somebody, carp(?), asked about ideal speaker placement in room. and then also asked about hearing that speakers should be some distance from the rear wall.
if the speakers are placed 1/4 distance from the front of the room and 1/4 distance to the wall, in a rectangular room, such placement will cancel out many room resonances aka eigenmodes aka room modes.
for what is a mode: http://www.hunecke.de/en/knowledge/room-acoustics/room-eigenmodes.html
for more discussion: http://www.harman.com/EN-US/OurCompany/Innovation/Documents/White%20Papers/multsubs.pdf
so, in some sense yes, there is an ideal placement for speakers in a room. now, why should they be forward of the front wall.
through some combination of the haas effect and the precedence effect reflections less than about 1ms are get grouped together tightly, from 1 ms up to about 15ms or so depending on content get grouped together loosely, and from about 15ms or so up are heard as reflections.
what this means is that if two speakers are in a field and playing some music, the imaging will be super tight as there are no reflections. all instruments will appear to be the right size and placed in space properly.
if you have reflection, such as a speaker near a wall and it is with about 3 feet or so, the arrival time delay is on the order of 4-5 ms of delay from the first wavefront. you would think that you would hear these two sounds as independent clicks, but what happens is your brain combines them into a single sound having a size roughly equal to the distance between the wall and the speaker. so instruments are no longer tight, but larger and fuzzy. imaging goes way down. things just don't seem right. kind of like looking through a camera lens that is out of focus.
put the speakers in a baffle wall and all reflections are killed. imaging is tight. very similar to being in a field. that is why it works so well.
but, if you have to have your speakers out in the room, move them VERY much forward so that the distance from the speaker to the rear wall or the side walls is greater than about 8 feet and the reflections will be heard by your brain as echoes, not combined with the first wavefront into a big foggy image, but a small tight focused image with an echo behind it.
the Allison paper even mentions first best approach is in the baffle wall. cam man mentions that this was thx spec from the beginning and for good reason.
anyway, several different things going on.
Accurate sound reproduction from two loudspeakers in a living room: http://www.youtube.com/watch?v=VC-sxvNzC8I
by linkwitz may be one of the best introductions to some of the key issues that I have ever seen. I have watched it at least 5 times and pick up some nuance of something every time. it is REALLY rich and is very easy to miss big points the first couple times through.Edited by LTD02 - 10/16/13 at 2:35pm