It’s a tense moment during a suspense thriller. A cannibalistic serial killer has escaped from a maximum-security detention unit and eluded capture for long enough to work up a healthy appetite. Two small children are playing hide-and-seek in an overgrown lot behind their home.

They have forgotten mother’s admonition to stay indoors, but we haven’t. One is hiding in a dense shrub, holding his breath in case the other is nearby. It’s dead—quiet except for some distant crows. Off-screen somewhere, and very near, a twig snaps, and we jump out of our seat. Then we hear “I see you!” and the adrenalin stops flowing—for the time being.

Long before movies, surround sound was saving our ancestors from saber-tooth tigers, giving them a second set of “eyes” with which they could not only watch their tails, but tell where any threat was coming from. Today, our ability to hear sounds coming from beside and behind us protects us from such contemporary hazards as approaching trucks and stalking hoodlums. Welcome to the world of surround sound!

Learning to listen to acoustical space is a normal part of growing up. By the time we are only a few months old we have already learned to discern the direction from which sounds are coming. Later, we take it for granted that every third-dimensional space has its own characteristic “surround sound.” Only when that sound is unfamiliar or inappropriate or threatening do we think about it at all.

The Room Has a Voice of Its Own
Have you ever found yourself in a plush, upholstered room with heavy drapes, thick wall-to-wall carpeting, and lots of overstuffed furniture, and noticed how “funny” your voice sounds to you? That’s because, when you’re speaking indoors, you expeqt to hear the room you’re in as well as the sound of your voice in it. When you don’t “hear the room,” you notice it.

In most enclosed spaces, we expect to hear sounds from two sources: directly, in a straight line from the source to our ears, and indirectly, a fraction of a second later, as reflections from the walls, floor, and ceiling. The reflections are usually much quieter than the direct sounds, but we are aware of them, even though we may not realize it. When they aren’t there, we know it immediately. That’s why your voice sounds unfamiliar in a very dead room. You hear it from your mouth and, by bone conduction, from your vocal cords. But you don’t hear the room sound that usually accompanies it. Outdoors, we often hear reflections coming from nearby buildings. Think about the sound of footsteps in an alley between two buildings. In wide-open space, with nothing to bounce sounds back to us, our ears tell us we’re in open space. There, all we hear are direct sounds, from birds, dogs, vehicles, and passing airplanes. The nature of the sonic space around us helps us orient ourselves to our environment. It’s no wonder our senses rebel when halfofthat space is missing.

Sure, stereo through two channels is nice, and it can sound a lot more realistic than the single-channel “mono” sound reproduction our grandparents grew up with. But no one is ever fooled into thinking stereo is the real thing. That’s because our ears tell us that the half of a real space that is normally outside our visual field is missing from stereo.

This is particularly true with musical sounds. If you attend live concerts often, you’re accustomed to hearing your favorite performing group in an auditorium or stadium, which may have a long and very pronounced reverberation that comes back at you from all directions. A good recording of that group will have an appropriate amount of its natural room sound mixed into it, but stereo can’t put it where it belongs. It all comes from up front, between the loudspeakers. Yes, you do hear the sound of your listening room too, but that’s the wrong room sound; it isn’t the sound of the performing hall. So the reproduction comes across as being not quite right. The only way to reproduce room sound the way we expect to hear it is with a surround system.

Surround Sound is a Long-Running Hit
Surround reproduction is certainly nothing new. Bell Telephone Labs was playing with it 60 years ago, using an array of microphones feeding an array of loudspeakers in another room, and they reported the startling realism it could deliver. And it’s been a feature attraction in your nearest theater for the last 20 years. But only recently have we been able to enjoy it at home. We can thank Hollywood for that.

During the 1930s and ’40s, people who had been going to the movies every week were spending more and more time at home, listening to radio and records, and Hollywood wanted to lure people back to the theaters by giving them something they couldn’t get at home. They were half-heartedly considering multi-channel sound when Walt Disney Studios preempted them all in 1941 with a revolutionary full-length animated film, Fantasia, that featured symphonic music reproduced through four sound channels. It wasn’t true surround sound, though, because most of the directional effects were done by switching one or another of three sound tracks to one or another array of loudspeakers. All the front loudspeakers were paralleled, so there was no real stereo at all.

The first true multi-channel sound system accompanied something called “Cinerama” (1952), which used three side-by-side projected images to produce a huge, wrap-around picture. Six soundtracks on a magnetically coated film were synchronized with the optical projectors to provide three stereo channels up front and three separate surround channels at the sides and rear of the theater. It wowed audiences, but it never caught on because the cost of converting an average theater for Cinerama was outrageous, and most of them weren’t big enough to accommodate it anyway.

During the 1950s and ’60s, Hollywood introduced several widescreen formats—70mm, Cinemascope, Ultra-Panavision, Todd-AO, and so on—and many large theaters could accommodate these. All the widescreen systems used at least four discrete sound channels (three in front and one for the surrounds), and several used six. All depended on synchronized magnetic tape for the sound. The home audio consumer hadn’t been left out completely; he got two-channel stereo.

A Wiggle Plus a Waggle Gave Us Stereo
The LP had been around since 1948, and like the 78-rpm discs before it, it used what was called lateral modulation: The sound vibrations wiggled the groove from side to side. Radio broadcasts, on the other hand, used vertical modulation, because the grooves could be spaced closer together to cram more playing time on each disc side. Then it occurred to some bright soul that, since lateral modulation didn’t move the groove vertically at all, and vertical modulation didn’t move it from side to side at all, why not use each modulation “axis” to record one channel of a stereo signal?

In its final incarnation, the stereo LP used neither vertical nor lateral modulation. It was found that the channels could be made to match one another better if their modulation axes were symmetrical, so both were simply rotated by 45 degrees so that one wiggled diagonally up and down to the left of the groove path and the other to the right. The stereo LP was debuted in 1958.

During the ’70s, a young engineer named Ray Dolby perfected a system for encoding a surround channel into two optical tracks on 35mm film, making it cheaper than ever for small theaters to buy into surround sound. It also made it possible to record a surround channel on stereo LPs, and the record industry unleashed “Quadraphonic Sound” amidst much fanfare and foofaraw. Music lovers didn’t care and audiophiles didn’t like the fact that all the decoders sounded lousy. It was a spectacular failure.

It took home video to make surround sound respectable. When movie video cassettes went stereo, the encoded 35mm soundtracks were transferred directly to tape with their surround signal intact, but it was still some years before the publice knew what was buried there. When consumer surround decoders made it possible to recover the encoded signal, the home theater revolution was on its way.

Dolby’s Got You Surrounded
Today, almost every movie on cassette and laserdisc has Dolby Surround on it, and people who aren’t yet equipped to reproduce it are missing half the fun. With surround sound, you don’t sit outside the world of movie sound; you’re immersed in it. A voice, the ominous snap of a twig, the opening of a door, can come from behind you just as it does in the theater. Airplanes and spaceships thunder through the room and over your shoulder, and you can find yourself right in the middle of a noisy crowd or a wet, cold downpour.

The speakers needed to create surround, or “ambient,” effects aren’t space-hungry. They can be much smaller than the front speakers, and may be mounted high on the walls or from the ceiling. Some can even be located right next to your favorite seat without calling attention to themselves. As long as they aren’t in front of you, the surrounds will do what they’re supposed to do: wrap you in sound.

The thing that makes all this possible is something called matrixing. When a Dolby Stereo film track is mixed at the stu dio, the surround information is subtracted from the front-channel left and right signals, leaving a “hole” in the L and R signals. In playback, both stereo signals are split into two identical L/R pairs. One pair is fed directly to the front stereo speakers. The signals from the other pair are subtracted from each other, and what’s left over is the information that was originally used to dig the hole—the surround signal. (This is an over simplification. For a more detailed explanation, see the adjoining sidebar, “The Mathematics of Dolby’s Mono Surround Sound.”)