YOU CAN EASILY SEE what a set designer, lighting designer and the costumiere do at the theater. The work of the sound designer is more ephemeral -- invisible, in fact -- but no less important to creating the "world" of a play.

David Crandall has created the sound design for two current plays, Woolly Mammoth's "Harvey" and Source's "The Golden Boy," revivals that called for entirely different audio approaches. Before designing the sonic architecture, Crandall says he first reads through a script and imagines the sounds emanating from the play. Then, after consulting with the director, he researches and chooses music, if appropriate, orchestrates sound effects and oversees the placement of speakers and other sound devices in the theater.

Crandall worked as an acoustician for a concert hall in Kentucky, and his first theatrical work was for Studio Theater's 1984 adaptation of Arthur Miller's Holocaust play, "Playing For Time." "That was also one of the hardest things I've done," Crandall says. "We had to create whole scenes that happened offstage, and with an almost-bare stage, I had to suggest things entirely with sound," like the squeal of boxcars, the rumble of trucks and the cries of prisoners, guards and dogs in the concentration camp arrival sequence.

As the "overture" and incidental music for "Harvey," Crandall chose excerpts from Igor Stravinsky's 1945 "Ebony Concerto," which sounds like sophisticated cartoon music and happened to be period-accurate. "It was being composed at the same time Mary Chase was writing her script {"Harvey" won the 1944 Pulitzer Prize for drama}. I first tried a new recording -- digitally recorded, beautifully performed -- but it didn't capture the feeling, so I used the old Woody Herman record.

Sound effects required in the play included the sound of boxes falling upstairs. "We used a contact mike on an old oak desk -- you never use what it's supposed to be," Crandall says. "They were testing fire alarms in my apartment building, and I walked out in the hall with my tape recorder -- it was perfect for the fire hall bell in act two. And for Miss Tewksberry, the awful opera singer, we coached {local singer/actress} Pam Bierly on the kind of character we needed, and she sang while I played piano. Then I slowed the tape down a little and fiddled with it to make her sound heavier than she is. They were going to list Pam as the singer in the program, but I said, 'Why don't you offer her that choice?' "

For director Dot Neumann's production of 1937's "Golden Boy," Crandall was faced with two scenes that take place in the dressing room beneath the ring at a boxing arena, with offstage sounds coming through the door and fight sounds coming from overhead. "We split the signal, sending the full range from the side, and putting the low frequencies over the stage.

"It's got to sound not like it's coming from two speakers, but like it's coming from the space," Crandall says. "{'Harvey' director} Jayme Koszyn and I were talking, and I said, 'What we're doing is joinery -- that's what they used to call carpentery. You take this piece of wood, and that piece and fit them together so it looks like they grew that way."

"The best compliment I ever got," Crandall says, "was when I was adjusting the level of a car horn cue in a play. And the director said 'Where's that sound cue?' And I said, 'I've been playing it for 15 minutes.' She said 'I thought that was some guy in the alley with his car.' It's just like with lighting designers. If people go home from the play saying 'What a great light show' -- they didn't watch the play. If they don't notice what you did, in a way, that's great."

Rabbit Rituals: Each night before "Harvey" begins, the Woolly Mammoth actors gather in their comfortable new Green Room for a preperformance pep rally they call the "'Harvey' huddle." They form a circle, one actor dons the ritual rabbit ears, and they yell "Pooka Power!" -- a Pooka being the invisible six-foot rabbit Elwood P. Dowd claims to see. Actors are notoriously superstitious, and when a sign went up on a nearby building during rehearsals, the Woolies took it as a cheering omen. It read: "Harvey Construction."

On Sunday at 6 p.m., Woolly Mammoth will present "Inside 'Harvey,' " a novel lecture-demonstration in which the actors discuss and demonstrate various ways of performing scenes -- and then perform them at the 8 p.m. show. The session is free, but ya gotta pay to stay for the show; reservations are necessary. Call 393-3939.

Making the "Scena": Director Robert McNamara is starting a new theater company called "Scena," which will produce new and classic European plays. The troupe, which includes actors Jane Beard, Chris Henley, Brian Hemmingsen and Richard Mancini, is renting Source Theater's Main Stage from December 15 through February. The plays will be performed in rotating repertory, starting with "Help Wanted" by Franz Xavier Kroetz, who wrote the powerful "The Nest," produced last season at Round House. Next up is Samuel Beckett's "Endgame," and a Polish absurdist comedy called "Tropical Madness" by Stanislaus Ignacy Witkiewicz.

Sweet Charities: The seventh annual Holiday Performance is coming to town. On December 14, actors, directors and technicians from more than a dozen area theaters will collaborate on a stage adaptation of the 100-year-old Sherlock Holmes tale "The Adventure of the Blue Carbuncle," which has a Christmas setting. The performance is sponsored by the League of Washington Theaters; the ticket price is 10 pounds of nonperishable food, which will be distributed by Capital Area Community Food Bank. Call the Cultural Alliance at 638-2406 for details.