RIAA equalization
The frequency curve every vinyl record is cut with and every phono preamp has to undo — bass down, treble up, and then the reverse.
A groove carrying full-strength bass would need such wide excursions that a side would hold a few minutes and the stylus would jump out. A groove without boosted treble would drown in surface noise. So the cutting lathe cuts the bass and lifts the treble, roughly ±20 dB across the audible range, and playback does precisely the opposite. Surface noise gets turned down on the way back.
Before 1954 nearly every label had its own curve, and collectors needed preamps with a selector knob to pick the right one. The Recording Industry Association of America then standardised a single curve, defined by three time constants: 3180, 318 and 75 µs. That is why a turntable cannot simply be plugged into an ordinary line input. It needs a RIAA stage, built into the amplifier or in a separate box, and without one everything sounds thin and shrill.