Two of the people who invented post-World War II classical music appeared simultaneously in Washington last night: John Cage at the Terrace Theater in the Kennedy Center's American Composers Series and Vladimir Ussachevsky at the Library of Congress.

Cage needs no introduction; Ussachevsky does.

Cage is quite simply the ranking iconoclast and idea-man of contemporary music and the father figure of post-avant-garde experimentation. He has been called the most influential American composer in the history of music.

Ussachevsky, in 1951, began to experiment with a new toy, the tape recorder, as a medium for creating new music and became one of the founding fathers of electronic music -- a once weird and now accepted form that can be heard today in popular film sound tracks and television commercials. He composed the incidental music for Orson Welles' New York production of Shakespeare's "King Lear" and the sound track for the film version of Sartre's "No Exit," as well as concert pieces for tape, or for tape and live performers. While Cage was giving a speech and hearing a concert of his music at the Terrace, Ussachevsky was speaking on "Musical Imagination in the Machine Age" at the Library of Congress and hearing a concert of his music.

This is how it all sounded:

Most of the instruments for John Cage's "Branches," a dance piece composed for Merce Cunnningham in the 1970s, are moderately exotic: rattles, strings of wooden beads, clappers, sticks set loosely into a framework so that they click delicately when shaken, a gourd, a wooden box containing beads or possibly pebbles, and a couple of twangy pieces of metal.

But that's not all. There is something different, something organic -- four varieties of cactus in assorted sizes and shapes that produce an intriguing array of groans, plunks, throbs and rumbles when they are stroked, scratched, rubbed or plucked. They are wired for sound (painlessly, one hopes), so that the sonic results can go far beyond what would be expected watching the physical actions.

How do cactus virtuosos perform? Very carefully. The four cactus-players of the Nexus Percussion Ensemble approached their instruments delicately, even lovingly, choreographing their hand movements with exquisite care. The results were a beautifully subtle sound texture that had, in fact, something organic about it -- nothing as developed as the sound of a human voice, but sometimes a feeling like the beating of a human pulse.

The piece seemed to be in three sections: two extended movements of percussion music separated by a shorter, contrasting middle section that sounded like an excerpt from Cage's famous 4'33" (alias "Four Minutes, 33 Seconds"). The four players sat completely still with meditative looks on their faces amid total silence -- which was not, of course, total silence. First, one could hear the sound of the Terrace Theater's ventilation system, then subdued coughs, a rustle of paper, a tentative beginning of what might have become applause from someone who thought the music might be over, the sound of people shifting in their seats, clearing their throats.

In this bit of self-plagiarism, Cage demonstrated once again that silence is quite a noisy business and implicitly asserted (if he did not quite prove) that any sound can be accepted as music. The gimmick probably works best, in fact, as a contrasting section in a piece that also includes voluntary, controlled noises--though I would not say that it is pointless as a stand-alone composition. In "Branches," it helped to emphasize the unusual delicacy of most sound textures in the outer movements, and it actually enhanced the impact of the music.

"Branches" was, for most of its length, an intriguing, refreshing experience. But, like the other two compositions on the program ("Third Construction" for percussion ensemble and "16 Dances" for four percussionists, piano, trumpet, flute, violin, cello and conductor), it overstayed its welcome. Cage seems to compose for people who are somewhat more patient than the average concert audience, or who are able to immerse themselves in the mystique of the music to a point where their capacity for boredom is suspended.

On the whole, the ideas underlying Cage's music are more interesting than the music itself, which may help to explain why he is an enormously influential composer whose music -- even the music that makes noise -- is rather seldom heard. But having survived past his 70th birthday, he has developed a significant following. The audience, which looked like a combination of the standard chamber music crowd with a contingent of (fairly affluent) punk and New Wave rock fans, applauded him with unusually warm and obviously heartfelt enthusiasm.

J.M.

In the public mind, electronic music has gained a name for utter impenetrability rivaled only by easier things such as Sanskrit or, even more aptly, the defense budget.

Vladimir Ussachevsky, a droll and graceful man of 71, admits it is partly his fault. One night 30 years ago at a concert at Columbia University he started electronic music as we know it. His first pieces performed with knobs and dials didn't become instant hits.

"I remember that night at Columbia, May 9, 1952," he told an audience that gathered last night to do honor to the man and to hear his highly wired works. "Afterward there was a panel, and Virgil Thomson was the moderator, and he was handling things with all his usual diplomacy. But somebody stood up, referred to my music and asked 'Have you visited any of the other planets recently?' "

Then, standing there last night on the Coolidge Auditorium stage, he spread the lapels of his natty dark suit with vest and declared, "You will notice I am not in a spacesuit tonight either."

But have no doubt about it, genteel and professorial as the first chairman of the Columbia-Princeton Electronic Music Center -- electronic music's colossus -- may appear, he's still way out there on the fringe instrumentally.

Last night Ussachevsky compared the enormous expansion of music's sonic range that has been brought about with magnetic tape, the synthesizer and now the computer to the impact of the grand piano, and how the grand's development made possible an era of keyboard music of "unprecedented depth and richness."

But he knows better. Discovering the limits of this musical technology is not even close. "It will be years before we know enough about the computers," he granted.

Thirty years ago, it was mostly cut and paste with the newly created tapes. But it gave composers a direct control over the exact sound of music for the first time, without the unreliability of the pesky performer. And because tape could be run at any speed, and either forward or backward, the composer could do just about anything with the sound he wanted.

Ussachevsky played a few examples on his machine. There was this huge gong sound. And then there was a pause and there was a huger one. A huge Chinese gong? It was the triangle. The first time it was played at a speed that made it sound four octaves lower than it really is. And the second time it was five octaves lower. Single-pitched instruments could suddenly be made any pitch.

In fact, technology will "probably" make it possible to take any sound, he said, and make any other sound from it.

After his speech there was a concert of his works.

One of the most famous came first, "Piece for Tape Recorder." Among the basic ingredients: a tugboat motor, lots of gongs, melodies constructed of doctored sounds of cymbals, and that was just the beginning. "I hope it adds up to a musical experience," Ussachevsky told his admirers.

But the work is not so esoteric -- it has been rather popular as background music: "Once on TV in an Egyptian march, once in a robbery scene and also in the whorehouse scene in the movie of Joyce's 'Ulysses.' "

In more recent works, Ussachevsky has been more inclined to use tape in chamber combinations with other instruments -- especially the oboe, for which he has an apparent passion. The "Pentagram" for oboe and tape had some wonderful combinations and was brilliantly played by oboist James Ostryneic.

There was a lovely choral work, which sounded like a blend of Stravinsky with Ives, called "Spell of Creation." It was so lovely that Norman Scribner and his chamber choir repeated it, with the composer still at his knobs and dials.

Finally there was the world premiere of a new work for oboe and piano called "Triskelion," a fancy word for a three-movement rhapsody or sonata. But there was no taped music. "I just wanted to be sure I could write an instrumental work," he observed with a twinkle. He sure can. It had all of his fascination with wild sonorities without any of his normal tools. And at the end, just to clear the air, he moved into a perfectly delightful jazz romp that sounded like about two bars Gershwin for every other bar of Schoenberg. It was great fun.

L.T.