I intend to build some shallow on wall speakers as I need to listen relative close to the wall (7-8ft) and want to avoid all the problems with speakers positioned close to walls. Many designs I found (commercial or diy) are just shallow boxes with walls perpendicular to the front baffle. In contrast to in wall speakers there is a distance between wall and front baffle which will cause diffractions, back reflections, comb filtering and psychoacoustic merging of direct and back reflected sound. This effect will be dependent on depth of the shallow box in a frequency dependent manner. If it is only 3-4" deep I assume that it will matter only above 600 -1000Hz (1/4 wavelength).
I can think of 2 options to avoid these mid and high frequency effects:
edgeless design i.e. 1/4 circle shaped sides. This would avoid edge reflections but much more involved to make.
covering the sidewalls with (wool) felt, potentially slightly extending past the front baffle. Felt is an effective absorber at higher frequencies but will it be sufficient?
I can think of 2 options to avoid these mid and high frequency effects:
edgeless design i.e. 1/4 circle shaped sides. This would avoid edge reflections but much more involved to make.
covering the sidewalls with (wool) felt, potentially slightly extending past the front baffle. Felt is an effective absorber at higher frequencies but will it be sufficient?