Other sound sources will interact too, making lobes and nulls of their own. The issue is essentially one of path length. Where path lengths between sound sources are equal, sound combines constructively. As you move in a direction that makes one sound source closer than the other, you eventually may cross a point where conbination is destructive, forming a null. The wave interaction is constructive as long as the difference is less than 1/4λ, but once it nears 1/2λ, the interaction becomes destructive. So in the room, there are pockets of energy lobes and some dead spots, called nulls. The position of these lobes and nulls is set by driver positions and phase.