When you walk into a movie theater, you usually see 12 to 16 speakers hanging on the walls. In addition, there are typically four systems behind the screen and hidden from view. What’s up with that? How do we get today’s 5.1 channels distributed to them and what’s changing?

The movies pioneered multichannel sound. Fantasia in 1940 had five loudspeaker channels^[i], and that was the first introduction of surround sound to the public.  But even in 1940, the engineers found that if they played the two surround channels out of just two loudspeakers, those sitting nearby would get creamed with level, while those sitting closer to the front hardly heard them in a mix. So the idea of an array of loudspeakers to play the surrounds has old roots. In fact, the two theaters equipped for Fantasound used many loudspeakers to reproduce the two surround channels.

So the idea that the number of loudspeakers you see represents the number of channels is wrong; many may be driven by one input signal. The most common configuration today is 5.1, where there are three main channels, left, center, and right, behind the screen, along with one or more subwoofers. So that’s 3.1 of the channels counted. The other two channels drive the left and right halves of the array of loudspeakers in the room, with the back wall speakers split between left and right.

Several years ago, pressure to get sound literally to come from behind you led sound designer (and multiple Oscar^®[ii] winner) Gary Rydstrom to ask for a separate rear channel^[iii]. The problem for cinema is that if you sit in the middle of the house and sound pans from the fronts to the surrounds, it only seems to come from your sides, because those are closer to you than those on the back wall. So it’s not really surround. By having a separate back surround, the effect can be overcome. Commercial implementations of this are Dolby Surround EX and dts Surround ES in theaters. These might be called “quasi 6.1” systems, because there is not perfect separation of back from sides, but usually good enough for the effect to work.^{[iv] [v] (more…)}

My first “beyond 5.1″ experience was in January 2000 at the Alexis Park Hotel in Las Vegas. My colleague from the USC Immersive Audio Lab, Tomlinson Holman, was in the small room next door explaining why more channels are needed. This was followed by a demonstration of the world’s first 10.2 system to a few members of the press.

The world of sound had come a long way from the introduction of the first stereo LP in 1957. But in fact, with 10.2 we were moving back in time. It was in the late 30s that researchers at Bell Labs had looked into the question of how many loudspeakers are needed to reproduce a seamless, realistic sound scene. The answer at the time was discouraging: it turned out to be an infinite number–not exactly a high SAF (spouse approval factor) idea. Two channel stereo, however, was the only way to deliver sound to the consumer and it was limited by the LP groove that had two sides. Certainly better than mono, it didn’t come close to delivering an immersive experience.

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