Every week for 15 weeks starting in December 2007, Adam Sonderberg–best known as one-third of Haptic–and Salvatore Dellaria posted a new piece on their Web site, creating a series of experimental sketches as part of a project named “Untitled_Ongoing.” Sonderberg and Dellaria, who’ve worked together in various contexts since 1998, hoped to rework and develop the material for an eventual album, but heard as a collection the pieces can stand on their own–the MP3 files remain available at their site.

Many of the track titles simply describe instrumentation (“Snare Drum,” “Piano”), while others reference a musical approach (“Musique Concrete,” “Indeterminacy”). You can certainly identify specific sounds here and there–guitar-amp noise, metallic rustling, bowed percussion, urban field recordings–but the work revolves around abstract texture and gesture. Most of the pieces range in duration from two to five minutes, and manage to convey a rewarding sense of development in that brief time.

Those experiments weren’t used on the recently released Safety (either/OAR), the superb second album by the Dropp Ensemble, a studio collective with only Sonderberg and Dellaria as steady members (“Dropp” is pronounced “drope,” and means “drip feed” in Swedish). But in the four new pieces here, you can hear where they might’ve been going with them. The album includes contributions from an international cast of experimentalists (Olivia Block, Christian Weber, Tomas Korber, Jason Kahn), who mostly sent sound files via e-mail for Sonderberg and Dellaria to process and adapt.

Compared with the duo pieces, the Dropp Ensemble material is infinitely more elaborate and elusive. Much of Sonderberg’s music puts great emphasis on resonant, rumbling low end, and these beautifully billowing, fluid works are no exception. Sometimes you can make out what’s clearly an instrument, like Weber’s arco double-bass scrapes on the brief opener, “Inlet,” but most of the time the various components are too slippery to indentify–just tones, washes, and flutters. Close listening reveals a wealth of patiently shifting detail–the way rhythmic fluctuations in long tones change pulse, the exquisite layering of disparate colors, the aural friction created by the pairing of oppositional sounds. It’s a minimalist gem.

According the promotional material for the new album, Dellaria is “now purportedly retired from music,” but Sonderberg continues to play. On Saturday he’ll perform in a duo with Haptic cohort Joseph Clayton Mills at Enemy.

Today’s playlist:

Various artists, Obsession (Bully)
Culture, Culture & the Deejays at Joe Gibbs 1977-79 (17 North Parade)
Louis Moholo-Moholo and Marilyn Crispell, Sibanye (We Are One) (Intakt)
Han Bennink, Michiel Borstlap, and Ernst Glerum, Monk Volume One (Gramercy Park)
Charles Mingus, At UCLA 1965 (Sue Mingus Music/Sunnyside)