Young concertgoers get acquainted with the symphony

''Hum this note very softly,'' said narrator Steven Aveson at the outset of a recent youth concert in Boston's gilt-edged Symphony Hall. The full house of elementary school children, who could barely see over the backs of the seats, hummed in response.

''You've just heard the end of a famous symphony by Ludwig van Beethoven,'' he explained, touching off a ripple of surprised giggles.

While the musicians played selections illustrating theme and variations, the students brought their own interpretations to the music. One youngster in the balcony mimicked the conductor's motions to the rollicking tempos of Charles Ives's Variations on ''America.'' On the main floor, a blue-sweatered boy played an imaginary violin in his seat during a sweeping Haydn piece.

''As much as the youth concert is an educational experience, kids have to come in here and have a good time,'' says Anita Kurland, administrator of youth activities at the Boston Symphony Orchestra.

In the past decade, the Boston Symphony, like most other major or regional orchestras around the country, has taken a more direct educational role to acquaint children in the community with symphonic music and to build future audiences. Today, many youth concerts, traditionally handled by adjunct women's committees, are planned and produced by full-time staff professionals.

''As (school) music departments dwindle, parents, classroom teachers, and orchestras have to fill the gap,'' Mrs. Kurland says.

Even if a parent or teacher has a hard time distinguishing Mozart from Mahler , there are a variety of ways to help children enjoy symphonic music and become comfortable with concertgoing.

As a start, symphony orchestras may hold special family programs on weekends in addition to school-day youth concerts. A few orchestras, such as the Buffalo Philharmonic Orchestra and the Philadelphia Orchestra, are experimenting with ''tiny tots'' programs for toddlers.

Symphony public relations departments often provide free informational materials such as advance program notes for youth concerts, posters of the hall, and illustrations showing the seating arrangement of the orchestra. Many orchestras also offer pre- and post-concert activities such as instrument demonstrations and tours of the hall.

To prepare a child for a youth concert, many symphony youth directors recommend contacting the symphony to find out what selections will be on the program, then playing at least some of the pieces for the child ahead of time. Parents may want to point out key parts to listen for, or make up simple lyrics to the main melody of the symphony for the child to sing.

''There is nothing like hearing a symphony live in a hall,'' says Anita Kurland. ''The thrill is greater if the children have heard the piece on a record player before.''

Record jackets often contain a great deal of information about the music and the composer that parents can share with children. Libraries are also good resources for books about instruments, music vocabulary, and the lives of composers.

''Any kind of preparatory work with kids pays off,'' says Becca Smith, director of the department of educational activities at the Cleveland Orchestra.

Explaining basic concert etiquette can help children know what to expect during the concert and how to behave. The most basic rule to emphasize is the importance of listening quietly. It's also useful for children to know that flashing lights or bells signal that the performance is about to begin and that they should stay in their seats until intermission.

''Children sometimes do not know when to clap,'' Ms. Smith says. ''Explaining visual cues for them to watch for during the concert is helpful. Parents can tell them a piece has not ended until the conductor has his arms down by his side and perhaps has turned around.''

To acquaint children with concertgoing, Joanna Cortwright, educational consultant with the Minnesota Symphony, suggests taking children to as many types of performances as possible. Summer band shell concerts and other informal events are good forerunners to the more formal experience in a symphony hall.

After children attend a youth concert, she recommends that parents play some of the pieces to reinforce what the children have just heard. She also suggests finding inexpensive recordings that children can play on the stereo themselves.

Most important, many symphony youth directors agree, is the adult's own attitude toward classical music and concertgoing.

''The adults accompanying children have to have a positive attitude themselves,'' says Anita Kurland, who would like to see the perception of going to the symphony as elitist disappear.

''Very early kids decide whether or not they are going to be receptive (to symphonic music),'' Evelyn Minie notes. ''A lot depends on what the atmosphere is surrounding their experience. That's where the parent's responsibility comes in.''