What is 440 Hz?
First of all, Hertz (Hertz, Hz) refers to the number of cycles per second. Usually, when tuning an instrument, the La (A) sound is used as the basic sound.
Therefore, when we say A=440 Hz, we mean that the La (A) sound wave above the center C (yellow in the figure below) vibrates 440 times per second.
A440 is the musical pitch corresponding to an audio frequency of 440 Hz, which serves as a tuning standard for the musical note of A above middle C, or A4 in scientific pitch notation. It is standardized by the International Organization for Standardization as ISO 16.
But why is it exactly 440 Hz? In the past, there was no international standard tuning. So a flute from France was tuned to a different note than a flute from England. This meant that a French flute couldn't be played alongside an English flute. They would sound out of tune with each other. So a touring musician would have to take several flute extensions with him to be able to play in tune in different locations. Pitches varied from country to country, from village to village, even within the same village. And there was a tendency towards pitch inflation where the same note was constantly retuned higher to sound more bright and brilliant, especially for stringed instruments. Naturally, singers weren't very happy about this. Over the past 500 years so since medieval and early Renaissance time, the pitch of A and therefore all the other notes have risen by about a minor third, but with large variations throughout. Indeed in the early 17th century, the pitch levels ranged by as much as a perfect fifth. For example, Baroque musicians tuned the note to roughly 415 Hz, which is what Bach used. Handel in 1740, used a tuning fork which set a to 422.5 Hz, while one found in 1780 had an at 409 Hz. Some countries set their own standard notes, for example, the French government passed the law in 1859 which set note A at 435 Hz.
This tuning fork was made in the 1800s in England, the engraving on it says that it is the note A, tuned to the old Philharmonic pitch which was 452 Hz. This was then replaced in 1896, by the new Philharmonic pitch with A as 439 Hz. This obviously wasn't very efficient. So after a lot of discussions and international conference in London in 1939 decided that the A above middle C was to be tuned to 440 Hz in all countries from then on. This is known as the international standard pitch or concert pitch. And 440 Hz was chosen because it was a nice round number and it was about the average frequency used for note A across Europe at that time. So a Stradivarius violin made during the 17th and 18th centuries was actually designed for a much lower pitch and therefore a lower string tension. To tune a Stradivarius up to today's concert pitch requires modifying and strengthening the violin. This in turn affects the sound quality so a Stradivarius violin today does not sound like a Stradivarius violin when it was first made. And of course, this means that all music written before 1939 is now technically out of tune. So for example, in Mozart's time in the Holy Roman Empire, the note we now call A would actually have been closer to a B flat. So when we listen to Mozart sonatas today we're actually hearing them about a semitone higher than he would have intended.
This also means that those people who believe that different keys have different moods are wrong. E flat major isn't any more serious than A major and E major isn't any more joyful than G major. This mood key idea was first published in the 18th century well before the international standard tuning was fixed in 1939. Further, this was before equal temperament tuning became popular and widely used. Prior to equal temperament note to diatonic keys on the piano sounded the same. Different scales had different intervals between the notes. But with equal temperament, all major scales sound exactly the same and so are completely relative and none has a different mood to any other. What matters is the modulation or the changing of keys, not the key itself. But even though a whole bunch of musical chaps came together in 1939 and agreed to set a standard international frequency for note A at 440 Hz. Today many orchestras tune to a slightly higher frequency, European orchestras for example, often tune the note A to 444 Hz. So pitch inflation continues. In 1936, the American Standards Association 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 (reaffirmed by them in 1975) as ISO 16. So A440 serves as a tuning standard as we know nowadays.
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