If a guy was to study the human anatomy of the ear, and ponder on it for a while, you might come to a couple of considerations as to the real nature of sound and the failings of all sound reproduction in general.
The ear can do something that no microphone or loudspeaker can do, it can hear in phase and out of phase material at the same time. You may ask “how is this possible?” The answer lies in how the ear is constructed. Yes , the usual eardrum we all know behaves like a microphone, but the ear has also bone and sinus cavities that are also integrated into the act. This means that while the eardrum is hearing the usual sine wave pulse of sound and converting it into electrical signals, the bone structure is also transmitting information and converting that into electrical information. The real crutch is that the bone transmitted information can be out of phase with the eardrum information and because they are occurring on different physical planes, do not cancel each other out as with out of phase air bound sound waves.
This mixture of in phase and out of phase sound, is what gives us humans the ability to pinpoint sound locations instantly. It actually creates a 3D vector in our minds and we can easily target the directionality of any sound source. If we were to get a head cold, these abilities are hampered as one of the vectors is lost and the ability to identify the source is retarded.
How can a stereo system overcome this? The microphones used to record the information do have both elements of in phase and out of phase material transmitted to the recording medium, but the real suspect is the loudspeaker. Its divers can only move in one direction at any given moment, and most of the depth clues are in the lower frequencies. So how is the anti-phase material going to get transmitted within any particular cone movement? The only way to solve this problem is with time.
As Jim has suggested, moving the speakers away from the rear wall is one way, moving them farther apart is another way. Moving the speaker away from the rear wall mirrors the rear radiation distance and then hence increases the “time” that the wave will reach a listener. The speaker cabinet also has an influence on this as the density of the cabinet material used will slow down the energy wave.
Say we are sitting 8 feet from a speaker s front baffle that is only 1 foot away from the wall. Rear radiated sound will have 10 feet to travel (one foot to the wall, 9 feet to the listener (not including other reflections)) and this is not enough time to recover out of phase sound. If we move the speaker farther out, say 4 feet, rear radiation will still have to travel 4 feet to the wall and then 9 feet to the listener, but the direct radiation is now only 5 feet away. We now have a more reasonable time shift between direct and rear radiated sound. This method is kind of fooling our ears into thinking that there is something much larger than it really is and we enjoy sound field depth. This is also why reverberation is used in musical recordings, to create the illusion of depth.
