Falsetto is sometimes said to be a style of singing, and other times a vocal register (lying above the model register in frequency). It is most predominant in men, where, by vibrating only the edges of the vocal folds, the voice is able to reach notes in the female soprano or alto range, about an octave above head voice. The tone of falsetto is usually breathy or fluty, although in some singers it may have a ring to it. It is not the same thing as a pure countertenor voice. Unlike the head voice, falsetto cannot blend with the chest voice (i.e., it is not part of the modal register), nor can the sound intensity be increased beyond a certain point. Although falsetto is much more pronounced in males, there is such a thing as female falsetto.
The anatomy of falsetto
In falsetto, the thyroarytenoid (TA) muscle relaxes completely, and so the length of the folds depends solely on the degree of contraction in the cricothyroid CT muscle. Since the TA muscle is lax, it also doesn't cause the vocal fold cover to stiffen or thicken. Only the outer layers of the cover vibrate, and almost all of the sound energy is in the fundamental frequency. There are very few higher overtones, and they are quite weak compared to the fundamental. The folds are open for a large portion – usually over 70% – of each cycle.