It is a technique to mix motion picture sound tracks. The front left and right carry much of the stereo music, the center channel is weighted toward dialog, and the rears mimic reverb found in concert halls of the front stereo pair. The creativity of the man mixing the sound can place dialog anywhere, Foley (sound effects), music, live sound, and ambient noise are often purposely mixed into the audio file. The .1 is a sub woofer, but it is not really for low frequency music as many would believe. A good set of front stereo speakers can cover to 40 Hz for nice bass response. The cinema .1 is for LFE (low frequency effects.) These are the room shaking frequencies from explosions, fast trains, helicopters over head, earthquakes, and your mother-in-law strutting into the room.
In most productions, sound effects like people walking, horses running, doors opening, and the like are not recorded live. The recordings of real objects are not what our ears want to hear for the visual cues from the screen. People called Foley walkers use all kinds of creative techniques to make recorded sounds that our ears and brain think are natural. Most of the talking in pictures are not from the sound recorded on the set. A system called ADR (automatic dialog replacement) is used for that. During post production the actor sits in a small sound controlled booth (like a telephone box if you are old enough to remember) and watches a film (really a video these days) of his performance and hears the original sound on his cans (headphones.) He then repeats the words to match his lips, and it is that studio recorded sound that is mixed into the show reel for distribution. 7.1 is the same thing, but there are two additional discrete channels. Most cinemas to day have a Dolby and/or DTS decoder and 8 channels of discrete amplification. The size of the house (number of seats and volume of the space) will determine the size of the amps and the speaker boxes behind the screen, on the walls and at the rear. A small screening room with less than 8 seats might do well with 100 watts (real watts) per channel, whilst a large genuine IMAX theater can have a 15,000 watt sound system. If you have a chance to go to the most attended IMAX theater in the world, Ramesh Prasads theater in Hyderabad has a 12,000 watt sound system. It is the best in Asia as far as I am concerned from a cleanliness and audience experience point of view. It is the worlds largest 3D theater. See a film there, and you will know what 6 channel surround sounds like.
