Tim Berne, a D.I.Y. Jazz Institution

For the saxophonist and composer Tim Berne, the Brooklyn bar Lowlands is a favorite neighborhood hangout. Lately, it has also become his musical laboratory.

During the past two years, Berne has regularly walked the block and a half from his Gowanus home to the cozy establishment on a sleepy stretch of Third Avenue. Tall, with a moseying gait and a mop of gray hair, he blends in easily. There is no stage, so he and his bandmates, a rotating cast of newer and longtime associates, set up on the floor, amid purple Christmas lights and an illuminated Miller High Life sign.

Passers-by might expect a classic-rock covers gig, but that changes when Berne begins warming up. His alto sound, chiseled and neon-bright, cuts through the space like a laser beam. It’s an instant reminder that one of the true thought leaders in progressive jazz — an unassuming yet undeniable force in the music for more than four decades — is still operating at peak strength as he approaches his 70th birthday on Wednesday. (He returns to Lowlands on Oct. 22.)

“That Lowlands gig, he takes it as seriously as if he was going to be playing at Carnegie Hall,” the guitarist Bill Frisell, a frequent Berne collaborator in the ’80s who has re-entered his orbit, said in a video interview. “It’s like his life is on the line.”

Frisell said that after their early work together, he didn’t keep close tabs on Berne’s music. “I heard him again, and it was like, ‘Man alive,’” he said, going on to describe “how these germs of ideas that were there at the beginning had blossomed and expanded, and his sense of all the parts — the melody, the rhythm, the harmony, the counterpoint — all of that had just kept on getting richer and richer.”