Right now there are eight or nine world-class countertenors, more than at any time since Handel's heyday--and in Handel's heyday, in the mid-18th century, they weren't countertenors in the modern sense but castrati, men who'd kept their otherworldly alto voices with the help of a boyhood surgery. German-born Andreas Scholl, a relative youngster at 33, is already among that select number. Though many of his peers sometimes sound wan in their upper register or have a limited emotional range, Scholl is famous for the robustness and wide expressive spectrum of his "head" voice. His ordinary speaking voice, or chest voice, is a baritone, but like most countertenors he contracts his vocal cords to half their normal length when he uses his head voice, causing them to vibrate at twice the usual rate--a fact Scholl has had verified with fiber-optic instruments. But anatomy alone can't explain the clarion, exquisitely modulated sound that has made him a darling of connoisseurs and conductors of Baroque music. Scholl, who began singing at age seven in his hometown's 650-year-old choir, made his recital debut in Paris in 1993 (filling in for his mentor, Rene Jacobs), and five years later sang his first major operatic role, in Handel's Rodelinda at the Glyndebourne Festival. Scholl often favors lesser-known works from the vast Baroque repertoire, a preference reflected on his recent solo CDs and also the program for his Ravinia debut, which includes three rarely performed Vivaldi compositions: a motet set to Filiae maestae Jerusalem, a nine-movement setting of the psalm Nisi Dominus (which appears on his album of the same name), and Cessate, omai cessate, one of Vivaldi's nearly 40 cantatas. Providing accompaniment is the early-music ensemble Europa Galante, based in Italy and renowned for its mastery of Vivaldi; founder Fabio Biondi conducts and doubles on the violin. Monday, August 6, 8 PM, Martin Theatre, Ravinia Festival, Green Bay and Lake Cook Rds., Highland Park; 847-266-5100.