One of the wonderful facets of free improvisation is that, unlike more formal music, practitioners aren't limited to certain instruments.

Thus you have this unbridled session of stirring improv performed on shakuhachi or Japanese bamboo flute; accordion retuned with just intonation; minimalist piano and string bass. The background of the four musicians couldn't be more different either. Bay area shakuhachi player Philip Gelb, who brings a unique Occidental concept to his instrument, is as likely to collaborate with multi instrumentalist Joe McPhee, or interactive electronics composer Chris Brown as with koto master, Shoko Hikage. Accordionist Pauline Oliveros has been composing so-called serious music for 50 years and has a long history of creating electronic and minimalist works.

Canadian born, Los Angeles-based Dana Reason works regularly with Oliveros and Gelb, as well as other explorers such as trombonist George Lewis, and is most interested in the byproducts of the piano that lie in between the black and white keys. American bassist Barre Phillips, who recorded a solo session as long ago as 1968, expatriated permanently to France around that time. Over the years he literally worked with everyone in avant jazz, improv and New music from saxophonist Archie Shepp to guitarist Derek Bailey.

Simultaneously backdrop and foreground, the effort makes you want to begin again when the CD finishes. Perhaps it's because the 12 tunes are all instant compositions, recorded live on the spot by the four. Louder most of the time than one might figure, considering Oliveros' commitment to deep listening and minimalism, even the quietest passages feature the sort of aggression one associates with free jazz, despite any denials towards the music these four would probably proffer.

Certainly all have been exposed to jazz, and Phillips has played it for a long time. Moreover like a bassist functioning in a jazz combo, the vigor of his long-lined pizzicato forays seems to be the fulcrum on which the compositions revolve. It's probably him, in fact, who adds the percussion-like underpinning on some of the tracks, Not that anything swings in a jazz sense, but the proceedings certainly move along at a powerful clip, lacking those awkward, prolonged silences that sometimes arrive in more self-conscious new music.

Gelb, too, is a marvel. If you didn't know his implement of choice was the shakuhachi, from the evidence here you'd think it was the metal flute, the soprano saxophone or perhaps both. Capable of high-pitched, ethereal, overblowing asides and basso tones, he still makes it a point not to stand out from the ensemble, but to blend with the others.

Banishing any thought of Lawrence Welk -- or for you veteran jazzbos Art Van Damme -- from her accordion association, Oliveros appears to have discovered the perfect musical outlet. Without abandoning its traditional resonance she can make the squeeze box sound like a very large harmonica or a sideways, elastic piano. The only dilemma created by this style, though, is that Reason, the band's regular keyboardist, finds herself mostly confined to contributing single notes or the occasional quick run reminiscent of John Tilbury's work with AMM.

As a group effort, these 62 minutes plus of creativity pass by like five, without, it seems, a bum note on disc. Overall, this product of two young virtuosi and two veteran tonal explorers as a quartet of unconventional instruments is memorable in its audacity.