While volume/amplitude is often shown on a vertical scale, the usual displays and diagrams show the sound spectrum from bass to treble on a horizontal scale. The most important instrument for intervening in mixing and mastering at this level is the equalizer, or EQ for short. In different fine divisions (besides lows, mids and highs still low or high mids etc.) mostly stepless curves are used today.
Here are three tutorials to get you started – by Martin Wolfinger, delamartv and Jonas Wagner from the Recording Blog.
Like levels and compressors and limiters for volume, this balances the interaction of multiple tracks on the scale of frequencies so that not too many signals compete with each other in the same range.
This balancing also includes rather local and sometimes somewhat extreme reductions of frequency ranges – where they lead to particularly noticeable disturbances. It is dull sound qualities that are created in this way in the overall picture and can be prevented with the EQ. For this purpose, individual tracks or the entire mix are scanned for such disturbing frequency ranges with the EQ – one makes particularly loud on a trial basis, which is then lowered in its amplitude at this point on the horizontal.
Philipp Ernst from abmischenlernen explains it:
To recognize this better in the interaction of the tracks, today additionally advanced plugins help, which mark the competing places in the frequency band by simultaneous comparison of tracks and corresponding visualization. The Anglo-Saxons call this problem “masking”, as explained here by Plugin Boutique using the example of the plugin “Neutron 3” by iZotope:
A first step in most sound recordings is usually a volume boost; hence the first part of the “Mixing Essentials” on the following topic. As opposed to just ‘turning up’ an entire volume, compressors are used – sometimes already between the instrument and the recording device. The compressor raises the volume of the weaker parts of the sound. This is because what can sometimes be heard well by the human ear in real space tends to be drowned out on the electrified signal path; and even more so in the interaction with other audio tracks.
KEYS author Holger Steinbrink explains a few basics here:
Philipp Ernst from abmischenlernen.de can also be consulted on the topic:
Rick Beato on more specific aspects:
As mentioned here in passing, compression goes from a strong flattening to clear upper limiting of the amplitude for which you use a limiter. At MixbusTV the difference is explained:
Joe Gilder once again summarizes basic functions and recommendations for the use of compressors in a different way. It is about the factors attack and release, about the application to single tracks or several in a mix bus, order of equalizer and compressor as well as the question of volume vs. sound quality.