Although used in the traditional music of Mongolia, Tuva, and Tibet, overtones have also been used in the contemporary compositions of Karlheinz Stockhausen ("Stimmung"), as well as in the work of David Hykes. By carefully controlling the configurations of the vocal cords, a singer may obtain "undertones" which may produce period doubling, tripling or a higher degree of multiplication; this may give rise to tones that fairly coincide with those of an inverse harmonic series. Although the octave below is the most frequently used undertone, a twelfth below and other lower undertones are also possible. This technique has been used most notably by Joan La Barbara..However, undertones may be generated by processes that include more than the vocal folds. For instance, the ventricular folds (also called the "false vocal folds") may be recruited, probably by solely aerodynamic forces, and made to vibrate with the vocal folds, generating undertones, like those found, for instance, in Tibetan low-pitched chant. By overstressing or by assimetrically contracting the laryngeal muscles, a multiphonic or chord may be produced. This technique features in the 1968 composition "Versuch über Schweine" by the German composer Hans Werner Henze. In voice pathology, there are various descriptions of somewhat similar effects, such as those found in patients with diplophonia, a condition that produces a "double voice", i.e., two or even more simultaneous pitches. A form of alaryngeal speech that has a high pitch that can be used for speaking and singing. It is most familiar as the voice of Donald Duck. Besides producing sounds with the mouth, singers can be required to clap or snap their fingers, shuffle their feet, or slap their body. This is usually notated by writing the appropriate word over a note. These gestures are sometimes written on a separate one-line staff as well. Inhaled helium is occasionally used to drastically change the timbre of the voice. When inhaled, helium changes the resonant properties of the human vocal track resulting in a very high squeaky voice. In Salvatore Martirano’s composition "L’s GA" the singer is required to inhale from a helium mask. Conversely, an unnaturally low voice may be achieved by asking the singer to inhale sulfur hexafluoride. This technique is less popular than helium inhalation, in part because of the inherent risk of the gas displacing oxygen in the lungs.