Acoustics is All About Minimizing Reflections
Getting great acoustics comes down to one simple concept: controlling reflections. As you probably know, sound covers a pretty broad spectrum from the lowest low bass rumbles of an explosion to that sparkling high hat or tinkling of glass. The critical human voice falls in the middle.
As the low bass tone from your speakers or subwoofer(s) bounce around the listening room, depending on the room’s dimensions, those tones will build up in certain areas of the room and cause two problems. Some low tones will be totally canceled out by the reflections and you’ll hear very little bass, yet other tones will get more pronounced and overwhelm other frequencies. These are called modes.
Rooms with equal dimensions tend to have more problematic modes. Also, as a general rule of thumb, the smaller the room, the more issues you will have. Higher frequencies can also reflect, bouncing around your room degrading the sound quality and making it very difficult to understand the details in a movie.