Audio mixing (recorded music) explained

Audio mixing is the process by which a multitude of recorded sounds are combined into one or more channels, most commonly two-channel stereo. In the process, the source signals' level, frequency content, dynamics and panoramic position are commonly being manipulated and effects such as reverb might be added. This practical, aesthetic or otherwise creative treatment is done in order to produce an elevated mix that is more appealing to listeners.

Audio mixing is done in studios as part of an album or single making. The mixing stage often follows the multitrack recording stage and the final mixes are normally submitted to a mastering engineer. The process is generally carried out by a mixing engineer (a mixer), though sometimes it is the musical producer who mixes the recorded material.

Prior to the emergence of DAWs (Digital Audio Workstations), the process of mixing used to be carried out on a device known as an audio mixer, sound board, desk, or mixing console. Nowadays, more and more engineers and independent artists are using a personal computer for the process (commonly referred to as mixing in-the-box).

History

Mixing as we know it today emerged with the introduction of commercial multitrack tape machines, most notably the 8-track recorders that were introduced during the 1960s. The ability to record sounds into a multitude of channels meant that treating these sounds can be postponed to a later stage - the mixing stage.

Equipment

Mixers

See main article: mixing console.

A mixer, or mixing console, or mixing desk, or mixing board, or software mixer is the operational heart of the mixing process. Mixers offer a multitude of inputs, each is fed by a track from a multitrack recorder; mixers would normally have 2 main outputs (in the case of two-channel stereo mixing) or 8 (in the case of surround).

Mixers offer three main functionalities:

Mixing - summing signals together, which is normally done by a dedicated summing amplifier or in the case of digital by a simple algorithm.
Routing - allows the routing of source signals to internal buses or external processing units and effects.
Processing - many mixer also offer on-board processors, like equalizers and compressors.

Outboard gear and plugins

Outboard gear (analog) and software plugins (digital) can be inserted to the signal path in order to extend processing possibilities. Outboard gear and plugins fall into two main categories:

Processors - these devices are normally connected in series to the signal path, so the input signal is replaced with the processed signal (e.g. equalizers).
Effects - while an effect can be considered as any unit that affects the signal, the term is mostly used to describe units that are connected in parallel to the signal path and therefore they add to the existing sounds, but do not replace them. Examples would include reverb and delay.

Common classes:

Processors:
- Faders - used to attenuate or boost the level of signals.
- Pan pots - used to pan signal to the left or right and in surround also back and front.
- Equalizers - used to manipulate the frequency content of signals.
- Compressors - used to manipulate the dynamic content of signals. Among many applications they can even the level fluctuations of a singer, or reshape dynamic envelopes of percussive instruments (e.g. adding attack to a snare).
- Gates - used mainly to attenuate low-level signals, for example, the kick spill on a snare recording.
Effects:
- Reverbs - used to simulate the boundary reflection created in a real room, but that adding a sense of space to otherwise 'dry' recordings.
- Delays - most commonly used to add distinct echoes as a creative effect.

Mixing Domains

The process of mixing often accounts for a few mixing domains:

Level - concerned with the relative level between instruments and their dynamics.
Frequency - concerned with the spectral content of the various instruments and the overall mix.
Space - concerned with the spatial aspect of the various instruments. The space domain is often further subdivided into two sub-domains:
- Stereo - concerned with the horizontal panoramic aspects of instruments.
- Depth - concerned with the front-back aspects of instruments.