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.
This is helpful for people setting up their surrounds, another tip that hopefully people will know is that many receivers have a calibration feature that helps adjust things. Often speakers can't be 100% set ideally in a room (especially one that isn't a dedicated theatre) and Pioneer's MCACC or other brands audyssey calibrations use a microphone to help determine and adjust for different listening distances, room equalizations etc.
This is also helpful if using different size/brand speakers for front and rear and centers to adjust relative volumes.
Disclaimer: I've been in the forum long enough to know a couple things, but have yet to actually do a speaker build myself =)
Good info. However, it's always been my way to get things working the very best you can without room calibrations, first. I've always found that DSP works best as the "finishing touch" on a setup, but never very well as the answer to sloppy installations.
Just as you can't rely upon a computer program to design a good speaker (they're an excellent starting point, but that's all they are) you should not rely on DSP to correct anything big... just the small stuff that can't be handled any other way.
One of the problems with over-use of DSP is that you can end up with amplifiers clipping rather badly. 12 db of boost calls for nearly 11 times as much power which can result in a 50 watt amp that actually clips at a mere 5 watts of output. Of course this is also a problem with equalizers, too. That traditional smile people used to put on the sliders probably caused more distortion than tonal correction.
So, my suggestion would be to use patience, get the room as good as you can, tracking your changes in REW... but don't apply DSP except as the finishing touch.