Long wall placement has a LOT of advantages so long as you can maintain a decent distance between you / the speakers / back wall. Here's the first and primary reason why this is true.
When you have your speakers placed on the short wall, nothing you can do can increase the distance from the speaker to the side wall. Since there is a short path to the side wall from the speaker, you end up with more reflections coming back to you at a higher intensity that are closer in arrival time to that of the primary wave. Do you want to fathom a guess as to what this does to frequency response and the soundstage / imaging ?
Placing the speakers further from the side wall, you have a longer path to the point of reflection. Not only have the intensity of these signals been reduced, but they are now at a much softer angle. In effect, the reflections are reduced in quantity, intensity and less concentrated towards your seated listening position. On top of this, the arrival times from these reflections are spread much further apart from the primary signals that you've heard, minimizing their effects.
If you really want to get into it, short wall / long wall placements require different types of room treatments in terms of diffraction and / or absorption. To take that a step further and really get into specifics, this will vary according to the radiation pattern of the speakers too. This is why, and i've stated this MANY, MANY times, there are no "universal formulas" that work when it comes to the room acoustics / speaker placement for an individual room / installation. The thing that comes closest in terms of "universal recommendations for speaker placement" is the program called "Computer Aided Room Acoustics" aka " CARA", but even that is only as good as the specific data that you program into into it. Sean
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PS... I've been an advocate of long wall placement for quite some time, long before i understood room acoustics. I followed what my ears told me and theories that i learned later verified what i was hearing. As a side note, long wall placement is not as critical with smaller speakers, especially those that have acoustic treatments on the baffle.