After composing 1980’s The Expanding Universe on software proprietary to Bell Laboratories, New Yorker Laurie Spiegel designed her own computer-music system, Music Mouse. (It was eventually commercialized for Macintosh, Amiga, and Atari.) The long-out-of-print Unseen Worlds, from 1991, is proof of her instrument’s expressive capabilities: Metallic drones shape-shift like thunderheads, while plucked and hammered string-like sounds suggest futuristic harpsichords. The album is darker and more ominous than her debut, less minimalist and more atmospheric; “The Hollows,” in particular, points the way for decades of dark ambient to come. But these compositions are as searching as any of her other work, with an emphasis on unfamiliar timbres and unexplored terrain; the brief “Strand of Life (‘Viroid’),” meanwhile, rises like a single blade of grass from an alien landscape, beautiful in its familiarity and simplicity.