Equalization (EQ)

In audio, EQ is the process of adjusting the levels of different frequency bands.

In the analog world, this is done using resistors, capacitors, and inductors (an RLC circuit – or at least resistors and capacitors, an RC circuit). I don't wanna get into the math or circuit diagrams here because it's scary to me, but the practical result is that the circuit takes an audio signal (as voltage), splits it up into frequency bands, which naturally causes a kind of phase shift¹. This is by design².