In most stereo systems, the best listening position — the “sweet spot” — is on the mid-line between the loudspeakers so as to form an equilateral triangle. If you’re listening from the sweet spot, you’ll hear the best illusion of sound-stage and depth. The difference between the sweet spot and other positions is noticeable, and sometimes quite dramatic. At the right position, the music seems to jump to life.
Room correction works by measuring the sound from each loudspeaker to a microphone at the sweet spot, and correcting for anomalies (in time, phase and frequency). As sound travels from the left speaker, for example, it will usually take several different paths to the listener: direct, but also reflected from walls and ceiling. The correction filter cancels part of these reflections, leading to a cleaner sound.
The loudspeakers themselves also introduce some anomalies, affecting the direct as well as reflected sound. This is especially true of normal inexpensive speakers. For example, the box might resonate slightly, introducing a characteristic tone of its own. For speakers with multiple drivers (tweeter, mid, woofer) the sound from each will often arrive at slightly different times, muddying the signal. Again, the room-correction filter will correct for part of these anomalies.
When the filter corrects for loudspeaker-based anomalies (which affect the direct sound path), the sound in the whole room improves. It doesn’t matter where you’re sitting. This is why room correction makes such a dramatic improvement to inexpensive sound systems.
Correcting for reflections, on the other hand, has its greatest effect at exactly the sweet spot. Outside that one listening position, the cancellations and the reflections don’t quite line up.
So one side-effect of room correction is that the sweet spot shrinks. If you’re exactly in the right place, the effect is very dramatic. But move a meter or so either side, and the illusion is less complete. The size of the sweet spot will depend on your system and your room, and on the strength of the room-correction filter. You might find that for casual or social listening you prefer to use a “soft” filter (correcting direct sound more than reflections), where for late-night solo sessions a “strong” filter works best.
