Way back in the 1950's when leading edge scientists and technologists were developing the successor to the old-style 78 rpm record, they found a problem. They wanted a 'long playing' record, capable of up to twenty minutes or more a side. But they had difficulty getting the turns of the groove close enough. The low frequencies were causing the cutting stylus to move a long distance from the spiral track, taking up a lot of the available space.
The answer - cut the bass!
Actually, this wasn't the whole of the answer or plainly the long-playing record would never have taken off. The other part was to put the bass back in on playback.
It sounds good in theory - simply reduce the level of bass during the cutting process, then boost it back up again on replay.
The problem is that the simplest way of doing it would result in a 6 decibels per octave curve. This means that high frequencies would be recorded at full level, but for every descending octave (halving of frequency) below that, the level would fall by 6 dB (which coincidentally is a halving of the level too).
The problem with this is that if you start at a reasonably high frequency - say 10 kHz, then halve the level at 5 kHz, then halve it again at 2.5 kHz etc., the level down at the lowest frequencies is virtually nothing. Try and boost that up and all you get is LF noise.
The answer is to use a more complex filter to equalize the signal. This results in a curve where the lowest frequencies are around 40 dB lower in level than the highest. 40 dB is a lot to make up, but it is possible. This 40 dB boost in the low frequencies explains why record players are so prone to low frequency rumble.
This EQ curve was ratified by the Recording Industries Association of America and is known as the 'RIAA curve'. Every record now made conforms to this curve. Every DJ mixer and vinyl preamp contains a filter that inverts this curve, giving the flat frequency response we desire.
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