Demystifying the Sweet Spot
I'm pretty sure most people will know this already but when setting up a stereo or surround system there is a "Sweet Spot" --best listening position-- in which all the speakers will deliver the best sound.
What may not be apparent is exactly how this works to produce a seamless "Soundstage" in the room.
First, we need to understand that this "soundstage" we hear is not a function of the speakers or the amplifiers or even the playback device. This information is burned into the recording itself, at the studio where it is mastered and mixed. Thus it can vary drastically from one piece of music (or movie) to the next and there isn't much we can do about it.
When stereo music is mixed some of the sound is played from the left speaker, some from the right. But there is also a component of the sound that is played equally in both channels. The overall effect is to create a phantom centre channel that appears to be directly in front of the listener, spreading the sound across the speakers. A similar effect happens with the rear speakers in a surround system.
We can, however, position our speakers to give the best average effect.
In a home theatre setup you want your listening position to be directly in front of the display screen. In a stereo system you want it on the left-right center of the room or listening area.
Per the drawing, you want to keep lines A and B the same length, and on the same angle to the listener. Lines C and D should also be the same length and on the same angles. But the A-B and C-D pairs do not need to be the same.
The sweet spot is where all the lines meet.
By moving the front speakers closer together or further apart we can fine tune the strength of the phantom centre image. Moving them closer makes it appear louder with less difference between the left and right speakers. Further apart makes it quieter with increasing left-right separation. Thus we can manage the balance between the left, centre and right sounds by the way we position the speakers.
Of course a similar exercise can also be done to manage the rear image from surround speakers, moving them in or out with respect to the listener.
In a surround system, the distance from front to back is less important than the position of the surround speakers relative to the listener. In this case we want the rear surround speakers to be beside and slightly behind us, not at the back of the room. In 7.1 systems, we additionally want the side surrounds to be half way between the front and rear speakers. This, because sound coming from directly behind us is actually distracting, taking our attention away from the screen.
We can also fine tune the tonality --the relationship between bass, mid and treble-- by rotating the speakers in place. Bass tends to generalize into the room, so it is our constant. Mids and highs tend to be more directional. Thus pointing the speakers more or less towards the listener gives some control over the tonal balance of the room.
Make changes slowly and with patience. Move the speakers then wait a few sessions before deciding if it's better or not. You are looking for the best overall performance, not something from a single song or movie.