Established by the Recording Industry Association of America in the 50s, RIAA equalisation offers longer recording times, improved sound quality and reduced playback wear.
An RIAA EQ curve preemphasises the high frequencies in program material by applying a +20dB high-frequency boost with a corresponding -20dB cut to bass frequencies. By applying the RIAA EQ curve directly before cutting the master lacquer, the bass is reduced significantly, meaning grooves can be spaced more closely for longer recording times. To achieve a flat frequency response on playback, an opposing de-emphasis curve is applied to the program material by the playback equipment. This not only balances out EQ applied before cutting, but also majorly attenuates any high-frequency artefacts present.
Though this process shouldn’t theoretically have an overall effect, there’ll usually be some phase distortion introduced by the EQ filters used for preemphasis and de-emphasis, as well as rumble as the turntable’s motor is amplified by the large de-emphasis bass boost.