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It’s time for a basic science lesson, but don’t worry, this one is about why some seats in your home theater room get better sound than others. Specifically, why the people in some seats might say your audio system has no bass while others have to hold on to keep from being shaken out of their seats during the action sequences of your favorite surround sound movies.
To explain this, we have to talk about something called a standing wave. Standing waves are what cause excessively high peaks and excessively low valleys for a given frequency in your home theater room. OK, I’ll back up even further. Sound is transmitted through the air in a series of high and low pressure waves. Visualize these sound pressure waves as the up and down waves on a body of water. The higher the wave’s peak, the louder that frequency sounds. If the wave has very little pressure behind it, it is nearly flat and sounds very quiet. These waves normally reduce in intensity over distance as the friction and loss associated with travelling through the air dissipates the wave. However, your home theater room is very small compared to the size of some of these waves.
A 50 Hz tone for example, measures roughly 20 feet long, and by the time you reach the lower end of the human hearing range, 20Hz, the wavelength is more than 56 feet long. This means that a single wave at 20Hz is rebounding off the back wall of your home theater room and colliding with itself. If the peak of the wave meets another peak (or a valley meets a valley) then the bass note at 20Hz is essentially added together to make the 20Hz note seem unnaturally loud at the point where these two parts of the wave cross.
Unfortunately, when a peak meets a valley, they subtract from each other’s height or amplitude making the pressure in that area closer to a flat line. These are the locations where little or no bass will be perceived by the listener. It is much less of a problem for higher frequencies since higher frequency sound waves are much shorter. At 6000Hz, for example, the wavelength measures roughly two and one eighth inches. So standing wave interference is typically a problem only for bass or lower frequency notes.
Standing waves can occur between the loudspeaker and any reflective surface in the room, or indeed, between any two reflective surfaces such as the ceiling and floor, but don’t worry, if your home theater room is experiencing a standing wave problem, all is not lost. You can easily add bass traps, or absorptive panels on some of the reflective surfaces to reduce the intensity of the reflections and thus the standing wave problem. You can purchase bass traps from a top quality home theater store or online from such places as ReadyAcoustics.com. Often these sources will provide some guidance about where to place them in your room, or you can use the tried and true, trial and error method. Keep trying the panels in different spots until the bass level is consistent throughout the listening area.