EQ's crucial mix job is reducing frequency masking, not beautifying soloed sounds

Frequency masking desensitises perception of a frequency region of one instrument when another has lots of energy there (cymbals above 5 kHz mask a vocal’s brightness). EQ’s crucial mixdown task is therefore reducing masking between competing instruments to achieve a stable balance, not improving each instrument’s soloed tone — which is why EQ presets are useless at mixdown, since no designer can predict the masking of your specific arrangement.

Examples

A vocal bright when soloed sounds dull once cymbals are added; you restore it by cutting the cymbals above 5 kHz or lifting the vocal there, not by “fixing” the vocal in solo.

Assessment

Explain frequency masking and describe how EQ reduces it between two instruments sharing a range.