IEC RIAA Curve
In 1972 an alternative version of the replay curve (but not the recording curve) was proposed by the International Electrotechnical Commission, differing from the RIAA replay curve only in the addition of a pole at 7950 µs (20 Hz). The justification was to reduce the subsonic output of the phono amplifier caused by disk warp and turntable rumble.
This so-called IEC amendment to the RIAA curve is not universally seen as desirable, as it introduces considerable amplitude and -more importantly- phase errors into the low frequency response during playback. The simple first-order roll-off also provides only very mild reduction of rumble, and many manufacturers consider that turntables, arm and cartridge combinations should be of sufficient quality for problems not to arise.
Some manufacturers follow the IEC standard, others do not, while the remainder make this IEC-RIAA option user selectable. It remains subject to debate some 35 years later.
Read more about this topic: RIAA Equalization
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