Multiphonics are an extended technique in instrumental music in which a monophonic instrument (one which generally produces only one note at a time) is made to produce several notes at once. Multiphonics in wind music are primarily a 20th century technique, first explicitly called for in the Sequenza for solo flute by Luciano Berio and Proporzioni for solo flute by Franco Evangelisti, though the brass technique of singing while playing has been known since the 18th century and used by composers… (More on Multiphonic)