Sound above 400Hz is considered to be directional. In fact, the higher the frequency, the more directional the sound waves tend to be. In a home theatre or 2-channel listening room, these waves bounce off the nearby walls and produce very noticeable second order reflections which compete with the direct sound from the loudspeakers.
You can easily control these reflections by strategically placing acoustic panels along the side walls to absorb them. As a rule, you should mount them at ear and/or speaker height and position them along the wall to capture the early reflections from the loudspeakers.