Sound travels at approximately one foot per millisecond. For two sounds to appear unrelated, they must be separated in time a minimum of 10 milliseconds (it’s a function of how our brain perceives and processes sound). For that reason it is recommended dipole planar loudspeakers be positioned a minimum of 5’ from the wall behind them. Why? The sound emanating from the rear of the planar travels the 5’ distance to the wall, is reflected off the wall and travels the 5’ back to the rear of the planar, 10 milliseconds after the same sound left the front of the planar. The rear wave therefore sounds like an event separate from the front wave; if the time difference is less than 10ms, the rear wave results in a smearing of the front wave.
But there’s more to the story: When the rear wave arrives back at the planar, if that wave and the front wave are out of phase with each other at any given frequency, the combination of the two may create a hole in the frequency response of the loudspeaker. Or they may combine to create a peak in response. We of course want neither of those to happen.
The way to prevent the rear wave from creating the above scenario---known as comb filtering---is to either absorb or diffuse the rear wave. Absorption needs no explanation, but it needs to be said that it may cause the sound heard in the room to be too "dead", lacking spaciousness, one reason for choosing a planar loudspeaker after all. A better tactic is to diffuse the rear wave; the random scattering of the rear wave prevents it from returning to the planar out of phase with the front wave, and keeps the sound produced in the room from becoming "dead" or lifeless.
