The Sound Mixer’s On-Set Responsibility
The role of the production sound mixer on set is a demanding one that blends technical skill with artistry. Their primary responsibility is to capture clean, intelligible dialogue. To do this, they employ multiple microphones simultaneously—typically a boom microphone positioned over the actors and individual lavalier microphones hidden on each actor’s body. The production mix, also known as the ‘mix track’, is the real-time blend of these sources that the mixer creates live as the scene unfolds.
Using a portable mixer/recorder, they listen intently through headphones and adjust the levels of each microphone. They might favor the rich, natural sound of the boom mic as the primary source, but skillfully mix in a small amount of an actor’s lavalier mic if they turn their head away from the boom. This live mixing is a performance in itself, aiming to create the best possible representation of the scene’s audio in the moment. This track is what the director and script supervisor listen to on their headphones during the take.
The Mix Track and ISO Tracks
Modern digital field recorders offer a powerful feature: multitrack recording. This means that while the mixer is creating the live production mix, the recorder is also capturing each individual microphone’s signal onto its own separate, isolated track. These are known as ‘ISO tracks’.
A typical audio file delivered from a scene with two actors might be structured as follows:
| Track Number | Content | Description |
|---|---|---|
| Track 1 | Production Mix | The live mix created by the mixer on set. |
| Track 2 | Boom Mic (ISO) | The isolated recording of only the boom microphone. |
| Track 3 | Actor 1 Lav (ISO) | The isolated recording of only Actor 1’s lav mic. |
| Track 4 | Actor 2 Lav (ISO) | The isolated recording of only Actor 2’s lav mic. |
This provides the best of both worlds. The production mix is immediately available for dailies and for the picture editor, while the ISO tracks provide the ultimate flexibility for the audio post-production team.
Use in Post-Production
The production mix serves as the primary audio guide for the picture editor. It’s a single track that’s easy to manage and gives a clear representation of the dialogue. However, once the picture edit is locked and the project moves to the sound department, the dialogue editor will almost always disregard the production mix and work directly from the ISO tracks.
Working with the ISOs allows the editor to have complete control. If the boom mic sounds best for a line but was slightly obscured by a noise, the editor can seamlessly switch to the actor’s lavalier mic for that single word. They can pick and choose the absolute best audio source for every syllable, a level of control that would be impossible if they only had the baked-in production mix. The production mix is an excellent on-set guide, but the ISO tracks are the key to a pristine final soundtrack.