Carnegie Hall reopened in late 1986 with a musical question: Had an extensive interior renovation harmed the hall's magic sound?

No way, officials said. Hadn't management removed padding from new seats after tests showed they absorbed more sound than the old ones? Two years later, many critics have decided Carnegie's old sound went the way of its old seats.The problems raise the question of whether acoustics is science, art or magic - or luck. "To a certain extent you're shooting craps," admits Cyril Harris, an acoustician for almost 40 years.

When you sit in a concert hall you hear two kinds of sound: direct, which comes straight from the stage, and reflected, which bounces - off the wall, the ceiling, the shoulder of the person in the next seat - before hitting your ear.

The first reflected sound should hit the ear 20 to 60 milliseconds after leaving the stage, and it should reverberate for just under two seconds - the approximate time in such great halls as Amsterdam's Concertgebouw, Boston's Symphony Hall and Vienna's Grosser Musikvereinsaal.

Most of the world's great halls date from the late 19th century, and share a rectangular layout; irregular, ornamented interiors that scatter sound; heavy plaster and wood, which reflect sound well; and fewer than 2,800 seats.

Acousticians have learned how to shape walls that scatter sound in the fashion of three-dimensional wall surfaces. They also use "quadratic residue diffusers" - textured panels on ceilings or walls that spread sound.

Lasers can show how light waves bounce around a room, giving a clue to how sound waves move through the space; computers make it feasible to analyze huge amounts of test data; models can highlight gross defects before construction.

Electronically assisted resonance was first used in 1964 at the Royal Festival Hall in London, whose problems stemmed from a low ceiling. Rather than raise the roof, management installed speakers, each with a microphone and an amplifier tuned to a certain low-frequency band to increase reverberation.