Don't forget the reflection points from the rear wall. In a wide room, these can be more intrusive at the listening position than the reflections off the side walls.
As a rule of thumb, if the distance the sound travels from the speaker to the reflecting wall and on to the listener, is more than 5 ft. longer than the distance traveled directly from the speaker to the listener, the ear will be able to distinguish this reflected sound in time and not confuse it with the image. That is why it is important to get speakers away from side and rear walls if possible.
You can see that if a speaker is more than 2 1/2 ft. off any wall, this criteria is always met. Still, the clarity of the image will always benefit with some absorption in the room, and putting it at the reflection points is a good place to start. Experiment.
As a rule of thumb, if the distance the sound travels from the speaker to the reflecting wall and on to the listener, is more than 5 ft. longer than the distance traveled directly from the speaker to the listener, the ear will be able to distinguish this reflected sound in time and not confuse it with the image. That is why it is important to get speakers away from side and rear walls if possible.
You can see that if a speaker is more than 2 1/2 ft. off any wall, this criteria is always met. Still, the clarity of the image will always benefit with some absorption in the room, and putting it at the reflection points is a good place to start. Experiment.