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: