Other than the occasional roar of a University Avenue bus outside or the whir from the air vents, Cheri Smith's Vista College classroom is as quiet as a sunglasses shop in a rainstorm.

Smith teaches a beginning course at the Berkeley community college in American Sign Language, the primary form of communication for some 300,000 deaf Americans, according to one estimate.The entire class is taught in sign. Because her students are just starting out, Smith will occasionally write an instruction on the blackboard. But no speaking is allowed.

Like most ASL students today, Smith's pupils have no problems with their hearing. Rather, they make up part of the growing number of college students taking sign language to fulfill foreign language requirements, prepare for careers in interpreting or teaching, or learn about what many view as a fascinating, mysterious form of communication.

"I'm personally quite amazed at how many want to learn it," said Smith, who works with some of the 350 to 450 Vista College students who take the courses each year.

When she began teaching at Vista 13 years ago, she and her colleagues designed a sign language curriculum, with the help of a U.S. Department of Education grant, that is now used by many major colleges around the country, Smith said.

Faculty members at other Bay area schools said the interest in ASL classes has grown considerably over the years.

"There's definitely an increase in the interest," said Barbara Franklin, a professor of special education at San Francisco State, where American Sign Language has been taught for at least 30 years. Over the past five to eight years, the number of sections offered has gone from just one or two to five, Franklin said.

David Perlmutter, a professor of linguistics at the University of California, San Diego, said some students took ASL because it was useful for their chosen career path, "but I think most of them are just fascinated with it. It is fascinating to see how a language works in a totally different modality."

The university began offering sign language courses just two years ago but already has seen a growth in enrollment, he said.

Megan Dunn, 24, of San Francisco, said she had signed up for Cheri Smith's ASL class at Vista because she planned to pursue a master's degree in special education and eventually teach elementary school students.

"I figured it would come in handy more than Spanish or anything else," Dunn said.

Other students in Smith's class said they planned to use sign language in their current work as police officers, social workers, actors or performance artists.

One student, an 85-year-old woman who shows early signs of Alzheimer's disease, told Smith she was in the class to learn something completely different, hoping that would forestall the symptoms of the disease.

Nationwide, the number of students taking sign language classes at two- and four-year colleges jumped from 1,602 in 1990 to 4,304 in 1995, according to the most recent figures available from the Modern Language Association.

Contrary to public perception, American Sign Language is not just a stripped-down form of English or a visual spelling out of English words. Linguists consider it a complete language in its own right.

"It has everything that other languages have," Perlmutter said. "There are particular syntactic patterns, there are ways of making new words out of smaller parts, there are ways of putting words into phrases and phrases into sentences."

Deaf people tell jokes in sign, curse and "recite" poetry. Deaf children often sign themselves to sleep or talk to their stuffed animals in sign language. Adults who are deaf may mutter to themselves with their hands; some deaf people stutter in sign.

"The words are visual rather than auditory; that's where the difference lies," Perlmutter said. He estimated that ASL was one of the five or six most commonly used languages in the United States.

Though hard numbers are impossible to come by, approximately 2 million Americans are profoundly deaf, according to a 1989 study by the National Institute on Deafness and Other Communication Disorders, a division of the National Institutes of Health.

For deaf people - especially those born deaf - American Sign Language is as integral to deaf culture as English is to mainstream America. Many learn it as very young children, particularly if they are born deaf to deaf parents.

For non-deaf students, ASL can be a window into deaf culture.

"These students have to learn not only a language but a way of behaving," said Smith, the Vista College instructor.

For instance, they learn that a flash of the classroom lights means that time's up or that they should stop what they're doing. They learn to be looked at, closely, when they are communicating. They learn that the deaf community is collectively oriented, that people are expected to share more information about their lives than would be expected among hearing people.

Those who succeed in learning ASL have a world of opportunities available to them, Smith said.

The job market for sign language interpreters has ballooned, said Carolyn Beichle, director of Bay Area Communication Access, a private company in San Francisco that provides interpreters for schools and businesses.

Her clients include doctors' offices, hospitals, companies doing large-scale training and coordinators of conventions like MacWorld (the computer industry attracts many deaf people).

Part of the demand comes from federal law; the Americans with Disabilities Act requires employers, public agencies and private entities like restaurants, hospitals and hotels to provide accommodations for hearing-impaired people.

Staff interpreters make between $30,000 and $45,000 a year; free-lancers command from $30 to $45 an hour, Beichle said.