The lights dim, the murmuring of the audience fades to quiet, and, after a few silent anticipatory seconds, the wind, string, and brass instruments strike the first festive notes of Petrouchka. This is the Hopkins Symphony Orchestra beginning its Annual Concert for Children and Families, a free performance that draws hundreds. Composed of Hopkins undergraduate and graduate students, faculty, staff, alumni, and community members, the HSO is celebrating its 30th year. Musicians performing in HSO concerts each year are chosen by competitive audition. This year, almost 130 musicians are on the roster, and about 50 of them are Hopkins undergraduate students. Music director Jed Gaylin says the orchestra is dedicated to performing and promoting orchestral and chamber music from the standard and contemporary repertoires at a high artistic level.