Commissioned by London's Barbican and Amsterdam's Muziekgebouw to commemorate Terry Riley's 80th birthday, these two long pieces tunnel deep into the pulsing rhythms and cycling arpeggios that mark the electronic pioneer's work at its most meditative. On "Outdoor Museum of Fractals," James Holden is joined by tabla player Camilo Tirado, whose bright, slapping tones lend extra definition to Holden's swirling modular-synth patterns. On "555Hz," Luke Abbott channels gravelly synthesizer patches and slowly oscillating drones into mesmerizing waves of sound. Both pieces offer a fitting tribute to a minimalist master.