Distance to side wall and back wall.
Most speakers need to be a minimum of a foot or two away from the side and back walls to reduce early reflections.
Differences among speakers can also influence positioning, so you must always read the manufacturer's specifics before starting to position the speakers.
A figure-of-eight pattern may be less critical of a nearby side wall, but very critical of the distance to the back wall.
The reverse is true for dynamic speakers that exhibit cardioid patterns. In general, the further away from reflective surfaces, the better. It is also crucial to keep the distances from the back wall and side walls mismatched.
If your speakers are set 3 feet from the back wall, do NOT place them 3 feet from the side walls, place them at a different distance.
Another crucial aspect of the listening position and speaker placement is that the distance from your listening position to each speaker be absolutely identical.
It has been calculated that an error of less than ½" can affect the speaker sound imaging, so get this absolutely correct.
Distance to speakers from listening position.
Once you have established the above, you now need to sort out the distance from the listener to the speakers.
I work off an equilateral triangle with the seating position being at the apex of this triangle. The distances must all be equal.
The other factor to consider is the distance between the speakers. Too close and you will get a narrow soundstage with the focus being very central. Widening the distance between the speakers will afford you a wider stereo width, but too far and you will lose the integrity of the soundstage.
This is the angle of the speakers facing the listener.
There are a number of factors that influence the angle of the speakers. The room, the speakers themselves, and your preferable listening angle I always start at an excessive toe-in and work outwards until I can hear the soundstage perfectly.
Tilt is also crucial. Depending on the make of the speakers, most speakers are meant to be level set, but some might require tilting and in most cases, the tilt is rear high. If you have to have the speakers tilted then start off level and work from there. Personally I prefer a level speaker setup.
You will find that the optimum listening height is that of the speaker's centre being at exactly ear height. However, certain speakers have their own specific height recommendations. You will find that with 3-way systems that incorporate top, mid and sub woofers, the listening height is more customized to account for the woofer placements in the speaker cabin or housing.
I find that keeping the seating position 1-3 feet from the boundary wall gives me the best bass response, and because the distance is too short for the brain to measure the time delay and thus locate the source of the reflection.
The listening position is at the rear of the room with the speakers facing and forming the equilateral triangle setup, and the listening position forming the apex of the triangle. The elliptical shape denotes the soundstage and as you can plainly see, the side and rear walls do not interfere with the soundstage.
As you can see, I have created this soundstage using the longer walls as the back and front walls, instead of creating the soundstage with the listening position on the shorter walls. This allows me to position the speakers as wide as is sonically possible and thus affording me a wider stereo field.
Place the listening chair near the rear wall, because the distance (1 to 3 feet) is too short for the brain to measure the time delay and locate the source of the reflection. Also, it places you at the room boundary where the perception of bass is greatest.
Please make sure to take care in optimizing your listening environment.
Once this has been achieved, you can mix far more accurately and truthfully.