Trem opens with calliope toots that shade off into the protests of unoiled door-hinges; rills of water briefly well up then sink down again. Behind this foreground a series of low, metallic clangs is audible, watery and ambiguous in pitch, rather like the unpitched percussion in The Sinking of the Titanic and other Gavin Bryars pieces. The improvisation now turns toward more familiar prepared-piano sounds, harmonics released patiently into the air from strings that are simultaneously plucked and muffled. Pitch is constantly in flux, and though the sounds are produced acoustically they mimic the textures of electronic music: a low drawn-out sound, for instance, will warp in slow-motion as if the tape were playing at half-speed. The improvisation is rounded off by an unearthly shriek ground out at length with the bow.

Little of all this is identifiable as issuing from a harp; the same goes for most of the rest of Trem, harpist Rhodri Davies' first solo disc. He bows or strikes strings as often as he plucks them; the instrument is fettered with metal clips or played in conjunction with percussion; the tuning key is used to unpitch the strings and is itself applied percussively to the instrument. (None of this is spelled out in the sleeve notes: I'm relying on my memory of live performances.) One track, "Trem" itself, stands apart from the rest. Its most prominent feature is the quiet but grating white noise of a prerecorded tape. Davies intervenes in this enveloping sound-environment with small dabs of percussion.

The eight performances on Trem were recorded on three different occasions at London's All Angels Church, where Davies and Mark Wastell curated a much-lauded improvisation series for three years. The disc is in many ways an encounter between the twin resonating chambers of the chapel and the harp itself. It's a mesmerizing study in catacoustics: even the simplest sounds ripple and bend, as if traveling through water rather than air. Harpists will undoubtedly be giving Trem close study in years to come for its mapping out of new territory for the instrument; the rest of us can simply sit back and enjoy one of the most compelling improv releases of the past year.