IT'S EARLY Sunday evening and musicians are trickling in to the Civic--musicians in flip flops, musicians in blue jeans, musicians in stylish haircuts. Slowly the orchestra comes awake in a chaos of indelicate morning noises: bleeps and sour yawns and fragments of melody abandoned before they've started make sense. Tuning up, the two harpists strain to hear the bell-like tones of their instruments, then fall to chatting. In front of the auditorium's empty section D the cellists lean their slick plastic cases against the seats and stand talking like mothers at a soccer game.

Responding to some unseen cue, the milling musicians take their places on the risers. A few words, on this first day of rehearsal, from Ellen Primack, executive director of the Cabrillo Festival of Contemporary Music. Then Maestra Marin Alsop sets down her Diet Coke and takes the podium.

It's hard to hear what she's saying to the orchestra--most of us who have come to this open rehearsal are seated in the back of the auditorium--but we get the gist. "...Twenty percent Death of A Salesman," she's saying, and it's soon clear what she means. Brett Dean's Moments of Bliss, which receives its U.S. premier on Saturday night, rises and falls repeatedly in ever more frenzied cadences of classical, jazz and pop culture sound bites, straining for something just out of reach.

This is the first time these musicians have played it together, and to my ear it's astoundingly polished, but Alsop detects rough places. She asks a bass player for more attack, goes back to the score's letter D, then letter F. The audience is rapt.

By the time I leave, maybe 60 music fans have crept into the auditorium to watch this process. Open rehearsals are a tradition at the Cabrillo Music Festival. If you can afford one concert but not three, you can hear the other two nights' offerings anyway. The festival even offers pre-rehearsal talks to help listeners frame the compositions (this week's is Thursday at 6:45pm, when Phil Collins discusses Azul). The free offerings don't end there, either; today at 5:15pm the festival presents "In the Works," with new conductors directing the work of new composers. It all adds up to an extraordinary experience anyone can afford.

OPEN REHEARSALS are Aug. 5-6 and Aug. 11-14 at the Civic, 307 Church St., Santa Cruz. For schedule see www.cabrillomusic.org and click on "education."

Send a letter to the editor about this story.