Last week we told you about a choir singing in the dark; this week there's something much odder, chamber music in the dark. Inspired by the same Holy Week tradition of Tenebrae services, the String Quartet No. 3 (2001) by Georg Friedrich Haas requires the musicians to play in darkness, as pitch as possible, the better to heighten the mystery of the unusual, sometimes inexplicable sounds it calls for. Advocates for the work, the Jack Quartet has played it across the country; of a New York performance last fall, the Times' Allan Kozinn called the piece "a soundscape like that of a forest at night" and added "It would be hard to think of a more involving way to hear a new work." They'll bring it to the seventh-floor Top of the Town room at the Sorrento Hotel as part of the hotel's "Night School" cultural series (900 Madison St., brownpapertickets.com, $15–$20, 7 p.m. Mon., May 2). The next night, the quartet joins cellist Joshua Roman to play Gesualdo, Ligeti, Xenakis (whose complete quartets they've recorded), and more at Town Hall (1119 Eighth Ave., brownpapertickets.com, $15–$20, 7:30 p.m. Tues., May 3). Among Jack's colleagues in the contemporary-music world is a quartet called Ethel, which draws vigorously on popular-music styles as well as improvises and composes. They'll play here this week as the guests of Simple Measures, a concert series devoted to deformalizing the chamber-music experience. Hear Ethel play Philip Glass and much more at the Mt. Baker Community Club (2811 Mount Rainier Dr. S., 7:30 p.m. Fri., April 29) or Fremont Abbey (4272 Fremont Ave. N., 7:30 p.m. Sat., April 30. $10–$28, simplemeasures.org). Geez, with a little coordination and the addition of a local ensemble or two, this week could have turned into a hot little string-quartet festival, providing an excellent overview of what hungry young musicians are doing to drag this tradition-weighted genre into this century.