In the last section of this four-part series, I told you that, given "perfect" speakers, perfectly positioned in the "perfect" room, what you should hear when you play your system in mono - even if the music's as big as a massed choir or a full symphony orchestra - is an acoustic "image" about the size of a U.S. half-dollar coin (3 cm diameter), floating at ear level directly in front of you, regardless of where between your two speakers you are sitting.
If that's not what you hear, then there's a problem that - odds are - we can go at least a long way towards fixing. To make the fix, the first thing you'll need to do is to figure out the problem's source; and in doing that, the first thing to rule out is your electronics. Is your system solid-state? If so, you're probably fine. If it's tubes, though, when was the last time you set your bias or changed your tubes? Not all tubes (even of the same brand and number) "wear" identically, so there could be some differences between what's going to one speaker and what's going to the other. Check it out!
If it's not your electronics, the next thing to consider is your speakers. Regardless of what the manufacturers claim, unless you're using "omnis" (omnidirectional radiators, like MBLs or Ohm "Walsh" drivers) or full-range single-driver panels (like some electrostatics or "magneplanars"), it's almost certain that there will be "time alignment" issues with your drivers, at least at some listening positions - especially if you are running a separate sub-woofer. Without seriously re-engineering your speakers, there's nothing you can do about driver alignment, so just "shine it on" - all it can do is to make your "half dollar" a little bigger, and even getting your mono image down to a consistent "basketball-size" can make a great improvement if it's already bigger than that or has no real shape at all.
After your speakers, the only things that remain to consider are your listening room and where your speakers are placed in it. The last part of this series will go into speaker placement in some detail so, for now, let's just concentrate on the room you listen in.
When you're in your favorite listening position and you play your system in mono, if you don't hear a point source (of whatever size), what do you hear, and where does it sound like it's coming from? If what you hear is a localized source of any size, your listening room is probably at least somewhat acoustically symmetrical already and your goal should be to get it as much more so as you can.
Having a room that's physically symmetrical is a good start, but physical symmetry is no guarantee of acoustical symmetry. As an example, consider this: What if your listening room has a perfectly regular shape and your speakers are placed away from the back wall and equidistant from the side walls, but one side wall has either a large soft wall-hanging (making it absorptive) or is covered in irregularly-shaped wood blocks or stones (making it diffusive), and the other side wall is either smooth or a picture window (either one of which would be highly reflective); will your room be acoustically symmetrical? Of course not, and you'll easily be able to hear it.
Even in a room that's otherwise terrific - perfectly symmetrical, with no hard/soft anomalies - you can still have problems if your speakers are not symmetrically-placed in the room: Having them a significantly greater distance from one side wall than from the other will change what you hear at your listening position, and should be avoided if at all possible.
An easy way to understand room acoustics is this: Your speakers are, in fact, air pumps that create waves of positive and negative pressure in the air of your room. Some portion of that wave energy will go to your ears directly, but the rest of it will bounce around off the walls and furniture, either to be lost or to be heard by you as echoes. If you consider the air in your room to be the ocean; the room's walls to be the shoreline, the sound to be the ocean's waves, and the room's finishes, furniture, and features to be rocks or byways that will direct, reflect, or disperse the waves' pattern, you'll have a good analogy for what happens when you listen to your system.
If you'll keep this analogy in mind, you'll find that optimizing your room is actually a fairly rational process: The primary goal is to get the wave energy bouncing around your room not to interfere with your enjoyment of the wave energy coming directly to your ears, and the way to achieve it is, on the one hand, to get your room as acoustically symmetrical (relative to your listening position) as possible and, on the other hand, to try (without making your room too "dead") to minimize that bouncing wave energy as much as possible.
In making room changes, always remember that there are only three options for dealing with stray energy: You can reflect it, absorb it, or diffuse it. The first thing to do is to look at the locations of both your speakers, and of your listening position relative to them. Keeping your speakers' dispersion pattern in mind and remembering that "the angle of incidence equals the angle of reflection", locate the "first bounce" point for each of your speakers, on the nearest side wall, on the floor, and on the ceiling. Are the textures or reflective/absorptive/diffusive characteristics at those points the same for both speakers? If not, make them so, using hard surfaces to reflect, soft surfaces to absorb, and irregular surfaces (large, leafy plants are great for this!), to diffuse the sound energy. Making the right choices in just that one thing will go a long way toward both improving your room's acoustic symmetry and (to the degree that you use absorptive or diffusive surfaces) reducing stray energy. It will certainly help to reduce the size of your mono playback "image", maybe even down to "half-dollar" size!The last section of this article will show you how to use an altogether different mono technique to finish "dialing-in" your room and get your speakers placed exactly where they should be. See you then.