Dolby Atmos

What is Dolby Atmos?

Dolby Atmos is the cinema and home audio format Dolby introduced in 2012. Pixar's Brave (2012) was the first feature released with an Atmos soundtrack, and within a few years the format became the default premium mix for large studio releases.

The core idea is object-based audio. Earlier formats, from Dolby Stereo to 5.1 and 7.1, were channel-based: the mix was a fixed set of tracks, each one feeding a specific speaker or speaker group. Atmos keeps the sound and its position separate. A car pass-by is stored as audio plus metadata that says where it is at every moment, and the theater's processor decides which speakers play it.

That adds a dimension older systems lacked: height. Atmos rooms have speakers in the ceiling (or, at home, upward-firing drivers that bounce sound off it), so a sound can move over the audience, not just around it. For the wider picture of how sound shapes a film, see the sound in film hub.

How does object-based audio work?

An Atmos mix has two layers:

• The bed — a channel-based foundation, similar to a classic 7.1 mix plus overhead channels. Ambiences, most of the music and broad effects live here.
• Objects — individual sounds with position metadata (left–right, front–back, up–down, plus size). A fly circling a character's head, a bullet ricochet, a plane crossing the ceiling.

In cinema, an Atmos program can carry up to 128 simultaneous tracks of beds and objects, and a theater processor can drive up to 64 individual speaker feeds. Every surround speaker can be addressed on its own, so a sound can travel down a side wall speaker by speaker.

The renderer is what makes it scale. The same master plays in a large Atmos auditorium, a 7.1.4 living room, a soundbar or headphones; each device renders the objects to its own speakers, or to a binaural signal for headphones. Every consumer version is derived from one master.

Why filmmakers mix in Atmos

Atmos is a tool for space and point of view, not for loudness. The height layer is best at things the audience would naturally hear above them: weather, aircraft, a creaking ceiling, birds in a canopy, the reverb of a cathedral or a cave.

Gravity (2013) is the textbook case: with no air in space to carry sound, the mix follows the characters' perspective and moves voices and music around the audience as the camera drifts, making disorientation physical.

The discipline is restraint. Dialogue almost always stays in the center channel so it stays locked to the faces on screen; a voice that wanders into the ceiling breaks the illusion. Many of the best Atmos mixes keep the overhead layer quiet for long stretches, so the moment a sound finally passes over the audience it lands as an event. The raw material is the same as in any mix — Foley, effects and the film score — split into stems that can be placed individually.

Atmos is mostly about diegetic sound, the world of the story. Score can bloom into the heights, but pinning instruments to points in the ceiling pulls attention from the picture.

Dolby Atmos at home: 5.1.2, 7.1.4 and soundbars

Home Atmos layouts are written as three numbers. In 7.1.4, 7 is the ear-level speakers, 1 is the subwoofer and 4 is the height speakers. A 5.1.2 system has five ear-level speakers, a sub and two heights. The mix does not change between them; the receiver renders the same objects to whatever is connected.

Height can come from ceiling speakers (the most accurate), from Atmos-enabled modules that fire upward and bounce off a flat ceiling, or from a soundbar that combines upward and side-firing drivers. Streaming services deliver Atmos inside a compressed stream, while Blu-ray discs usually carry it in a lossless track.

For filmmakers this matters at delivery: a home Atmos mix has to be checked on small systems too, not only on the stage.

How to plan a film for an Atmos mix

Atmos is finished on a mixing stage, but good Atmos starts in prep. The sound mixing team can only move sounds that exist as clean, separate elements, so:

• Record space. Capture overhead and wide ambiences on location — rain on a roof, wind in trees, a warehouse's roof creaks — as separate tracks, not only the dialogue.
• Mark height moments in the script and boards. A helicopter over the characters, a storm, footsteps on the floor above: note them where the story needs them, so picture and sound can support each other (a low angle looking up gives the ear a reason to go up).
• Budget the room. Atmos needs a stage with certified Atmos monitoring; home mixes are commonly done in a 7.1.4 nearfield room.

In FlashBoards you can keep this plan visible next to the picture: put the storyboard frames or generated shots for a sequence on the board and add sound notes beside them — which shot carries an overhead pass, where the room should feel closed in — so the director, editor and mixer read the same map.

What was Dolby Stereo?

Dolby Stereo, launched in the mid-1970s, is the format Atmos descends from. It fit four channels — left, center, right and a mono surround — into the two optical soundtrack stripes of a 35mm print. The four channels were matrix-encoded into two tracks, and a decoder in the theater unfolded them again; Dolby noise reduction cleaned up the hiss that had made optical sound dull.

Star Wars (1977) made Dolby Stereo the sound audiences expected, and within a few years surround had become standard in cinemas. The same matrix later reached homes as Dolby Surround and Pro Logic.

Digital 5.1 in the 1990s and 7.1 in 2010 added discrete channels; Atmos changed the model by storing positions instead of channels. Alongside large-format picture such as IMAX and 3D film, it is one of the premium formats that bring audiences to theaters for the experience.

Further reading

Tomlinson Holman, Sound for Film and Television (Focal Press). Dolby Laboratories technical documentation on Dolby Atmos for cinema and home entertainment.