Introduction to Sound Synthesis Mixing and dynamics
Mastering
Mastering is the last stage before music is released. A finished mix is adjusted as a whole, in tone, dynamics and loudness, so that it sounds consistent with other releases and suits the places it will be played.
Mixing balances the parts of a piece against each other. Mastering works on the result as a single stereo signal. Its usual tools are an equaliser, to adjust the overall balance of bass, middle and treble; gentle compression, to reduce the difference between loud and quiet passages slightly; and a limiter, a compressor with a very high ratio that stops the peaks from going over a ceiling, which lets the whole piece be made louder. Mastering also means preparing the files for release, at the right sample rate and format and with the tracks of an album at consistent levels.
The tool below masters a short loop. The meters show loudness in LUFS, a measure of how loud the music sounds, and peak level in dBFS, how high its highest samples go. Normalisation plays the result back as several streaming services do, turned up or down to a loudness of −14 LUFS.
A mastering chain
A loop passes through low and high shelving equalisers, then a limiter with adjustable drive and ceiling. Loudness and peak level are measured, and normalisation can play the result at −14 LUFS.
Listen for Raise the drive with normalisation off and the loop gets louder as the limiter holds the peaks at the ceiling. As the drive goes higher, the drum hits are turned down further and lose their sharp onsets. Turn normalisation on and the loop plays at −14 LUFS whatever the drive, so extra drive adds no loudness, but the drum hits stay flattened. The shelving controls raise or lower the bass below about 100 Hz and the treble above about 8 kHz across the whole loop.
Loudness and peaks
A master's level is usually described by its peak level and its loudness. Peak level, in dBFS, is how close its highest samples come to the digital maximum of 0 dBFS. It matters because a peak above 0 dBFS is clipped, but two masters with the same peak level can differ greatly in loudness. Loudness, measured in LUFS under the ITU-R BS.1770 standard, is found by filtering the signal with K-weighting, a highpass near 38 Hz and a shelf that raises frequencies above about 1.7 kHz by about 4 dB, and averaging its power over time. The filter is a rough model of the ear's sensitivity, so the result follows how loud the music sounds. The difference between the two numbers is a measure of how much dynamic range the master keeps. A large difference means the peaks stand well above the average level, and a small one means limiting or compression has brought the peaks down towards the average.
The loudness war
Through the 1990s and 2000s, masters were pushed steadily louder, by limiting harder so that the average rose while the peaks stayed at the ceiling. Listeners tend to prefer the louder of two tracks in a quick comparison, so there was pressure on mastering engineers to match the loudest releases. The heavier limiting reduced dynamic range: drum hits lost their sharp onsets, and quiet passages were brought up close to the level of loud ones. This is called the loudness war. Streaming services now measure each track's loudness and play it back at a common level, about −14 LUFS on several of them, so a master made louder than that is turned down. With normalisation on in the tool, extra drive adds no loudness but still flattens the drums.
Equalising, checking and delivery
An equaliser in mastering usually makes small, broad changes, a decibel or two with gentle shelves, because each change applies to every part of the mix that has energy at those frequencies. A mastering engineer also listens on several systems, checks the mix in mono, sets the gaps between album tracks, and produces the final files with the right sample rate, bit depth and loudness for each place they will be released.
Common questions
What is mastering?
It is the last stage of production before release. A finished mix is adjusted as a whole: its overall tonal balance with equalisation, its dynamics with compression and limiting, and its loudness, so that it sounds consistent with other music and suits the places it will be played.
What is LUFS?
LUFS stands for loudness units relative to full scale, the unit of a loudness measurement defined in the ITU-R BS.1770 standard. The measurement filters the signal with K-weighting, which reduces the lowest frequencies and raises those above about 1.7 kHz by about 4 dB as a rough model of the ear's sensitivity, and then averages its power over time. A peak meter shows only how high the highest samples go, and a LUFS reading follows how loud the music sounds much more closely.
What does a limiter do in mastering?
It is a compressor with a very high ratio and a fast attack that stops the signal from exceeding a set ceiling. Turning up the level into a limiter makes the music louder on average while its peaks stay at the ceiling, and the loudest moments, such as drum hits, are brought down towards the average level.
Why do streaming services normalise loudness?
They do it so that listeners do not have to adjust the volume between tracks. Services measure each track's loudness and turn it up or down to a common level, about -14 LUFS on several of them. A master made louder than that is turned down, so it plays at the same loudness as a quieter master but still has the flattened peaks that the limiting produced.
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