User:Lukasdoro/Franco-Belgian acoustic index

The Franco-Belgian system is an acoustic index that assigns the number 1 to the C of the second lower ledger line in the key of F, giving it the name of C1.

Thus, it is necessary to use negative numbers, so the lowest C of the pipe organ (two octaves below the C cited above) and the lowest A of the piano are respectively C-1 and A-2.

Also the middle C of the piano (261.63  Hz ) would be a C3, and the A of the same octave (the famous A440 Hz) would be called A3.

This system is used in France, Belgium, Spain, Italy, Brazil, Argentina and other parts of the world, although it often coexists with other acoustic indices, and in particular the scientific acoustic index.