Mwata Bowden tends to get overlooked when the discussion turns to modern masters of the A.A.C.M. (Association for the Advancement of Creative Musicians); perhaps it's because of his quiet demeanor and unprepossessing approach to performance. But don't be fooled: beneath that surface lies one lionhearted woodwinds player. Often concentrating on rhythmic elements in his own playing, Bowden brings an uncluttered directness to a wide variety of saxophones, clarinets, and flutes, on tunes that are distinctly structured but still allow for extended, open-ended soloing. His improvisations on all his instruments share a sort of short-legged phrasing--his ideas come in small bursts, almost as if they were out of breath--and this imparts a terrific urgency to his statements. Like Bowden himself, his music is unintimidating but impressive. His band, Sound Spectrum, is locked in with Yosef Ben Israel on bass and Avyeerel Rah on drums, and it gets a lot of mileage out of Ari Brown, who usually solos at the piano but occasionally uses his tenor sax to augment Bowden's horns. This is the second event in the A.A.C.M.'s spring concert series celebrating the organization's 25th anniversary, ten concerts in all through the end of June. (The series kicked off April 25 with Edward Wilkerson's Eight Bold Souls, the much-praised band in which Bowden plays a key role.) Thursday, 8 PM, Hot House, 1569 N. Milwaukee; 235-2334.