Five feet from the front wall

@markalarsen 

That would be a mistake. When seated at your listening position the front wall is in front of you and the rear or back wall is the one in back of you.

@ditusa 

There is much more to imaging than lateral position, there is image size, location in depth in the sound field and the third dimension which is the sense that the instrument is a 3 dimensional object in space. Early reflections will disturb all of them even if lateral location is not affected. As far as stability of the image is concerned, especially with point source speakers, The image is extremally fragile when out of the listening position even by a few inches. 30 degrees or more off axis you can even lose the far speaker completely. Because lose of volume with distance is much less severe with line sources they create a more stable image and you never lose the far speaker of axis. The image is still perfect only at the listening position. This is why Line Arrays are used exclusively at large concerts. We were at the Arctic Monkeys concert at Red Rocks last night and even the subwoofers were linear arrays just outside of the main Linear Arrays. 

Why don't we just say the wall behind the speaker?

To call the wall behind the loudspeakers the rear wall makes no sense. Sure, that wall is behind the speakers, but so what? The wall the listener is facing is the front wall, the one behind the listened the rear wall. Period.

5’ from the front wall in a 10’ deep room.....?

Just a random concept to be or not to.... ;)

A Hall effect....

Regarding the wall behind dipole planars: One of the hardcore Maggie owners on the Planar Speaker Asylum completely damps the wall behind his very-modified Tympani T-IVa’s (he also braces the panels to the walls), other owners prefer diffusers.

Both absorption and diffusion can work; when choosing between the two, one consideration is the acoustic properties of your entire room. If you have a very lively room, absorption is probably the way to go; if it’s on the dead side, perhaps diffusion. Too much absorption in a dead room can lead to lifeless sound.

My room is definitely on the dead/warm side, so I went with diffusion. I have stacked Vicoustic Multifuser DC3’s behind my planars, and have found them to work great. Diffusers don’t absorb high frequencies, they scatter them randomly, keeping the inherent sound of the room intact. The scattered reflections don’t head straight back to the planar (or listener), so it’s back wave doesn’t compromise the sound coming from it’s front side. But that back wave does enhance spaciousness and depth, not a bad thing imo.