system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what does "mastering" do to audio?
gpt
Keeping it relatively simple for ELI5, mastering audio is basically the process of preparing a mixed song or album for duplication and distribution.

You can break mastering up into 2 broad sections.
Aural and Technical.

Aural would be the manipulation and processing of a song/songs to get the best possible listening quality on as many different devices as possible. People can listen to a song on anything from a clock radio, cheap earphones, home hifi, car stereo, on their tv etc... All of which react completely differently and a given song will sound different on every system. There are techniques such as EQ and compression that can be applied to a song that can help it to translate as well as possible between different systems. A lot of bass would sound great on a good hifi or in a recording studio but would not sound good at all on a clock radio or cheap earphones. So mastering engineers use many techniques to "smooth out" the song dynamically (called compression), and also techniques to balance the song sonically (EQ).

Limiting is also applied which is a technique which basically makes a song louder. The louder the better with modern music it seems, but there are draw backs and this is an ongoing argument in the mastering industry. Limiting a song too much can cause it to distort, or can lose clarity in drum strikes etc. There's a happy middle ground here but these days most songs are over compressed and limited.

On an album the mastering engineer would ensure that the transition between each song is as the artist wants. So they will do fade in or fade outs as appropriate. They would also ensure that an album has an overall "tone" or "sonic signature", which basically means that the songs sound consistent in both volume and tone. If one song was bass heavy and was followed by a song that was lacking in bass the listeners attention might be drawn to that, and we want to avoid that.


Technical side to mastering would include>

* Embedding ISRC codes which stands for International Standard Recording Code. These codes are embedded on a CD and simplify the collection of royalties.

* Track order on an album.

* Gap times between each track. How much of a gap a listener hears before the next song.

* Creating hidden tracks.

* Creating a disk image that complies with international CD standards (Red Book).

* In recent years mastering engineers often supply tracks for online releases only and may be required to encode the song in the various versions that are required. MP3, m4a, flac etc...

Another big aspect to mastering is the listening environment. Mastering engineers use extremely expensive speakers called reference monitors. Although these speakers can be loud and can have massive bass response, they are more useful because they somewhat mimic home hifi speakers (but are much much higher quality), these speakers are clinical and are comparable to using a magnifying glass. Unlike recording studio speakers which no average person has in their living room. A mastering studio would sound very very nice and would be a lovely environment to sit and listen to an album. A mastering engineer would be an expert and know exactly how a song should react/sound in their studio and know how to manipulate it to get it to translate well onto car radios/headphones/home stereo etc...

Finally a mastering engineer is often a fresh pair of ears who only hears a song or album at the very last step. Everyone else involved to this point may have been listening to the songs for months and may not notice little errors. A fresh perspective can often discover the strangest mistakes and errors that need to be fixed. A mastering engineer could often send a mixed song back to the mix engineer if something needs to be addressed.

It is a very important job and a role that many claim they can do but few do it well.