The back vowel “U” is sung with the thinner vocal cords characteristic of falsetto adjustment. This makes “U ” very useful in working out the man’s “passaggio”.
CARUSO, in fact, in his vowelisation exercises always sings a “U” from f’ onwards (345 Hz).
In the 1931 recording of the Faust-Aria, GIGLI places a brilliant high C on the “U” of “fanciulla”.