EQ cannot isolate a single instrument because every instrument's harmonics spread across the spectrum

A common beginner belief is that EQ can surgically remove or isolate a specific instrument from a mixed track — ‘just cut the horns’. This is impossible because every instrument produces not only a fundamental frequency but a series of harmonics (overtones) spread across the spectrum, and the character of most melodic instruments is contained across the whole range. Cutting the midrange to remove horns also strips harmonic content from every other instrument sounding there. The practical implication for DJs: an EQ adjustment always affects the character of the entire mix, not just a target instrument. A fairly pure bass sound is the exception — it does not carry far into the higher spectrum.

Examples

A DJ tries to ‘cut the horns’ out of a soul track by reducing the midrange. Instead, the whole track loses warmth and definition because the vocals, guitar, and bass all have harmonic content in that range.

Assessment

Explain why EQ cannot isolate a single instrument from a mixed recording. What is ‘harmonic content’, and which kind of sound is the partial exception?