The open rehearsal "is something the public doesn't get a chance to see very often," says Miles Hoffman, music director of the Library of Congress Summer Chamber Festival. A traditional feature of the 4-year-old Summer Chamber Festival, open rehearsals offer the curious public "a chance to observe musicians making the acquaintance of the piece."

As rehearsals go, Hoffman predicts this Wednesday's will be "the real McCoy." It will be "the rough cut, earliest stage" of rehearsal, with the musicians assembling to practice John Harbison's Piano Quintet for the first time. Hoffman describes the Harbison Quintet as "an attractive, emotionally persuasive modern piece with an appealing mix of styles. It is modern but not forbidding."

With the musicians in blue jeans and T-shirts the atmosphere is casual. In fact, at last week's rehearsal of Faure''s "La Bonne Chanson," mezzo-soprano Constance Beavon's little daughter wandered up on stage, where, roosting restlessly on her mother's left hip, she chirped "I want it slow" and pressed her hands to her ears during crescendos. Beavon sang all the while, and ended with a "Sorry, guys" after her little critic left. The musicians, with pencils behind ears and shirt sleeves rolled up, cracked jokes, yawned and amiably debated the interpretation of notes and measures.

Next Wednesday's rehearsal, at 11 a.m. in the Library of Congress' Coolidge Auditorium, is open to the public. Concerts will be June 18, 21 and 25, at 8 p.m.