From a production perspective, creating a contemporary music release can be roughly divided into three main phases: recording, mixing, and mastering. In the recording phase individual instrument and vocal parts are recorded into a Digital Audio Workstation (or onto a physical format). In the mixing phase these parts are combined and balanced to the elicit the desired experience for the piece of audio. And finally, in the mastering phase the many songs of a project are brought together into a cohesive whole and considered in the context of the wider musical and commercial world. The mastering engineer is the person who finalises the audio, both sonically and in terms of format, utilising analog and digital audio signal processing to optimise audio for the maximum enjoyment of the listener, however they choose to listen.
Key to the mastering process is the engineers fresh perspective, trained ears, and specialised tools that ensure a level of professional quality and listener satisfaction in the finished audio media (music, podcast, film sound, etc.). To this end, a great mastering engineer possesses a wealth of accumulated listening experience that allows them to have a sense of how any recorded audio should be adjusted in order for it to translate to the end listener optimally. In most cases this listening experience has come from a true love of music and many years of listening to music of many genres and styles.
As a mastering engineer begins to work professionally they start to accrue thousands of dedicated listening hours in the same listening environment, which in the best cases is a professionally designed acoustic space with a very accurate monitoring system. Having heard a great deal of music and other audio in this same accurate listening environment a mastering engineer develops a mental sonic imprint of how certain program material, of varying genres and styles, could sound.
When a finished mix is presented to the mastering engineer, the mastering engineer considers this new piece of music in the context of this mental sonic imprint and makes subtle changes to sonic characteristics of the music such that it sits comfortably for them in their listening environment, and therefore comfortably in the wider world.
In making changes to the sonic characteristic of audio material the mastering engineer requires specialised signal processing tools, which can include:
Equalizer (EQ) — A filter that adjusts certain frequency bands by adding or removing energy from the specified frequencies. Perhaps the low frequencies of some piece of music are generally overpowering the midrange, a mastering engineer might use an EQ to reduce the level of the problematic low frequencies.
Compressor — A processor that sdjusts the dynamic range of a signal relative to the input level of the signal. A compressor begins to act on a signal when it reaches a given input threshold. When this threshold is crossed gain reduction is applied to the signal. A compressor also allows the mastering engineer to adjust the speed with which this gain reduction occurs, essentially shaping the envelope of transients above the threshold.
Limiter — A processor that prevents a signal from exceeding a specified level. Often in mastering this level is somewhere close to digital zero (-1.0 dBFS, for example). The limiter provides a safety that prevents levels going over the nominated ceiling (which if left would produce obviously poor playback artefacts) and also allows the average signal level to be increased above what would otherwise be possible. In the latter situation the overall level of the material is increased but anything hitting the ceiling is attenuated.
Although these signal processors are also used when recording or mixing the equalizers, compressors, and limiters in a mastering studio are often made to higher specifications and closer tolerances to facilitate very detailed work. Additional to the common mastering processors above, mastering engineers may employ processor made for harmonic saturation, and stereo width adjustment, and in the majority of cases some combination of the processors listed is used. Processing can occur in the digital and/or analog domain.
A mastering engineer is the final person to adjust the sonic characteristic of audio material and finalise digital audio files (or analog media) ready for production and/or digital distribution. This may include addition of metadata, the production of a DDP file (used to produce CD), or the cutting of a lacquer (which is needed to produce vinyl records).