in 1954, the recording industry association of america (riaa) introduced a standard equalization curve to be used by all record manufacturers. consequently, both low-quality and audiophile reproducers alike could replay any recording with the correct equalization. there are two versions of the reproduction riaa equalization curve. the first, is simply the inverse of the recording curve designed for cheaper equipment using crystal or ceramic reproduction cartridges. the second curve is intended for equipment fitted with magnetic reproduction cartridges where the output voltage is dependent on the frequency of the recorded signal (the voltage output is directly proportional to the frequency of the recorded signal; that is: the voltage doubles as the recorded frequency doubles).