Front or Back wall diffusers

As others said, every room & system is different. There are no cookie cutter rules. I personally do the following:

Diffusers in the front and back wall. Depending on the distance from the back wall, the impact they have there is bigger. My diffusers are actually combination diffuser absorption and bass trap (GIK Alpha 6” and 4”)

Absorbers and diffusers (a mix of them two) in side walls and ceiling. Lots of experimentation needed. Actual work. Listening and REW measurements. There is no free lunch.

Base traps in both front and back corners.

In my opinion and experience, the coverage area is important. You cannot just slap a few panels here and there and call it a day.

‘Good luck. Lots of work but rewarding at the end

To add my 2c’s.

I have 7 Stillpoint Apertures 2’s, they claim to provide absorption, diffusion and reflection.

2 are in the front between the speakers (see my system page.)

2 each on each side wall (4 total) at first reflection point.

1 behind my listening position.

4 ASC tube traps in the corners.

And a couple of absorbent panels behind the speakers.

ozzy

The need for acoustic treatments is room dependent and I just share my experience for whatever it is worth. Three critical elements are (1) diffusers on front wall, (2) bass traps on both corners and (3) the distance between the speaker tweeter and front wall. (3 ft as a rule of thumb) As a gentleman mentioned earlier, you do not want the reflected signal be perceived as a direct signal and mess up the image of sound. If you need to place speakers closer to wall, some form of absorbers (like curtains I used) behind the speakers is also recommended. As far as the back wall is concerned, if your sitting position is cleared roughly 1/3 of the room from the back wall, the treatment behind is less critical.  Lastly, do not overdamp your room!!

if your sitting position is cleared roughly 1/3 of the room from the back wall, the treatment behind is less critical. 

Do you know how long bass waves are? You must be referring to a castle.

yes, it can still have standing wave, nodes, etc., issues in the bass region. I apologize for the misspoken.