Whistle register

The whistle register (also called the flute register or whistle tone) is the highest register of the human voice, lying above the modal register and falsetto register. This register has a specific physiological production that is different from the other registers, and is so called because the timbre of the notes that are produced from this register is similar to that of a whistle.

In some sopranos, the modal register vocal production may extend into what is usually thought of as the whistle register. With proper vocal training, it is possible for women within all voice types to develop this part of the voice. However, some women are unable to phonate or comfortably produce sound in this register. Children can also phonate in the whistle register and, rarely, some men.

The whistle register is the highest phonational register, that in most singers begins above the soprano "high C" (C6 or 1,046.5 Hz) and usually extends to about two Ds above (D7 or 2349.3 Hz). It is created by using only the back of the vocal folds. The lower part of the whistle register may overlap the upper parts of the modal and falsetto registers, making it possible for singers to phonate these notes in different ways. However, fundamentally the whistle register is most commonly used to produce pitches above C6. As with the other vocal registers, the whistle register does not begin at the same point within every voice, and there are rare voices which can extend the whistle register much higher or lower than the range listed above. For example, most operatic coloratura sopranos can sing up to the "high" F above "high" C without entering into the whistle register.Mado Robin is one example of a soprano who often sings F above "high" C (F6) to B above "high" C (B6) without entering into the whistle register.

...
Wikipedia