The Kennedy Center's Terrace Theater, which has now enjoyed a mixed week of dance, recitals and chamber music, is one of the most beautiful theater-concert halls in the country. However, that hyphen is the key, not only to the various purposes for which the elegant new hall will be used, but also to a major question asked by many of those who attended any of the events during the opening week.

That question, always a vital one with the inauguration of any new hall, is, "How are the acoustics?" In the Terrace Theater, as in each of the Center's three other halls, the acoustics were planned by Cyril M. Harris of Columbia University, to whom the credit for the superb sound of the Kennedy Center Concert Hall, Opera House and Eisenhower Theater is due.

In last Tuesday night's piano recital in the Terrace Theater, Rudolf Serkin played Mozart's A Major Sonata, K. 331, and the posthumous sonata by Schubert in the same key. On Friday night the Tokyo String Quartet played music by Haydn, Ravel and Mendelssohn. And on Saturday, the Theater Chamber Players, heard for the first time in their new home, played music by Barbara Kolb, Stravinsky, Teizo Matsumura, Toru Takemitsu and George Crumb. Their program, with soprano Phyllis Bryn-Julson, string quartet, piano, flutes, clarinets, percussion, banjo and guitar, afforded a wide variety of sounds and dynamic levels.

Comments from a number of musicians in the several audiences, experienced listeners, and the performers themselves, brought out a strikingly uniform view of the new theater's acoustics.

There is general agreement that the sound is perfectly clear throughout the hall, that it is natural in its projection of whatever music is in process. There seems to be no difference in sound at any point in the hall. And there is admiration for the acoustical shell designed especially for this stage, a shell made of wood rather than the more usual Fiberglas.

After making all of these favorable points, a number of musicians said the same thing: that the sound is "slightly dry," "somewhat muted," "a touch lacking in reverberation." Phyllis Bryn-Julson, the excellent soprano on Saturday night, said, "It is a dream of a place to sing in, but I would be happier for my voice if there were a little more life to the sound."

The only exceptions to these views came from those who sat in the first four or five rows. And it is precisely there that the reason for the chief difference between the Terrace Theater and the Coolidge Auditorium of the Library of Congress -- which is almost exactly the same size -- lies: except for the wooden flooring at the front of the Terrace Theater, the area to be used for an orchestra pit when chamber operas or musicals are given, every inch of the new theater is carpeted! True, the carpet is very hardsurfaced. But it lies there in contrast to the bare floor in the Coolidge Auditorium, which still stands as the ideal for chamber music and intimate concerts in this or any city.

Those seated in those first rows on Tuesday night commented that Serkin's piano sounded as natural as if they were seated next to it.

The performance level on the three nights I attended was so uniformly high that two things were possible all the time: to hear the music in something close to perfection in realization, and to wonder why the slight lack of vibrancy in sound existed. Over and over the same answer suggested itself: The carpeting is a mistake.

Against this opinion is the fact that the Terrace Theater is to be used for drama, intimate Broadway shows like the Comden-Green revue that opened there Sunday, and chamber opera, as well as chamber music.

Speaking of this factor, acoustician Harris said, "I think that, in view of the various uses for which the theater is intended, the present acoustical design is the best possible compromise." He made the point that the Terrace Theater is not the same as a "multi-purpose hall" since that term usually refers to a large hall, a situation Harris said "never works."

Introducing Serkin on Tuesday night, Martin Feinstein snapped his fingers for the audience. "That shows you," he said, "how live the acoustics are." But actually, the snap of his fingers showed precisely how dry the sound was.

As far as the performances are concerned, Serkin was in his most genial form, though his Mozart had a severity about it that is not present when he plays the concertos. His Schubert sang nobly.

The Tokyo Quartet was ideal in the Haydn Quartet No. 35, and dreamlike in the Ravel. However, it was in the course of the Ravel that it became very noticeable that there was by no means sufficient difference in the character of their sound when their mutes were in place or removed. The hall consistently mutes too much of the life of the tone.

The same was true during the course of the Chamber Players' characteristically brilliant program, which closed with their celebrated performance of Crumb's "Night of the Four Moons." Every detail was clear, but the natural freedom of tone was restricted.

Washington has, in the Coolidge Auditorium, an ideal, a perfect model of sound for chamber music. The Terrace Theater can match that if minor adjustments are made. Surely the carpeting should be removed from underneath the seats.