The latest fashion for theatergoers at Arena Stage is a set of earphones. Last Sunday was the first time the headgear was seen there. Some members of the audience within earshot were wondering what it was all about, why some people wore the earphones that looked like Walkman sets and why one man -- who was Cody Pfanstiehl, Metro's director of public affairs -- was perched on a folding chair off the main aisle wearing what looked like a small feedbag over his mouth. The feedbag was a steno mask with a microphone; Pfanstiehl was talking into it, and from his neck there dangled a transmitter half the width of his tie.

He was describing the action on stage to blind and near-blind people in the audience who wore earphones wired to an FM receiver the size of a cigarette pack. The earphones were worn off-center so the listeners could hear the dialogue with one ear and Pfanstiehl with the other.

"The Phonic Ear," an innovation that the people at Arena Stage think may be a first, is available to 20 patrons at opening nights, Saturday matinees and Sunday-evening performances for any of the eight annual subscription plays.

Twenty minutes before a performance, a tape is transmitted describing the characters and the actors and actresses who play them, the costumes, the stage setting and lighting, and details about the playwright of the current production, such as, for example, that Shaw wrote "Major Barbara" late in life and that his mother drank too much.

As the play is about to begin, the live commentary starts. A voice speaks quietly, evenly, almost soothingly: The lights are dimming a bit now . . . There are two chairs on one side of the stage, two potted palms in diagonal corners . . . Now the lights turn all the way down. The place is in total darkness . . . The lights come up. Lady Britomart sitting at the table . . .

The narrator sometimes intersperses descriptions in staccato fashion, squeezing them adeptly between dialogue: Sarah. Purple dress. Leg-of-mutton sleeves. Flat, swooping hat with two shades of purple. Late in the play, in the munitions factory owned by Major Barbara's father, a large cannon is the centerpiece: Stagehands come out again dressed as Salvation Army people . . . And we see coming out of the corner . . . a gigantic cannon. About 41/2 feet high, 14 feet long. Spotlight. The lights come up. The cannon . . . pointing right at the audience . . .

"Andrew! Andrew!"

Lady Britomart, carrying a bunch of red, red roses. He takes the dozen long-stemmed red roses and smells them . . .

After the play, Oral Miller, a lawyer with the American Council of the Blind, said, "It's all the difference in the world." Though "Major Barbara" is very verbal and intellectual, he felt there were many details he wouldn't have known from just listening. "In other plays the need to describe would be even greater," Miller said. "In a fast- moving play like this the description has to be thrown in quickly." For example, how one character threw up his coattails to sit down was to Miller "a description you don't get without relying on a sighted friend . . ."

Scott Marshall, who works in legislative affairs for the council, said, "It's a very difficult job for the narrator to talk enough but not too much in order to give the important details but not mask the dialogue."

"I like live performances of things because you do get a much different feeling," he said. "There's a certain energy that is there when you are there in person rather than just when you are listening to something on the radio or TV. It's like going to a concert as opposed to listening to the radio." Until now, the blind and near-blind have had to rely on sounds of movementmade on stage and whispered accounts from their sighted friends. The new method, which uses volunteers who are trained commentators, may work better and should eliminate any shushing from the rest of the audience. At the moment there are 17 volunteer commentators. Part of the screening process was to watch and describe old movies, like "Love Story" and "African Queen" (which turned out to be the best, with quiet times in dialogue). In "Turning Point," recalled Pfanstiehl, there was a scene where "the eternal dance" was going on: "That one was interesting. It got to be almost explicit. We found you can't have any value judgments." They can't giggle or say things like "You won't believe this." One trainee who didn't make it was a critic, who during a ballet scene said "That's a lousy plie."

"We are developing a new kind of art in this commentary," said Pfanstiehl. At first they weren't sure they should describe colors; but most visually impaired people have had sight. "What you are talking to is someone with residual memory of what red is," said Pfanstiehl. A volunteer needs a good vocabulary and the ability to express himself in short, automatic phrases. As they go along they learn from each other -- key phrases like "he steepled his hands."

Richard Bryant, who does public relations for Arena and will be a commentator in "A Lesson from Aloes" at the Kreeger, said, "It's a little like sportscasting except you know what the play is going to be."

You have to ride the laughs. You learn to anticipate the sight gag, Pfanstiehl said: "You have to describe it a split-second before it comes on the stage so that the laughter comes for them at the same time as it does for the audience."

Arena's helpmate in the system's development has been The Washington Ear, a radio service for the visually impaired. Funds came from a grant from the Public Welfare Foundation. Margaret Rockwell, founder and president of The Washington Ear, said that she hopes that what the organization learns now with the Phonic Ear at Arena Stage can be used to describe other public events or even enhance television.

"I like to think that it's like the voice of a friend," she said, "quietly coming into your ear and telling you all you want to know." THE PHONIC EAR is available by reservation from the Arena Stage box office, 488-3300.