Stereo Speaker Placement 

This video is a comprehensive breakdown of the basics of speaker placement for quality stereo playback.
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"Stereo playback is over a hundred years old, but the principles behind the technology haven’t really changed. Two audio signals are played through two separate speakers that are positioned so that the combination of the two signals creates a coherent sound space.
It’s a simple illusion that makes the reproduced sound that much more vivid by letting the listener localize individual elements. As basic as this technology is, it only works if the speakers are positioned properly.
If you’ve researched room setup before you’re probably already aware of the equilateral triangle setup. Setting your speakers up so that the distance between your listening position and each speaker is the same as the distance between the speakers forms a symmetrical layout that ensures the sound from each speaker reaches the listening position in sync. Push one speaker closer than the other and the sound stage will be distorted as one channel arrives at your ears slightly sooner than the other.
Symmetry isn’t just important for the measurements between the speakers and the listening position, but their placement within the room should also be symmetrical so that the reflected sound from the surrounding surfaces also reaches the listener in sync.
So where you place your speakers is going to be based on your room size and listening position. Ideally your listening position should be placed on one of the room’s thirds and avoid the null in the center of the room.
It’s almost always a better idea to have your listening position face the shorter wall. With the speakers aimed down the length of the room, bass frequencies have moretime to develop and have less energy by the time they strike the rear wall which will make treating your bass issues a lot easier.
Based on these principles, the starting layout is always the same: An equilateral triangle formed between the listening position and the speakers centered along the length of the room.
You’ll also want to make sure that the speakers are seated at ear level with the listening position. Generally, Half way between the woofer and tweeter is the acoustic axis of the speaker. It’s also a better idea to have the speakers on dedicated stands then on your desk. (this could be expanded upon)
From here we can adjust the speakers to finetune our soundstage. Toeing the speakers in can reduce the amount of reflections off the side wall and help make the center channel clearer.
Bringing the speakers closer together will reduce stereo elements for a more homogenous sound while pulling them further apart will make stereo elements more distinct.
Finding the ideal layout for your soundstage requires experimenting
with your speaker placement, but as you move your speakers around, the reflections from the front wall, side wall, and floor can cause acoustic issues known as Speaker boundary interference Response or SBIR. The trick to speaker placement is finding the ideal balance between crafting a great soundstage and minimizing the unwanted effects of SBIR. Treating around your speakers with thick bass traps can help minimize SBIR leaving you more room to focus on your soundstage.
If you’re using a subwoofer and both the sub and speakers have an adjustable crossover frequency, then you can reroute frequencies to your subwoofer that might be causing SBIR issues, or leave out specific frequencies that might be causing issues.
Where you place the subwoofer is obviously going to have a substantial effect on your bass response. Placing the subwoofer in the corner of the room will excite all three of the axial room modes which will result in a very strong but uneven bass response.
Placing the Sub one fourth of the way out from the wall will have the Subwoofer on a null which will keep it from exciting that room mode.
By using two subwoofers we can use phase cancellation to our advantage in order to achieve a flatter response.
Placing the subwoofers in opposite corners will cause the subs to excite the same room mode but one will be 180 degrees out of phase with the other, so the mode gets canceled out. There are a few different placement options that can take advantage of this effect. All of them make use the second subwoofer to cancel out the resonance of one or more room modes.
The same concept can be applied to a room using four or more subwoofers, but you can expect dimensioning returns the more subwoofers you add.
The combination of a fine tuned speaker placement and a comprehensive treatment strategy is the key to having a great sounding room. Good speaker placement can’t eliminate all of the issues in a room, but it can give you a better starting point to focus treatment where you really need it."