Xerox Corp. chose National Secretary's Day last week to unveil what it calls the world's quietest ribbon typewriter, a machine nearly as quiet as a computer keyboard.

The typewriters use a patented technology that gives the printer heads the vibrational characteristics of a hammer weighing nearly 35 pounds, Xerox said. That makes them vibrate at a pitch nearly six octaves lower than usual, almost inaudible to the human ear.The quiet typewriters could help Xerox take some business from International Business Machines Corp., the market leader in top-of-the-line typewriters for Fortune 1000 companies, said Julie Voss, a consultant at Venture Development Corp. in Natick, Mass.

The big question is whether customers will pay the extra $300 Xerox is asking for the feature, said Anthony Polifrone, a vice president of Buyers Laboratory in Hackensack, N.J.

"There's no doubt that the machine is incredibly quiet," he said.

Xerox is the traditional No. 2 in the big-business market, although it fell to third in 1988.

The Xerox 62 Series typewriters have six patents and five more pending, which should make it impossible for competitors to copy the noise-reducing characteristics, said Andrew Gabor, the inventor.

Gabor is the co-inventor of the daisywheel printer system, which was brought to market in 1972 when he was at Diablo Systems. His company was acquired by Xerox a short time later. He is a senior research fellow at Xer-ox's Palo Alto Research Center in Silicon Valley.

Gabor at a news conference Wednesday demonstrated a 1960s-era IBM Selectric with a golf ball-type head (noisy), two earlier models of Xerox Memorywriters with daisy-wheels (quieter), and the new 62 Series (soft clacking).

Thermal printing typewriters, in which marks are burned onto the page, are even quieter than the new Xerox models, but they have drawbacks such as the inability to handle carbons and multipart forms.

The 6225 model is $1,995 and the 6240 model, which includes a disk drive for memory storage, is $2,595.