The whistle register (also called the flageolet or 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 are 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.
Physiology and definition
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). 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 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.
The physiology of the whistle register is the least understood of the vocal registers. Unlike other types of vocal production, it is difficult to film the vocal cords while they are operating in this manner as the epiglottis closes down over the larynx and the resonating chamber assumes its smallest dimensions. It is known that when producing pitches in this register, vibration occurs only in some anterior portion of the vocal folds. This shorter vibrating length naturally allows for easier production of high pitches.
Although the whole physiological production of whistle tone is not understood, it is known that when the laterals are active but the transversus inactive, a triangular opening is seen between the arytenoids, the vocal processes contact each other, but the posterior parts at the apex do not contact each other. The exception to this would be if the vocal folds are not stretched, as stretching of the vocal ligaments abducts (moves away from) the vocal processes.
Whistle register in children
Many babies and small children of both sexes can produce sounds in the frequency range of the whistle register without any conscious effort. Typically, the whistle register in children extends from the soprano "high C" (C6 or 1046.5 Hz) to two Gs above (G7 or 3136.0 Hz). Some children, however, can produce pitches that surpass the upper limit of the keyboard. Some medical professionals advise that children should be discouraged from going into the whistle register frequently for fear that such use of the voice could cause permanent damage to the vocal cords or vocal dysfunctions that affect speech and phonation. 
Uses of the whistle register
In European classical music, the whistle register is used primarily by coloratura sopranos. Many parts in the coloratura soprano repertoire extend beyond "high C" and often extend up to high F (F6). Although many coloratura sopranos use whistle tone vocal production to sing these notes, some operatic sopranos are capable of singing up to "high F" (F6) without utilizing the vocal production associated with the whistle register but remaining in the modal register. That being said, most coloratura sopranos do utilize the whistle register, particularly when singing staccato notes in rapid succession, during high trills, or other elaborate coloratura ornamentation in the upper tessitura. Rarely will coloraturas use whistle tone when doing high extended notes. However, singers like Mado Robin were noted for doing so. Probably the best-known example of the whistle register in European classical music is in the "Queen of the Night" aria (properly titled "Der Hölle Rache kocht in meinem Herzen") from the Mozart opera Die Zauberflöte; it calls for pitches up to F6.
In Western popular music, the whistle register is used with more variety and to produce much higher pitches than are called for in classical music. While it is mostly used by female singers, such as Shanice, Debelah Morgan, Angie Stone, Minnie Riperton, Tamar Braxton, Ariana Grande and Mariah Carey  there are few males who use it, such as Marq Torien, Brad Delp, and Prince. Adam Lopez holds the Guinness Book of Records title for the highest vocal note produced by a male (C#8) by making extensive use of the whistle register. Female singer Georgia Brown was listed in the 2005 Guinness World Records for highest note (G10) ever reached, but this claim was removed when the 2007 edition was issued.
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