Do Countertenors Use Falsetto?

In actual practice, it is generally acknowledged that a majority of countertenors sing with a falsetto vocal production for at least the upper half of this range, although most use some form of "chest voice" (akin to the range of their speaking voice) for the lower notes.

How can countertenors sing so high?

It evolves a slightly different vocal mechanism in terms of where the voice-box puts itself, to present the cords in such a way that the air passing through is only passing the thinner edges of the cords. So the sound is higher pitched. Countertenors are still quite rare, though.

Do female singers use falsetto?

The ability to speak within the falsetto register is possible for almost all men and women. The use of falsetto is considered uncommon in normal Western speech and is most often employed within the context of humor.