In some sopranos the modal register vocal production may extend into what is usually thought of as the whistle register. Women of all voice types can use the whistle register. With proper vocal training, it is possible for most women to develop this part of the voice. Children can also phonate in the whistle register and men can as well in very rare instances.
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 the vocal processes.
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 females, such as Mariah Carey, and Minnie Riperton, there are a few male singers who use it. Among male singers, Adam Lopez holds the Guinness Book of Records record (C#8) for the highest vocal note by a male, making extensive use of the whistle register. However, the female singer Georgia Brown, holds the record for highest note ever reached.