Following a period of studies at the Hochschule für Musik und Theater München and in courses given by Hermann Scherchen in Gravesano, Riedl, influenced by Carl Orff and Edgar Varèse, devoted himself as a composer particularly to percussion and Lautgedichte (sound poetry).
[1] In 1950 he was co-founder of the German Section of the Jeunesses Musicales, together with Herbert Barth, Reiner Bredemeyer and Eckhart Rolfs.
[1] Starting in 1952 he did pioneering work in the use of concrete and electronic sounds, joining Pierre Schaefer’s Groupe de Recherche Musicale in 1953.
[1] The concert series Neue Musik München / Klang-Aktionen initiated by Riedl in 1960[1] continues to this day.
[1] As a teacher Riedl influenced the work of musicians well-known today, such as Lorenzo Ferrero and Michael Lentz, for whose Bachmann Prize–winning book Muttersterben (2001) he created the music.