DAR Constitution Hall, at 18th and D streets NW, was once the epicenter of classical music in Washington. It was not only home to the National Symphony Orchestra for that ensemble's first 40 years, but it has been the site of countless recitals by the great, the near-great and the long-forgotten, both before and after the Kennedy Center opened in 1971.

And now preparations are underway to make it the temporary home of the Washington Opera. On Feb. 22 the curtain will rise on a new production of Giuseppe Verdi's "Aida," conducted by Heinz Fricke, directed by Paolo Micciche, with Maria Guleghina in the title role. The opera company's move -- roughly a mile to the east of its usual climes -- is made necessary by the renovation of the Kennedy Center Opera House, which is expected to consume the rest of 2003. And it will take place in a newly reconstituted Constitution Hall.

Longtime concertgoers have been shaking their heads. An opera house? With that tiny stage? In that vast, cavernous diffusion of bluish space?

An opera house. Over the past few months, big changes have been effected at Constitution Hall. Eighteen rows of seats have been removed from the main floor, and the hall's capacity has been reduced from a leviathan 3,700 to a merely gargantuan 2,900. A wide, long stage will thrust out into the auditorium, permitting the audience to surround the drama on three sides. For want of an orchestra pit, the musicians will be located at the back of the stage, behind the sets and singers. The conductor will keep an eye on the action by watching a monitor near the podium, while the singers will follow the conductor through judicious use of television screens located along the footlights and on some of the bridges over the doorways to the auditorium.

Complicated? To be sure -- but if it were easy, it wouldn't be opera. And according to the Washington Opera's new artistic administrator, Christina Scheppelmann, who oversaw a similar implementation during her tenure with the San Francisco Opera, "conductors are able to adapt well to this new orientation, and stage directors love it because it removes a physical barrier between the performers and the audience, allowing great intimacy."

The biggest question -- as always with Constitution Hall -- is the acoustics. As Ted Libbey observed in his engaging and authoritative history of the NSO, "the boomy vastness of the place was impossible -- the music simply died on the stage." Antal Dorati, who led the NSO from 1970 to 1977, put it even more strongly: The hall, he said, had "acoustics to take one's shirt off."

Mark Holden, the president of Jaffe Holden Acoustics, which has been engaged to improve the sound, acknowledged the challenge. "Constitution Hall was never designed as a concert hall or an opera house," he said in an interview. "It is very large and very wide, and it was designed as a big lecture hall or a conference room. And so, as a place to hear music, it's not a bad conference room."

First lady Grace Coolidge presided over the laying of the cornerstone for Constitution Hall on Oct. 30, 1928; it opened to the public the following April. The space was specifically planned to accommodate the meetings and presentations of the Daughters of the American Revolution, which owns it to this day. Still, as the largest auditorium in the nation's capital, it was quickly adopted for concert use, and the first musical event, featuring violinist Efrem Zimbalist and soprano Anna Case, took place Nov. 2, 1929. An ad hoc ensemble called the National Symphony Orchestra of Washington, D.C., played its initial concert at Constitution Hall on Jan. 31, 1930; the following year it was incorporated as the National Symphony Orchestra. Until the 1950s, the hall had a glass ceiling and a view of the stars.

Acoustician Holden declared himself heartened by the changes his company has made in Constitution Hall over the past few months. "Our first and greatest challenge was to find a way to quiet the hall down -- to keep the sounds of the old-fashioned exhaust fans and mechanical systems from overwhelming the sounds of the musicians. And then we added some acoustic reflectors on the back of the stage and on the ceiling to keep the sound from getting lost. We had a test concert here and we listened to the music from various positions throughout the hall -- and it did not sound bad at all."

The most controversial "improvement" to the hall would be the addition of what the Washington Opera calls "a subtle electro-acoustical enhancement system." This is in the process of being installed in the overhead rigging system but will not be activated unless the company deems it absolutely necessary after performing some fully staged rehearsals with orchestra. According to the Washington Opera, the "enhancement" system will be both nearly invisible and barely audible, assisting only with what the opera calls a "minimal sound dropoff when the singers turn away."

Holden draws a clear distinction between "enhancement" and "amplification." "There will be no microphones on the singers or anything like that," he said. "What we are talking about is area enhancement, area miking, just a little of it, so that people on the left side of the hall can hear what is happening on the right side."

Although the use of microphones and electronics is commonplace in popular music and on Broadway, it is disdained by many operaphiles as a corruption. "Some critics think it nothing less than the end of opera as we know it," according to Patrick J. Smith, the former editor of Opera News and the author of "The Tenth Muse: AHistorical Study of the Opera Libretto." "For me, it becomes a problem when the sound of the voice does not come from the throat of the singer, but seemingly either from the entire stage or from a loudspeaker positioned outside the stage area."

For Placido Domingo, the Washington Opera's artistic director, the question was simply one of necessity. "If people come to the opera, they have the right to hear what is happening," he said this week as he conducted a tour of the hall's reconstruction. "We will not know for another couple of weeks, until the rehearsals, whether or not we have to turn the enhancement on. But if we have to turn it on, we will.

"The year will go quickly -- and we'll be back at the Kennedy Center next spring, where no enhancement will be necessary," he said. "But who knows? A lot of work has been done at Constitution Hall, and it may be that we'll continue to do a production here every season. We'll see how it goes."

To be continued . . .

Placido Domingo and opera technical director Noel Uzemack check out the Constitution Hall changes.A computer-generated model of the renovated Constitution Hall, with a new design to accommodate opera.