Sound occurs when air is set into motion rapidly. Humans can hear sound if those cycles of compression and uncompression occur anywhere from twenty times each second to about twenty thousand times each second.When a piano string is set into motion, it vibrates up and down repeatedly. If the note A above middle C (C4) is properly tuned, that string will vibrate up and down 440 times in one second. That's what A-440 means also know as "Concert Pitch".
Every note on a piano is tuned using A-440 as the starting point. A-440 has been accepted as the universal standard for most of the century. Before that, it varied as much as a semi-tone higher or lower.
And even further back in time, there was no standard at all. Every village used a prominent local instrument, such as a church organ, as the standard for tuning its musical instruments. Pity the wandering minstrel!
History
Prior to the standardization on 440 Hz, many countries and organizations followed the 435 Hz recommendation the Austrian government made in 1885. The American music industry reached their own informal compromise of 440 Hz in 1926, and used it in instrument manufacturing. In 1936, the American Standards Association recommended that the A above middle C be tuned to 440 Hz.[1] This standard was taken up by the International Organization for Standardization in 1955 (and was reaffirmed by them in 1975) as ISO 16. Although still not completely universally accepted, since then it has served as the audio frequency reference for the calibration of pianos, violins, and other musical instruments.
