The Sonic Foundation
An immersive audio mix, such as one for Dolby Atmos, is not composed entirely of dynamic ‘objects.’ Instead, it uses a hybrid approach that combines objects with a more traditional, channel-based foundation known as the ‘bed.’ The bed is essentially a fixed surround sound mix—typically 7.1.2 or 9.1.4—that contains all the non-directional, atmospheric, and diffuse sounds of a scene. This includes the musical score, room tone, weather elements like wind and rain, and indistinct background noise like the murmur of a crowd. The bed provides the stable, enveloping sonic environment of the scene. It’s the canvas upon which the more specific and dynamic audio objects are painted.
The Hybrid Approach: Efficiency and Control
The reason for this hybrid structure is both practical and creative. It would be inefficient and creatively unnecessary to treat every single sound element in a film as a separate object. A sound designer doesn’t need to animate the position of every individual bird in a forest scene. Instead, they can create a rich, immersive forest ambience and place it in the bed channels, which then plays back from all the surrounding speakers. This creates the feeling of being in the forest. Then, only the specific, narratively important sounds—such as a specific bird call that a character reacts to, or the sound of footsteps approaching—are treated as dynamic objects, panned with precision to draw the listener’s attention. The bed handles the general atmosphere, while objects handle the specific actions. This gives mixers the best of both worlds: the efficiency and control of channel-based mixing for broad strokes, and the precision of object-based mixing for the fine details.
Crafting the Bed
Creating the bed mix is a critical part of the sound design process. It’s where the overall mood and texture of a location are established. A scene set in a cavernous cathedral will have a bed filled with long, reverberant echoes, while a scene in a cramped submarine will have a bed characterized by the claustrophobic hum of machinery. The musical score is almost always mixed into the bed channels, allowing the composer’s work to envelop the audience in a traditional, powerful way. The final immersive soundtrack is a seamless fusion of the stable, atmospheric bed and the dynamic, attention-grabbing objects that move through it.