The whistle voice is a unique texture of sound produced by an extreme stretch of the vocal cords, often to the point where the larynx rises slightly. This register is found exclusively in the female voice at the very "tip-top" of the range, typically beginning above High C (C6) or D6 and extending up to around A6.
In the world of pop music, Mariah Carey is the most famous example of a singer who can navigate these sounds with ease. In the classical world, the "Queen of the Night" arias from Mozart’s The Magic Flute provide a perfect example of this soaring, bell-like texture.
These ultra-high notes begin as tiny, pinprick-sized dots of sound felt way "back and up" in the head. If they remain small and squeezed, the voice becomes constricted. However, with slow, careful development—specifically practicing crescendos and decrescendos on these pitches—the whistle register can become a legitimate, healthy part of the full voice. Think of it as standing on the roof of your vocal house.
While stretching into the whistle register is exciting, extended periods of this extreme stretch can be risky. If the stretcher muscle is not carefully balanced with the closer muscle (the arytenoids), the vocal cords can begin to separate. This imbalance often causes the middle of the "full voice" to disappear or become breathy and weak.
Most women will not need these extreme notes for their day-to-day repertoire. However, if these notes are naturally accessible to you, it is beneficial to claim them. Stretching into the whistle register can improve your general vocal health and flexibility, provided you maintain a healthy balance across your entire range.