Originally Posted by Nick Di Lorenzo
The first point of reflection in your set up.
Treat with absorption.
If so why?
This is probably most confusing question today in small room acoustics (I'm still not sure why, probably because too much people ignore very specific properties of the small rooms
First reflections generate non resonant interference in the room, so even if we solve resonant ones (room modes) we will still have problems with notches in our response which doesn't resonate... but ruin our complete impression and make room still technically faulty for serious work.
Non resonant interference is sound waves canceling at the listening place because reflecting boundaries in the room. This is more intensive in the small room because closer proximity of the boundaries than in the larger rooms. This behavior may happen from about 80-500Hz, dependent of loudspeaker position in the room. Some of the problems may be solved with careful loudspeaker/listening place positioning (cancelling cancellations... literally). Very important thing in non-understanding of problems is mostly omnidirectional radiation characteristic from loudspeaker in this range.
Only way to "isolate" our listening impression from faulty mirrored "loudspeakers", because our walls, is absorption
, there are NO diffusers which may do this especially because their working range is not at this frequencies... there are no diffusers in small rooms which work below 500Hz... or someone ignore listener proximity problems with phase grating diffusers.
Also, frequency range of problems, and needs for isolation from mirrored images of "loudspeakers" causes fact that absorbers must be large to be effective, usually from floor to ceiling, and 2-4m width... because too large wavelength of frequencies of waves which is cancelled at listening place.
Why we need diffusers at first reflections? We need it to bring back reflections in the rooms above 500Hz, because we naturally need it (people are used to live from their birth, in more reverberant places), or room will sound "too dead" after all "ghost loudspeakers" images are only "deleted" by absorption.
Why diffusion and not only specular reflections? Because diffusers give much better focus (shortly).
So, answer is... following all described above... we need it both, always, in the small rooms. MyRoom Design
is logical sequel of this story.