Why do instruments get tuned according to the La?
We heard so much and used so many times the expression "the La of diapason" that we could think it came naturally. But no, we use it by convention.
Before a show, it is always a La, more precisely a La3 (the one written between the second and the third line of a stave) that rings in the room to tell us to stop talking. The instruments are being tuned. The conductor is about to come on stage.
The la3 is the empty string that the violin and the viola have in common. But, if those instruments can be tuned easily, with only a few peg turns, they are also extremely sensitive to temperature changes and therefore need to be frequently readjusted. Thus, it is the oboe, the longest to be tuned, that give the La to the string instruments and to the other stands.
And this famous La3 reigns way beyond the orchestra pit: it is the diapason of Western music. Which means it is the reference the most commonly used to tune instruments and voices.
Physics into music
Yes, but to what frequency must we set this famous diapason? Because, if some orchestras use a La3 at 440 Hertz (meaning 440 vibrations per second), others, like the ancient music ensembles, use a La set at 415 Hertz.
Why a La at 415Hz for baroque music? Was it is use at the time? The question is in fact anachronistic since the concept itself of frequency was discovered only in the 18th century. And before 1930, we didn't talk about Hertz (Hz) but about cycles per second.
A La for everyone
Until the 19th century, the diapason didn't exist (at least, at the sense that we understand it today, as a fixed note of reference). The instruments were tuned according to one another. They were tuned for example on the flutes, as their height is fixed, determined when they are made.
Every musician knows how to transpose, to adapt to the context in which he is performing. The diapason therefore changes according to the geographic spaces but also within a same space: the "chapel tone" is for example rather low, to facilitate the singing, when the "chamber tone" of the orchestras rings higher.
All for a diapason
The diversity of diapasons in use was a problem for the traveler musicians. How could we play in unison with instruments of another fabrication? How can one adapt their ear to a tone too different? As the exchanges between musicians became more intense, the idea of a bigger uniformity was starting to gain ground...
Finally, it was imposed during the 19th century, in particular due to the first industrial revolution and the birth of the serial production. More and more instruments were made and sent all over the work. It became possible to use the same flute or the same trumpet in Berlin and in Milan.
To save the singers
In the 19th, voices started to rise to defend the idea of a unique diapason, and not only for the makers. Imagine for a second a French soprano invited to play The Queen of the Night in Covent Garden and discovers there that her two virtuoso arias are executed from a La higher than the one in use in Paris. Her famous conter-fa could be in great danger.
Some singers and composers therefore joined the movement in favor of a regularization of the La. It was the case of Verdi and Berlioz, the latter even wrote to a minister that he was "convinced with reason that the progressive elevation of the diapason is a cause of ruin for the greatest voices".
If the old treaties and the studies made on the instruments of the time showed that the La cold reach 300 or 560 Hertz between the 16th and the 19th century, it did have a tendency to go higher during the period of Verdi and Berlioz.
Why? In theory, higher the diapason to tune an instrument is, the more brilliant the instrument will sound. In practice, the elevation of the reference La matches the introduction of steel strings, more resistant, but also the necessity for orchestras to adapt to venues that became bigger and bigger.
The time for normalisation
Facing this large variety of diapasons, the French government gathered in 1858 a commission of physicists and musicians, including Berlioz, Auber, Meyerbeer and Rossini. After a year of work and research, the La3 is set in France at 435Hz.
The initiative is inspiring but stays exclusive. It finally in 1939 that the famous La 440 is declared as the international reference by a group of international experts, in Londres. It would become an ISO standard and even a European recommendation.
Why 440? Some say it matches the European average. Other that it was adopted because the instrument makers sell to the United States, where the diapason was higher, in particular because of the influence of jazz.
It remains the use of the La 440 is not mandatory. Every musician is free to choose their own diapason!