Though her work is often characterized as minimalist, composer Eliane Radigue is a category unto herself. During the 1950s and ’60s, the Paris resident worked as an assistant to the originators of musique concrete, Pierre Schaeffer and Pierre Henry. But the music she composed after leaving their orbit employed long tones obtained from microphone feedback, tape loops, and synthesizers, and it’s both quiet and demanding; close listening is necessary to perceive the subtle shifts in tonal color that occur as these pitches gradually coalesce and disperse. Just after the turn of the century, Radigue transitioned from playing electronics to composing for acoustic performers, and the first instrumentalist she chose was American cellist Charles Curtis. The essence of their first collaboration, Naldjorlak I (composed between 2005 and 2008), is the wolf tone, which occurs in stringed instruments when the resonant frequency of a bowed string and the resonant frequency of the instrument’s body interact with each other to create a new sound–usually a raw maelstrom of string and wood noise. Cellists generally do whatever they can to avoid wolf tones, but on Naldjorlak I, Curtis sustains and modulates them for three quarters of an hour, obtaining rich layers of rasping and resonance that are every bit as entrancing as Radigue’s electronics of yore. On Curtis’s three-CD survey Performances & Recordings 1998-2018, just released by the Saltern label, he exercises similar devotion to the diverse requirements of a piece by 17th-century Scottish composer Tobias Hume, another by 20th-century 12-tone composer Anton Webern, a Velvet Underground-steeped original named “Music for Awhile,” and a more recent Radigue composition. Curtis will return to Naldjorlak I for this Thursday concert, which is part of the Frequency Festival (booked by former Reader staffer Peter Margasak). The festival also presents performances of Radigue’s music by violist Julia Eckhardt and trumpeter Nate Wooley on Wednesday, February 26, in the Bond Chapel of Rockefeller Memorial Chapel. Curtis will make a nonfestival appearance at the Art Institute that same day, playing over prerecorded drones in response to the work on display in the Alsdorf Galleries of Indian Art. v