The current issue of Strings, June 2009, asserts in a small review of the Bach Brandenburg Concertos that the conductor on this recording chose to perform them “in the French Baroque pitch of A = 392 Hz....”
I have some reasons to doubt that A as a frequency of 392 Hz was commonly considered anything but A flat.
Elsewhere I've written how C-256 Hz was common in the time of Bach. I've made the case that this pitch reduces mathmatically to one cycle per second.
Taking that pitch and multiplying by 7 we get the seventh, B flat. Then multiplying again by 7 we get the seventh of B flat, A flat.
That’s 49 vibrations per second. Double that for the octave and you have a tone that is 98 Hz. Double that and it’s 196 Hz. A final doubling gives you 392 Hz.
In short, if middle C in the time of Bach was 256 Hz, then the tone produced by 392 Hz would be perceived as an A flat.
I have a slightly extended version of this discussion at the Music Lesson Plan blog.
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