TRADITIONALISTS may still think of Broadway and 42nd Street as the busiest corner in Manhattan. Not if they spend a warm summer afternoon walking between 50th and 51st Streets along the Avenue of the Americas. There, the crowds that congregate for a visit to the Time & Life Building's street-level Exhibition Center, and pause to relax near the fountains in the plaza, are likely to rival any on Broadway.

During the past seven years, the Exhibition Center has offered shows that have ranged through history and art, from medicine and mathematics to modern sculpture in papier-mache. Previous visitors probably remember the reproduction of Michelangelo's frescoes in the Sistine Chapel shown at Easter time, or the animated diorama of the First Continental Congress. This week we are opening "A Science Tune-In: New Horizons in Light and Sound," created with the assistance of Bell Telephone Laboratories.

An audiovisual guide to the achievement and promise of contemporary technology, the "Tune-In" includes 20 demonstrations, and takes as its theme a quotation from Sir Francis Bacon: "The real and legitimate goal of the sciences is the endowment of human life with new inventions and riches." Visitors will be able to:

P:Prove for themselves that sound cannot travel in a vacuum.

P: Study a simplified model of the complex switching systems that make almost instantaneous coast-to-coast phone calls possible.

P:Tune in on communication among fish as well as on transmissions from distant galaxies, and hear, on immediate playback, the sound of their own voices as they would have been recorded on equipment available in 1900, 1920 and today.

P: See figures on a chessboard stand out in remarkable three-dimensional clarity on a two-dimensional "hologram."

P:Test their hearing acuity by trying to match the pitch of electronically generated tones.

P:Speak with remotely situated friends at the exhibit over a Picturephone that transmits both sound and visual images.

P:Control a laser beam with a system of gas-filled tubes and mirrors.

P:Check the ability of an electronic computer to guess their ages after they have answered "yes" or "no" to, at most, half a dozen questions.

The exhibit will run into September. And, just as if visitors need a reminder of the hot and noisy New York summer outside, they will be able to watch, on closed-circuit TV, the bustling Avenue of the Americas where it runs between the Radio City Music Hall and the Time & Life Building.

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