In the last three parts of this four-part article, I've told you about the value of using monophonic ("mono") playback through your stereo system for testing your system and your room; I've told you how to get it and how to use it, and given you some clues about how to "dial-in" your listening room. This final part will be about using a different kind of mono to get your speakers perfectly placed and to finish the job on your room.
For all that's gone before, we've used "straight" mono playback. Now, we want to change that, and put your two speakers out-of-phase with each other. To do that is simple: Just reverse either (but not both) the amplifier-end or the speaker-end leads on the speaker cable to just one (it doesn't matter which one) of your speakers by connecting the red cable lead to the black speaker or amplifier terminal and the black lead to the red terminal.
Once you've done that, you'll have a tool to provide the exact opposite of what you were trying for before: Instead of "perfect speakers", "perfectly" placed, in a "perfect" room giving you a mono "image" the size of a U.S. half-dollar coin, playing your system will then give you -- just at your speaker/listening room "sweet spot" - a mono image that's absolutely huge and seems to have no location at all.
If you were to feed two identical (both in content and amplitude) out-of-phase mono signals into a single channel amplifier at the same time, what you'd get is total cancellation and... silence - no information or signal at all except for the background noise generated by the amplifier, itself. When you do the same thing with your ears (but only in a "perfect" room, with "perfect" speakers, perfectly placed, and with yourself at the exact "sweet spot" in your listening room), because your ears aren't a single channel, but are naturally "wired" for stereo perception, the result is not silence, but a total cancellation of all differences (time, tone, phase, and amplitude) between the sound arriving at your ears from one speaker and that from the other. These differences are what would normally provide you with clues for localizing and "sizing" the sounds you hear, and cancelling them means that you will have no location clues at all: Everything you hear from your speakers will sound like it has no source whatsoever and is coming from all around you - possibly even from behind your listening position.
This is useful in two ways: Because a complete lack of "locatability" of the sound source (let's call it total "blobby-ness") can only come about as a result of perfectly identical sounds arriving at your ears at exactly the same time from both of your speakers, and because that can only happen if your room is perfectly acoustically-symmetrical and your speakers are perfectly placed, this out-of-phase mono playback is the perfect test for both your room and the placement of your speakers.
To use it, just sit in your normal position and listen. Assuming that you've done the basic steps toward getting your room set up that were given to you in the last part of this article, what you hear should already be at least reasonably "blobby". To improve it (to make it even "blobbier"), move one of your speakers (it doesn't matter which one) a little bit in some direction (right, left, forward or back).
What do you hear? After moving the speaker, is the sound more focused? Or less? Or does it stay pretty much the same? The object here is to destroy all focus entirely, so if there's no change, move the speaker more in the same direction. If it sounds worse (blobbier), you're on the right track; good! If it sounds more focused (less blobby), you're going in the wrong direction, so move your speaker back to where it was, and then a little more in the opposite direction.
By continuing this process, moving both speakers, each in turn, in small increments in all directions, you will eventually get to a point where ANY movement of either speaker in any direction will make the sound less blobby and give it a little more of a sense of location or source. Bingo! That's exactly where you want your speakers to be, and when you go back to in-phase mono, you'll immediately hear that your "half-dollar" - the size of your mono "image" -- has gotten noticeably smaller.
Before you go back, though; while your speakers are still out of phase, you may notice that something else happens once you get your speakers in just the right positions: you may suddenly notice that, instead of just being totally blobby, some point in your room is calling itself to your auditory attention. What that will tell you is that you have an acoustic anomaly of some kind that's calling to you to be fixed. It could be something (sonically) reflective or something - an open vase or urn, for example - that encloses a resonating air space that's "singing along with" the music. Fix it by moving it, covering it, or (as in the case of the "singing" vase or urn) simply filling it with something to keep it quiet.
There may be more than one of these anomalies, and, as you fix them, you may want to re-check and, if necessary re-diddle your speaker placement. Eventually, though, you'll get it as perfect as it's ever going to be. That's when to go back to "in-phase" mono and listen to your now tiny mono image. The real treat though, will be when you first go back to stereo and hear your system and your room imaging and soundstaging better than you ever thought possible!
The stereo will be great, but it's the mono, in both of its forms, that will be what gets you there!