Most tracks are recorded with -6dBFS maximum peak at 24-bits resolution for adequate headroom and gain staging. During audio mixing, the levels of your track can fall somewhere in the average mixing levels of -18dBFS to -10dBFS. Sometimes it can reach to -6dBFS peak and make sure you allow some headroom for your mixes.
2.) Extreme compression of master bus and tracks – don’t over-compress your mix to make it loud or to sound perfectly even. Leave that to the mastering engineer. Bear in mind that by over-compressing your mix, it will lose its dynamics that can make your mix sounds thin and unrealistic. It will worsen during mastering and this mistake cannot be reversed.
As a result, it will sound awful when broadcast on air (TV, radio, etc) because radio/TV will apply another stage of audio compression. Read this tutorial on how you can create a radio-friendly mix.
3.) EQ assignments of instruments – as a mixer you are responsible in assigning the best EQ settings for every track in the mix. For example, you want the bass guitar to sound above the kick drum. So you might want to boost the kick drum at 70Hz while the bass guitar somewhere at 150Hz. The purpose is to give clarity to these instruments in the mix.
Things like this should be finalized during the audio mixing process and no mastering tools can reassign the EQ assignments once it reaches the mastering stage.
4.) Serious mistakes in reverb and other FX settings – of course this can only be fixed by remixing the project.
5.) Out of tune instruments – this cannot be fixed because the tracks are already mixed down into a single waveform. This can be fixed using an auto-tuner during the mixing stage applied on affected tracks (vocals, guitars, etc). A much recommended method is by re-recording the track.
So what can be fixed?
Surprisingly some weakness in the mix can be improved during mastering such as:
1.) Over-EQ in a certain spectrum/band – this can be fixed by using multiband compressors. This works by compressing the signal within a certain frequency range. For example, the bass content is too dominant comparing to the mid and high frequency content of your mix. The mastering engineer can use a multiband compressor to compress ONLY the bass part of the mix. For more details, read this multiband compressor tutorial.
However, there is a limit as to how multiband compressors can fix an over-EQ problem (depending on what type of multiband compressors you are using). Some mastering engineers know this limitation and sometimes they return the mix to the studio to have that it remix to fix the serious EQ issues.
2.) Under-EQ – in the opposite, if the mix is lacking a certain aspect in some frequency range, this can be easily fix by boosting that range using a parametric equalizer.
Again bear in mind the limitation on using parametric equalizer in the mastering process. Boosting more than +3dB in any frequency band of the mix seems too much for some mastering engineers. EQ Boosting creates more side-effects than cutting. It is why they limit on how much they can boost.
If the mix has serious under-EQ issues, the best fix is to remix the project with the correct EQ settings.
3.) Stereo width – if the mix sounds thin and narrow, a mastering engineer can use stereo widening tools and other analog gears to add more punch and stereo separation in the mix. These gears can be expensive and mostly available in standard audio mastering facilities.
Credits: Crookwood mastering console
There are lots of mistakes in audio mixing that cannot be fixed during mastering. You should take note of that before sending your mix to a mastering facility.
Your objective in audio mixing is to make it sound as best as possible. This is attainable as long as:
1.) The mix and the individual tracks aren’t clipping.
2.) You have assigned appropriate headroom to the complete mix.
3.) You have applied the right FX for each track.
4.) There is an acceptable clarity of the entire mix (correct EQ settings for each track).
5.) You have correctly panned each instruments in the stereo field.
6.) The mix and the individual tracks retain its dynamics (not over-compressed).
7.) You are mixing the best recorded tracks of the project (not out of tune, recorded at 24-bits, perfect artist/musician performance, etc.)
Then send the complete mix to a professional mastering engineer that you can trust. They can transform your mix to sound a hundred times better.