The San Francisco Bach Choir and its Period Orchestra strive to employ tuning practices based on what we know of the historical practices of the composer’s time. For the Bach Cantatas, we do not use the modern tuning practice of setting the A above middle C to vibrate at 440 wave cycles per second (Hertz). Instead, we tune that A to 415 Hertz; in effect, we then move everything down by approximately one half-step. In addition, we do not use modern equal temperament. Instead, we use Vallotti-Young temperament, a system of tuning that prioritizes pure perfect fifth intervals. Compared to modern tuning, this system produces purer consonance and more intense dissonance.
Vallotti-Young temperament also produces irregular interval relationships among each of the key areas, or scales. Each key, therefore, has a personality or affect of its own; playing the same composition in different keys would produce subtly different intervals and a somewhat altered listening experience. Composers chose the keys of their pieces with these affects in mind. Our temperament choice helps us more closely recreate the key affects and harmonic qualities that Bach probably intended.
by Rachael Hutchings, May 2005