“ON AIR: The Sound of the Material in the Art of the 1950s to 1970s,” an exhibition excavating the pre-history of contemporary sound art, opens at Kaiser Wilhelm Museum in Krefeld, Germany. Assemblies, experiments, and media by Joseph Beuys, Bruce Nauman, David Tudor, and co-presenter Museum Tinguely’s namesake Jean Tinguley, demonstrate how “sounds, tones, noises, signals, and voices became ‘substantial’ sculptural material” in the second half of the 20th century.