The most important interior cabinet wall to treat is the back wall.
Driver cone materials are very thin and so are also quite acoustically transparent. The cone obviously vibrates both forward and behind, so equal amounts of acoustical wave energy goes into the room and the cabinet. So sound pressure levels inside the cabinet are relatively high compared to those in the room. Those high pressure waves inside the cabinet hit the interior walls and bounce back and forth. The pressure created helps dampen the cone vibration as needed for good performance. Lower frequency waves are longer and so they spread out into all directions (just like in the listening room). Higher frequency waves are shorter and tend to "beam" like light from a flashlight. Midrange/treble frequencies that hit the back wall first tend to bounce right back towards the driver.
Since the cone material is somewhat transparent those reflected sound waves come through the cone and into the room as a distorted and delayed copy of the original sound wave you're trying to listen to. The effect is a smearing of the sound. No-Rez should help. BTW designers of rear loaded horns, tuned pipes, and transmission lines account for this by slanting the back wall so that those "beaming" midrange/treble frequency sound waves are directed away from the cone. In the greater scheme of things, this is not highly significant, so it is often neglected in the design of sealed/ported speakers. But for audiophiles, good enough is never good enough.
