The main exhibitors' hall sounded like a giant birdcage yesterday afternoon at the Capital Hilton - a birdcage where someone had mixed LSD in the food and a pack of wild canaries were bombed out of their skulls.

From one corner, you might hear a fragment of Edgard Varese, a wisp of Debussy from another, and, floating in the distance, cool and gently arched, a melody of Johann Sebastian Bach.

The sixth annual convention of the National Flute Association was in full swing, and with dozens of flutists sampling new instruments - in a variety of keys and sometimes no key at all - the noise level often twittered at the pain threshold.

Most of the association's members (825 in a total membership of about 1200) flocked into the Hilton for a marathon weekend of concerts, lectures, formal receptions and informal parties, a young artists' competition and lectures on such arcane subjects as "Harmonic Implication of Unaccompanied Flute Music" or "Quantz Reconsidered."

A predominantly young crowd - quiet (except when trying out a new instrument), studious and gregarius - was divided almost evenly between students and professional players or teachers. Among the students, there was a large contingent of hero-worshipers, ecstatic because almost all the heroes - a who's who of American flute-playing, plus the legendary Jean-Pierre Rampal - were or would be on the scene, to perform, to conduct master classes or to engage in intense, specialized shoptalk.

"Did you hear Paula Robison Friday night?" was the comment most often heard in passing, but other magic names also hovered in the air: James Galway (who was unable to attend), Samuel Baron, outgoing president of the association, Julius Baker (who attended but did not perform), early-music specialist Shelley Gruskin, and Robert Dick who is the chief technician of the avant-garde. Above all, Jean-Pierre Rampal, who has restored the modern flute to the glamor it enjoyed 200 years ago. Rampal had agreed to hop on a shuttle after a Sunday afternoon concert in New York and fly down - without a fee - to play an evening program for this very special audience.

"I can hardly wait to hear him play the 'Ronde des Lutins'," said one enthusiast, referring to a difficult piece transcribed from a violin solo. "I've heard double-stopping on the flute, but I want to see how he handles those left-hand pizzicato effects."

Some flutists worry about playing before an audience of more than 800 other flutists, but others - like Rampal - love the idea. "I like it," said one performer, Robert Willoughby, who teaches at Oberlin. "I enjoy the feeling that here is a sophisticated audience, people who know exactly what I'm up to. Some of them sit there with scores, penciling in notes, some of them studying my fingering."

"At one concert," Samuel Baron recalled, "Ransom Wilson played a piece that everybody in the audience knew, and at a certain point he tried a very tricky new fingering. The whole audience gasped - 'oooh' ' simultaneously, and my wife, who doesn't play the flute and was just listening to the music, turned to me and asked: "What happened?'"

Conversation between the concerts, lectures and demonstrations often drifted to favorites topics of the flute-playing fraternity: does the material a flute is made from affect its tone? Does the flute attract a different sort of person than the trombone or the percussion? And, among the old-timers at the convention particularly, the phenomenal technical abilities of the younger generation of flute players. "The entrance examinations today are as tough as the graduation exams were a generation ago," said Baron, who teaches at Juilliard.

President-elect Robert Cole of the University of Wisconsin, who owns both a gold and a silver flute, said that there is a running argument between musicians and scientists about whether different materials produce a different tone.

"I have a friend who is a physicist who does acoustic experiments, and he has proved to his own satisfaction that you get the same tone whether the flute is made of lead, wood, hard rubber, copper or glass - but my ear tells me that certain metals have a characteristic response. Each of us has a certain concept of the sound he wants to produce some want to sound warm and some want to sound brilliant. I had a feeling that my sound was too edgy on my silver flute and I didn't have that problem with the gold one."

More important than the instrument (unless it leaks) is the player, he added: "Bob Willoughby sounds like Bob Willoughby no matter what instrument he is playing.

Willoughby (who also owns both gold and silver flutes) agreed that he preferred gold, "partly because of the tone but partly because when they make a flute out of gold they take extra care to make a specially good one."

Everybody who discussed the subject seemed to agree that flutist (who should be called flautists only in Europe) are happier and more friendly than most musicians, and the atmosphere at the convention seemed to support this opinion. Baron elaborated somewhat more than his colleagues:

"Flute-playing attracts an open, naive, trusting, nature-loving, poetic type - a little less . . . er, physical . . . than a trombonist" (his arm moved up and down vigorously, imitating the characteristic movement of a man sliding his trombone from note to note).

"Brass-players - trombone and French horn, for example - love to eat ahearty meal and drink a few beers, for example; they drive fast cars, and they're really living in the world.

"Now an obosit, I would say, is an intense person who suffers a lot; the glory of oboe-playing is what you can do to one note" (he vocalizes a note, bending its pitch, swelling and diminishing its volume - sounding, however, a bit more like a kazoo than an aboe). "And he is perhaps a bit self-centered. The flute is basically a simple sound-producing mechanism, it can reach people and imprint itself on their brains at a yound age; its like singing or a sound of nature.

"The flute is such a popular instrument because it belongs to classical music, it belongs to popular music, it's a jazz instrument, it's a rock instrument, it's afolk instrument. It's a backpacking instrument; people going on hikes take it along because it helps them commune with nature. It has mystical overtones, you meditate while you play.

"In symphony orchestras, I don't know whether flute-players end up being simple, naive, idealistic types - symphony orchestra flute players often end up drinking a lot, perhaps because they're always being drowned out. When you play in an orchestra, you realize that you're not the most powerful player there; you don't have the most influence on what's going on."

The instrument itself has been in a semi-eclipse since the 18th century, when Bach, Mozart and Haydn wrote for it music which remains the cornerstone of its literature. Frederick the Great played the flute, when he wasn't busy swapping philosophical ideas with Voltaire or trying to enlarge the boundaries of Prussia, and even composed music for it, as did one of his musicians, Johann Joachim Quantz.

In our own time, the flute has been reconsidered, and in the lighter textures of modern music it is coming back into its own. A good part of the convention's energy was devoted to considering newly published music for the flute and brushing up on the techniques that have made it a specially useful toy of the avant-garde: producing two notes simultaneously, incorporating the key-clicks into the music, bending the sound and talking into the flute, tapping it like a drum and blowing into the wrong end.

At the same time, the normally retiring flutist, taking a cue from Rampal and Galway and Robinson, no longer dreams exclusively of making a small, anonymous contribution to a bigger sound.

"When I was a student," Baron mused, "all the flute students dreamed of playing for Toscanini. Now, they dream about being stars in their own right, and they have the technique for it and some of them are making it."