Understanding Mixing & Mastering in Post Production

One rarely ever thinks about the stages that a film goes through in order to make it to screen, and whilst we may not realise it, everything we watch and hear has been through an intense process of mixing and mastering in order for it to sound just right.

What is mixing? Mixing is essentially adjusting the balance between sound effects, dialogue and music, and the balance of the individual elements within those groups as well.

This process is even more involved when a mix is done in surround, because there are 5.1, 6.1, 7.1 and even 10.1 sound systems, (The numbers relate to the amount of channels in the soundtrack and how many speakers the system the tracks are mixed for, are supposed to provide.)

Which Person Does What?

Mixing is the most important first factor. What you’re doing here is making sure that everything is level and balanced. The audience should be able to clearly hear each element, and the balance between them should enhance the overall experience. Ambient sounds and music cannot be louder than the dialogue, the same goes for any FX or diegetic sound that need to be happening in the piece you’re watching.

There are various people or groups involved in this process.

Firstly, your on set sound recordist and boom swinger make sure that everyone that’s on screen can be heard. This is called location recording. This process involves lapel mics, hidden mics and boom mounted mics to get sound recordings from as many sources as possible.

Second are your post production sound engineers who take care of your ADR, as well as creating extra sound effects with either foley or with processes like sound design. When creating effects, a post production engineer can create their own sounds in a process called foley recording. This involves being creative, and thinking outside the box to recreate sounds that are needed, for example; to get the sound of a fire, you would not expect the studio engineer to build a fire and record it. Instead you could use a thin piece of wrapping paper and crinkle it lightly to create the illusion of fire.

Similarly, to create the sound of someone getting punched in the face, you wouldn’t expect someone to get punched in the face, so a foley artist would record the sound of someone punching a cushion. For this you need a foley studio space and lots of props.

Sound design is when you use audio engineering and production skills to modify recordings and sounds to create the desired effect. For example; A simple transposition of a male voice down an octave will make it sound like a monster from the underworld is speaking, or a reverse cymbal sound creating a whoosh effect to enhance a logo or fast cut.

The last option in sound design is to use pre-created and packaged sounds which can be purchased.

Next is the music – Your composer and music director will have tracks that work or take the lead directly from the director in order to make sure the music is just right.

It’s important to find the right balance of the music in the mix – a happy medium between the music not being overpowering, but still being loud enough that it has the desired effect.

Music that has been expertly mixed already might not need to be adjusted that much, but basic volume automations can make a big difference when it comes to the general momentum of the piece. This is often done manually, by the mixing engineer, drawing or recording automations to lower and raise the volume of the music when desired.

There is also and automated way that this can be done – which is side-chain compression using the dialogue audio as a trigger for the compressor to reduce the gain on the music, essentially ducking the music under the audio when someone speaks.

Once this is done, the last (and arguably one of the most important parts of the process) is the mastering engineers. In layman’s terms mastering is making a piece of audio louder, and in a consistent format for playback on multiple devices.

Mastering in music and mastering in film, whilst serving a similar purpose, are not the same process.

Mastering audio for visuals involves making sure that the audio is prepared for distribution.

OMF Files

OMF, which stands for ‘Open Media Format’, is a file format type (like .xls for instance) which media professionals use to share information across different platforms. Why this is important, is so that when a file is imported the user can see where cuts are taking place, and what automated effects have been created (although instruments and VST’s will have to be set). What it does, is create a summary of the project, so that any user can easily see what is happening and allows the mixing engineer to focus on mixing and not have to spend time on editing clips and timing.

It is therefore assumed that when a mixing engineer receives OMF files, the timing and the edit sync is already locked.

Location vs Studio Recording

Being on location means there is the possibility of noise, at times so much that the sounds recorded on set are unusable. This is when you bring in ADR (We did an in depth blog about that process and what it involves). Whilst you will be able to use the original dialogue as a guideline for your actor to use whilst recording, studio recorded sound will allow you to properly integrate the dialogue into the final mix.

During the ADR process, the original location recordings can be helpful to the actor as it will guide them with regards to timing and delivery. Interestingly, the voice actor is not always the same actor as the actor on set, which is where the original audio helps.

Music Placement and FX

A sting/sounder is a short few seconds of music which specifically convey a moment, somewhat like a musical punctuation point. These can either be added to the score, or in some cases (especially in horror film) they are used on their own to create greater effect.

Sound effects go through exactly the same kind of process. They fall under ‘diegetic sound’, which means that they come from a source on screen. Effects are added on almost everything, from the sound of a women’s heels on a tiled floor, to rushing horses. When you watch a battle scene you may not be aware that many of the swords on screen aren’t real metal, so the sounds of metal against metal have to be added, in time, after the fact.

Formats and Volume levels

Integral to making a good mix and master for a film is the right project format and volume level.

The higher the volume of particular stems, the more you will have to compress in your final project, so it may become problematic. Rather get your sound files at a lower volume that can be drawn up.

Formats are also very important. This includes making sure that your sample rates are set at the same kHz. Khz Refers to the rate at which the project is being read by your processor, this is why they should be kept consistent.

Recording Format (also called bits) in this regard relates to “bits” of information in each sample. This is why 8 bit music sounds like it was made by a computer, and 24 bit can sound almost live.

Surround or Stereo

For 5.1 and above, you can’t just walk into a standard studio and encode your sound. Unfortunately the truth for most indy directors is that they simply won’t have the budget it takes for a surround sound mix. Even if they’ve created their film for a surround mix in post. Specialised encoders and facilities mean that the price tag can be too high.

A stereo mix is both simpler to do and cheaper, but that doesn’t mean it won’t sound as good. Stereo is still the most popular because most people don’t have surround sound home theatre systems, or they just watch things on their mobile devices.

We mix in stereo at Cosher Studios because we know that’s what most people have available out there.

Finalising Your Mix

The first thing to do when finalising your mix is to reference. It can be very helpful to know what the platform of playback will be when the project is complete. (Is it for cinema, radio or TV?)

If for example it will be on TV, it is quite easy to get your audio to playback on a TV that you have in studio, to test the sound on the TV’s built in speakers. However, if it is going to be show in a cinema, this presents a different challenge.

The solution is what Colin, our head producer calls ‘reverse referencing’.

This is where you take existing audio from a film and listen to it in your own studio, listening to the balancing, volumes, effects etc… to make sure that the audio is consistent and comparable.

It is important to remember that if you use a trailer for a film, that the DB output will be louder than the actual film.

When finalising, make sure there is no distortion or digital clipping. If you are having trouble mixing your stems properly, remember that referencing the Fletcher Munson curve, which, if you use it the right way, will help you find the sounds which are becoming problems in the mix.

Related Articles