Prolly like the F below middle C or lower, approximately. However, I would not recommend having a hard and fast "boundary" between high, middle, and low. It's more important to preserve the continuity of melodic lines. In a "smooth" melodic line (one in which each note is fairly close to the notes before and after it, as opposed to an "angular" melody, where there are larger leaps between notes) you should rarely (if ever) change waves in the middle of the melody. If you wanted to be really fancy, you could do something weird like gradually fade one out and fade another in.