

http://www.gikacoustics.com/speaker-boundary-interference-response-sbir/


I think that you can "simulate" the effect of diffusion by moving the panels out from the wall another 2-3' (not that you are going to leave them that way) which will then give you a different combination of direct sound, reflected sound and absorbed sound.

What’s really needed is a test tone such as found on test CDs, say 315 Hz, which works fine, and an SPL meter. Then, determine locations in the room where sound pressure peaks 6 dB or higher than the average sound pressure in the room at moderately high loudness. Those locations are generally where acoustic devices - panels, resonators, etc. - should be placed. Without a method for placing acoustic treatments it’s like shooting blanks in the dark. As the system evolves you should find that the locations of acoustic devices can change along with the system changes. Ditto speaker locations. It’s a fluid situation.

I will repeat my warning and caution about using QRD quadratic diffusors.  snapsc had a point about diffusors " out of phase wave interfering with the sound".  The QRD diffusors can definitely cause problems here if they are placed in certain spots.  I have had situations where some of the midrange frequencies were boosted and others were cut, making the sound completely terrible.  The QRD diffusors can also boost high frequencies to the point where it's too bright/harsh sounding.  These side affects are entirely dependant on where you put the QRD diffusor and the specific room you're operating in, so it becomes much of a test and listen exercise.