Loudspeaker placement and the room's dimensions will cause certain frequencies within a room to resonate. Resonance is when an object vibrates at a particular frequency. An object's size and what it is made of determines the frequency. The puddle of water, in the movie Jurassic Park, vibrating when the T-Rex comes near , is a good example of a resonance taking place. When we play music in our listening room, the air in the room will resonate at certain frequencies. The resonate frequencies determined by the room's dimensions and the materials of build.
These resonances are called modes. They can affect our systems' sound by increasing or decreasing certain frequencies. I'm not going to get into how to control theses modes in this article. That will appear in a future HI-Fi Highlights article. Instead I'm going to show you how to calculate these modes. This will be useful for determining what actions to take to help the problem or to help you with selecting/building a listening room.
The distance between the room's walls will determine the resonant frequencies.