Placement des enceintes dans une pièce rectangulaire

In a room that is longer than it is wide, it's better to place the speakers near the shorter wall so they fire the long way into the room as shown on the left in Figure 11 below. This puts you farther from the rear wall where the low frequency peaks and nulls are most severe.

Ideally we would like to try out different listening and monitor positions in an empty room, usually while playing a familiar mix and test tones. It takes three people moving about – two holding the monitors, the other listens. Despite the time and technical issues involved in connecting the monitors and moving them around, the resultant benefit from correct positioning could be priceless.
Usually there is only one dimension in question. Rarely we have a choice as for how high we sit, which consequently determines the height of the monitors. Since left/right symmetry results in more accurate stereo image, the listening position is often halfway on one dimension. Unfortunately, halfway between any two walls is often where profound frequency imbalance is caused by the fundamental room mode. One thing we can do is to experiment to see whether it is the length or the width of a rectangular room that gives us less problems. Since the height is fixed, and on the left/right axis we usually sit in the center, all we have to experiment with is the front/back movement of our listening position and our speakers. Some suggest that the ideal listening position might be one-third way on the dimension. Nothing, however, beats experimentation.