WEILL RECITAL HALL OPENS AT CARNEGIE

The other, smaller shoe fell yesterday, and now Carnegie Hall - all of it - has been restored and reopened to the public. Yesterday saw the formal unveiling of the old Carnegie Recital Hall, now formally renamed the Joan and Sanford I. Weill Recital Hall at Carnegie Hall.

Although the Weill Recital Hall may seem less central to the mainstream of New York's classical-music life, the 268-seat theater is vitally important to the recitalists and chamber groups that perform there. And here the renovations both visual and acoustical have been more radical than those effected in Carnegie Hall proper.

Yesterday's daytime events -which were to be followed by the Juilliard String Quartet in the hall's first evening concert - inaugurated a weeklong festival sponsored by Carnegie Hall itself.

The day began with a ribbon-cutting ceremony attended by the Weills, the hall's two managing directors -Judith Arron, who is artistic director, and Norman Belknap, who is involved with financial matters - and the singers Betty Allen, Martina Arroyo and Roberta Peters. Confusion Averted

''It's like a little jewel,'' Mr. Weill said proudly. The founder of the Shearson-Lehman Brothers investment concern, Mr. Weill donated $2.5 million to the Campaign for Carnegie Hall and serves as the drive's co-chairman. Although he had not earmarked his own gift specifically for the Recital Hall, Isaac Stern and James D. Wolfensohn, Carnegie's president and chairman, decided to honor him and his wife by renaming it. That act will also minimize the inadvertent or deliberate confusion by artists who appear in the small hall and then claim back home that they performed ''in Carnegie Hall.''

After the ceremony came a closed morning concert for emotionally and learning disabled children by the Annapolis Brass Quintet, followed by the day's two public events: the first of a series of lunchtime, hourlong recitals - yesterday by the American baritone William Parker in a recital of 20th-century American songs - and, at 3 P.M., an ''Informance,'' which is what Affiliate Artists calls its informal discussions and recitals. This one offered the violinist Liba Shacht and the violist Toby Hoffman.

The Weill Recital Hall is on the third floor of Carnegie Hall and has existed in a variety of forms over its 95 years; hence its restoration had more accumulated layers of history to peel away. When the ''Music Hall founded by Andrew Carnegie'' opened in 1891 (it cost $2 million to build, everything included, compared with the present $50 million restoration), what is now the Recital Hall was called the Chamber Music Hall. What is now the Carnegie Hall Cinema, in turn, was the Recital Hall. Eventually, the Chamber Music Hall became the Recital Hall, but over the years, architectural details were obscured or removed and a false proscenium arch was inserted.

The current Recital Hall restoration has been overseen by the same architects, James Stewart Polshek and Partners, and acoustician, Abe Melzer, who worked on the main hall, and the building was again carried out by the Tishman Construction Corporation. Several Phases of Work

The work involved several steps. The street-level lobby area and stairwell were redone three years ago. Now the orchestra-level floor of the hall itself has been raked and replaced with mahogany, the seats have been reduced in number from 283 to 268, the false proscenium has been removed, plastering details have been repainted and gilded, a new altar-like niche has been installed at the rear of the stage, a new stage - shallower and lower than its predecessor - has been installed, and three large chandeliers have been placed overhead, along with matching lighting fixtures elsewhere. The construction completed in the Recital Hall since May has cost $1.2 million, Mr. Belknap said.

The visual result is far warmer than the old Recital Hall. The cream-colored painting and lighting, which echo the main hall, lend the space a welcoming ambiance, and while the third-floor entrance is still cramped, the overall impact is far friendlier. One dressing room has been added backstage, but new lobby and backstage facilities are promised when the adjacent office tower is completed in several years. Until then, the old, unrenovated Blue Room and restrooms on the second floor will continue to serve Recital Hall audiences.

Unfortunately, on a first day's hearing, the revamped acoustics seem, if anything, colder and harder than before. Carnegie Recital Hall was a constricted acoustical space, and became overbearing with a loud singer or pianist or percussion-laden chamber group. Now, the slightly increased interior space and less absorptive materials used for the seats and carpeting have added a small amount of reverberation. But the end effect - certainly for Mr. Parker but even for a Mozart violin-viola duo -was harsh and wearing. Sound Has Changed

Mr. Parker and his accompanist, William Huckaby, said that the sound for the performers onstage was better than it had been before - ''clear as a bell,'' in Mr. Parker's words. But from out front, even applause sounded harsh and percussive, and when Ms. Shacht spoke from the stage, her words were overly resonant and unclear.

Mrs. Arron said yesterday that one disadvantage of a splashy opening festival like this week's was that the artists didn't have time to acclimatize themselves to the hall's new acoustics. Mr. Belknap added that as late as Sunday afternon, Weill Hall was full of dropcloths and ladders, and every painted surface was still wet to the touch.

''I wanted to open with a bang and get everyone's attention, but there was no chance for any of the artists to adjust to the hall,'' Mrs. Arron said. ''I hated the sound of the old Recital Hall. If eventually we decide that the new one doesn't work, then we've got to do more. I think we're very much committed to doing what we have to do.''

Before any further renovations need be undertaken, however, a consensus of opinion must be reached, and that will only take place over the next few weeks and months. This week alone offers 12:30 P.M. recitals by the pianists Jeffrey Biegel and Michal Tal, the cellist Sara Sant'Ambrogio and the Franciscan String Quartet, and evening concerts by the jazz pianist Ramsey Lewis, the Empire Brass, the baritone Benjamin Luxon with the folk tenor Bill Crofut and, on Friday, a chamber ensemble led by Mr. Stern.

Heady fare, and a fitting welcome back into our musical life for one of this city's smallest but most important halls.