Any stereo loudspeaker pair disappears when only the center channel plays. That's a fact we can all agree on, for there is no physical box to play from.
The trick is to get the same disappearing property when sound moves closer to far left or right.
Let's imagine a stereo setup that uses box speakers (not dipoles such a Magnepans, electrostats, or any panel type) with a very forward sound, meaning the soundstage comes forward from the lateral plane of the speakers. And whatever music we select should be simple, recorded in an acoustic space, preferably with a voice in the middle. This will be our baseline.
We can achieve this forward sounding stereo system by shoving our loudspeaker pair close to the wall behind them (sometimes referred to as the front wall, but more typically it is called the rear wall). When you do this, if you're in an experimental mood, you'll notice several things, depending on the recording you've chosen. Tonal balance changes might be the first and most obvious to you. Voices that once sounded natural become bass and midrange accentuated and lose full spectrum of sound. You'll also note the increased lateral spread of each instrument or voice, smeared, as if they had been spread wider with a butter knife. The sound now projects an image in front of the speakers, instead of being trapped between them, or behind. One last thing you should notice, the closer to the rear wall the speakers are placed the more they are divorced from their enclosures, though tonal balance is very wrong.
Want to understand what's going on even if you don't perform this experiment? Stand in the middle of any room with a clear path between you and a wall. Start speaking at medium volume and approach the wall. Note how the sound of your voice begins to change the closer to the wall you get. This happens because the wall becomes an acoustic amplifier, bouncing your voice back to your ears in ever increasing proportions. Because the wall is flat and relatively infinite in size, compared to you, smaller higher frequencies aren't amplified as much as larger lower frequencies, thus your voice becomes skewed. Try it. This same thing is happening with the speakers placed near your wall.
