Acoustics is all about managing reflections of sound
Acoustic problems are caused by sound reverberation in the room. Absorptive materials reduce the amount of sound that ricochets from one hard surface to another. The effectiveness of an absorptive material is measured by the Noise Reduction Coefficient (NRC).
An important point to note is that some reflections are actually important. Reflections aren’t all bad. If you place your speakers outdoors, you’ll find that the sound becomes immediately thin and loses all it’s bass. This is because there is no reflection outdoors.
Overabsorption of sound can make a room sound dead. A great solution for these areas is diffusion. A diffuser reflects sound in random, multiple directions, spreading acoustic energy around. Again, the key to great home theater acoustics is balance.