In your A/V receiver’s setup menu enter the distance between the best (center seat) listening position and each surround speaker. If each surround speaker is not equal distant to the main listener you can enter different values for each surround speaker. The surround decoder (DTS or Dolby) in the receiver will then automatically insert the appropriate degree of delay so that the sound from the surround channels reach your ears after the sound from the center channel and the main speakers.
The reason you must set this correctly involves compensation for a peculiarity of human hearing–our tendency to locate a close-up sound before one that’s a little farther away. For multi-channel surround sound, the illusion only works if the sounds from nearby surround speakers reach your ears after the sound from the main front speakers.
Our precise stereophonic hearing acuity results from near-instantaneous calculations performed by our brain when it measures the difference in timing of sounds that reach one ear and the length of time it takes the same sound to round our head and reach the other ear. Called “inter-aural time differences” (IAT), these immediately tell us which direction a sound comes from. Inter-aural amplitude (loudness) differences combine with the timing differences to give us our finely-tuned multi-directional hearing ability.
If you think for a minute about virtually any installation of main front speakers and side- or rear-located surround speakers, you’ll realize that in most rooms, the surround or rear speakers are often closer to the listening area than the front speakers.
By our laws of directional hearing (sound travels about 1 foot per millisecond), it’s apparent that any sounds coming from loudspeakers close to the listener will be perceived before the sound from speakers farther away–the main speakers at the front.
If a surround sound setup is to perform convincingly, the sounds emanating from the side or rear speakers must be delayed by 10, 15 or 20 milliseconds or more, so they arrive after the sound from the main speakers reaches our ears, otherwise your brain will tell you that the sounds are coming first from the surrounds and then from the main speakers and the whole illusion will collapse. In mixing Dolby Digital 5.1 movie soundtracks, there are times when the director intentionally wants you to hear the sound from, say, the right rear speakers first, as in a plane or spaceship that is to fly diagonally over the room and appear on-screen as it passes above us and moves on.
While every receiver offers manual settings, some of the latest A/V receivers will include a calibration microphone to measure and configure these setting automatically by generating a series of test tones. Even with the automatic option it’s still a good idea to double check the results to make sure you’re home theater system can sound its best.
[Reference & Diagram: Axiom Audio Blog, Video: Pioneer]