When you walk into an IMAX theater, it is clear that you are in for anuncommon cinematic experience. The steep rake of the seating and the hugedomed screen-typically 100-feet wide and 60 to 70-feet high-promise animmersive, "middle-of-the-action" feel for films such as MacGillivrayFreeman's 1998 film Everest. But what you don't see is the audio systemdesigned to deliver a sound stage big enough to complement the huge picture.

Birmingham, Alabama-based Sonics Associates, a subsidiary of Toronto's IMAXCorporation that designs and builds sound systems for IMAX theaters,creates the sonic canvas. "The director and sound designer should be ableto choose the ambiance they want to create without the room or our systeminterfering," says Lynn McCroskey, president and CEO.

Achieving this goal starts with close attention to the physical layout andacoustics of the theaters. "IMAX theaters must meet a NC-25 Noise Criteriarating," McCroskey says, "compared to NC-35 in typical theaters wherebackground noise from air conditioning or traffic might be 10dB higher. Wealso require 0.8-second mid-band reverberation time, which is very lowconsidering the cubic volume of our theaters."

The Sonics playback systems use six discrete channels: left, center, right,left rear, right rear, and top center. "Material below 80Hz from each ofthese channels is rolled out of the main speakers and summed into oursub-bass system," explains McCroskey. The company's latest formatspecification also allows for a discrete low-frequency channel, effectivelymaking it a "6.1" channel system. But McCroskey says, "Most IMAX producersare not yet working in that format."

The playback system differs in several respects from surround systems inconventional theaters. Perhaps the most unusual element is the top-centerspeaker. "The typical IMAX screen," McCroskey explains, "is close to aconventional 4:3 aspect ratio, but much, much bigger. So you have a greatdeal of vertical, which gives you the opportunity to do a 'voice-of-God'loudspeaker." Another difference is the use of point-source surround, asopposed to the multiple small surround speakers used in conventionaltheaters. "Conventional rooms," McCroskey says, "come in so many differentshapes that it is nearly impossible for them to make point-source surroundwork."

Overall IMAX system power varies depending on the size of the room, but itis typically in the range of 12,500 watts. "The power is not there for theloudness," McCroskey says. "It's there for clarity and freedom fromdistortion." The enclosures are three-way systems using componentscustom-designed and manufactured to Sonics' specifications. Sonics combinesfour low-frequency loudspeakers in each cabinet with nested high- andmid-frequency horns. McCroskey points out the trapezoidal dispersionpattern (narrower at the top than the bottom), designed to match thedistinctive shape of IMAX theaters.

Using a sub-bass system for the deepest lows, McCroskey says, minimizesphase coherence problems. "In most installations, we use eight sub-bassloudspeakers, each in a 16-cubic-foot enclosure," he says. "The enclosuresinclude a filtering labyrinth we designed that physically traps thehigher-frequency components that can otherwise cause overtones anddistortion."

Another distinction between IMAX and other theater surround systems is thatSonics uses no digital audio data compression. Both the DDP and DTAC linesare full fidelity, "double-system" approaches, meaning that the sound isnot recorded on the film itself. "DDP uses three CD-Audio discs with apatented sample-accurate playback synchronization system," McCroskey says.DTAC, the company's newest system, plays back audio files either fromDVD-ROM or from a built-in hard disk. In some older IMAX theaters, theoriginal 35mm six-track, full-coat mag-sound system used from 1971 through1988 is still in place.

"These days most soundtracks are produced in digital formats," McCroskeysays. "They are usually sent in on TASCAM DA-88, and we transfer towhatever format is needed for the theaters where the film will play. OnEverest, for instance, we created both DDP and DTAC discs."

The Everest ExperienceWhen it came time to mix Everest, the tightly controlled audioimplementation in IMAX theaters offered mixers a reliable platform to aimfor. But how does the environment affect the actual finished mix?

Ken Teaney, chief re-recording mixer for Los Angeles-based EFX Systems andWilshire Stages, mixed Everest with colleague Marshall Garlington. Heexplains: "One of the things I exploit heavily compared to 35mm is thatIMAX surrounds are full-frequency speakers-big with a lot of low end. Thatmakes it easier to use interesting panning and changes of perspective tosupport the sense of motion you get from that large picture.

"In Everest, for example, sound comes from all around you in the interiorhelicopter shots, but when you cut to the copter going up the canyon, thesound is distant in the center speaker. We try to do that kind of thing for35mm films as well, but most surround speakers can't reproduce the low end,so the sound gets thinner as it moves from front to rear. With IMAX, thesound doesn't change at all, just the placement."

As in conventional mixing, Teaney focused dialog in the center. "Most IMAXpictures are driven by narration and voice-over," Teaney says. "To supportthe story, we place narration and voice-over in the center and also about10dB down in the left and right. Synch dialog, however, we keep only in thecenter."

Meanwhile, mixers kept the music out of the dialog's way. "In Everest, themusic is only in the four corner speakers," Teaney explains. "That actuallymakes it bigger spatially with a wider stereo image. Steve Wood, who workedwith Daniel May to orchestrate and mix George Harrison's compositions, usedthis format very effectively in places like the storm scene where there isa Tibetan bell that swirls around the room. It's a perfect complement towhat we did with effects to put the audience in the middle of all that windand ice."

As for effects placement, Teaney generally uses left and right instead ofthe center. "But where the action is center-screen, so are the effects, orelse the illusion is gone," he notes.

Teaney uses the top-center speaker sparingly. "There's one scene where aclimber is belaying another climber; you don't see him, but you hear himabove her," he says. "In another scene, the avalanche starts in the topspeaker and pans its way down. But that channel is really only useful in afew specific instances."

Everest was the first IMAX film where the final mix was done on location inan IMAX theater, the Irvine Spectrum. That meant the mixers did not have tomake several rounds of notes while watching the film and then implementchanges back at the EFX dub stage. But they did have to set up facilitiesin a working theater that was not available until 10 at night (after theday's last screening). They worked from a 24-track premix made at EFX withsix tracks each for effects, backgrounds, and music; three for Foley; andone each for narration, voice-overs, and dialog. EFX made a discrete premixto 24 tracks: six each for effects, backgrounds, and music; three forFoley; and one each for narration, voice-overs, and dialog.

Teaney and Garlington noticed some interesting differences between thesound on site and the sound on their dub stage. "The left and rightspeakers were so much further out," Teaney recalls, "that we decided wecould push both the background effects and the music up a bit withoutinterfering with the narration. That was great because the higherbackground levels of wind swirling around you really make it more of anexperience."

During the mix, picture was video-projected (IMAX projectors only rollforward). But at five in the morning when the mix seemed right, sound wassynched to the IMAX print for a check pass. "It was pretty exciting tofinally hear and see it all together," Teaney says. "Any fatigue we had upuntil that moment was gone the minute that big picture began to play."