A440 is the 440 Hz tone that serves as the standard for musical pitch. A440 is the musical note A above middle C (A4).
In 1939, an international conference recommended that the A above middle C be tuned to 440 Hz. This standard was taken up by the International Organization for Standardization in 1955 (and was reaffirmed by them in 1975) as ISO 16. Since then it has served as the audio frequency reference for the calibration of pianos, violins, and other musical instruments.
Tuning a piano is an example of a simple frequency calibration that is actually done with the human ear. The piano tuner listens to a standard musical pitch and compares it to the same note on the piano keyboard. The piano is then adjusted (by tightening or loosening strings), until it agrees with the audio standard.
The smallest frequency offset that a piano tuner can hear depends on lots of factors, including the sound volume, the duration of the tone, the suddenness of the frequency change, and the musical training of the listener. However, the just noticeable difference is often defined as 5 cents, where 1 cent is 1/100 of the ratio between two adjacent notes on the piano's keyboard, i.e. 1/100 of an equally tempered semitone.
A440 is also the note most readily accessible (i.e. the note which they are the most able to pitch) of those people gifted with the talent of perfect pitch.
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