Not long ago, a teen-age girl underwent an operation on her ear. After the surgery, the world became a different place. She could hear all sorts of things she hadn't heard before. She could hear birds singing. She could hear the sound of her own footsteps. Doctors had replaced a tiny bone in her ear -- a bone as small as a grain of rice. The replacement now carried sound vibrations from the world outside into the girl's inner ear.

This patient was one of the lucky people whose hearing problem could be improved through surgery. Surgery cannot help everyone with hearing problems, however. But in laboratories around the world scientists are working on finding new ways to help. Experts argue about what deafness is exactly, but most agree that being deaf means being unable to hear human speech. Some people are profoundly deaf, and hear no sound at all; others have some hearing, but don't hear completely normally. Some people are born without the ability to hear; others lose their hearing due to accidents, illness or the effects of old age. In the United States, about 14 million people suffer from some kind of hearing loss. Perhaps as many as 3 million of these people are deaf.

Today, there are many different kinds of equipment available to help people with hearing problems. Some people with hearing loss wear hearing aids; these devices amplify, or make sound louder. More recently, deaf and hearing impaired people have begun to use computer keyboards to communicate. Writing on computers hooked up to telephones, they can chat with other people who have similar equipment, even though no sounds are made.

In 1984, the United States government approved a kind of artificial ear called a cochlear implant, and some deaf people are now using them.

To understand how a cochlear implant works, you need to understand something about the ear and hearing. Your ear has three major parts: the outer ear, the middle ear, and the inner ear. The outer ear collects sound, sends it through your eardrum and special bones in the middle ear, and on to nerve cells in the inner ear.

Unfortunately, all three parts of the ear can have things go wrong with them. If something goes wrong with your outer ear, your eardrum, or your middle ear, you may experience conductive hearing loss. That means that the structures of your ear are having trouble conducting, or passing, sound into your brain.

Deep inside your inner ear is a snail-shaped organ called the cochlea. Its job is to pick up sound vibrations conducted from the outer and middle ear, and send them on to the brain. When something goes wrong with the cochlea, you experience what doctors called sensory hearing loss. It gets that name because the cochlea's job is to sense sound. It does this with thousands of tiny hair cells connected to nerve endings that lead to the brain. The hair cells pick up sound and change it into electrical signals, which the brain can interpret as sound. When something happens to the hair cells inside the cochlea, sound signals stop getting through to the brain.

A cochlear implant can help. The implant has a microphone worn outside the ear. The microphone picks up sound and changes it into electrical signals. The signals travel along a wire that a surgeon has inserted through the ear and into the cochlea. The wire takes over the job the hair cells should do. It sends electrical signals to the brain.

The "artificial ear" is far from perfect. Cochlear implants available now can only produce sound in the form of buzzes or beeps. But for people with severe hearing loss, even those sounds can be very exciting. Deaf people who have received the implant can recognize some sounds for the first time, like car horns honking, telephones ringing or doors closing. And the implant sends enough sound to the brain to help some deaf people read lips more accurately.

A more complicated kind of cochlear implant is being developed right now. This device should make it possible for wearers to actually understand speech and hear music.

Doctors are excited about the cochlear implant. They hope that even better devices will be available before long. Until recently, there wasn't very much doctors could do for the profoundly deaf. Now that situation is beginning to change. By the time you grow up, today's cochlear implant may look old-fashioned. But for some deaf people today, it's an important first step on the road to hearing. Tips for Parents

So far, the cochlear implant isn't for kids. Dr. Joseph Nadol of Harvard Medical School and the Massachusetts Eye and Ear Infirmary's Cochlear Implantation Project reports in The Harvard Medical School Health Letter this month that the reason for this is the device's limited life span. Although some implants have been functioning for six years, there is no telling how long they'll go on working. Doctors prefer not to have children undergo the implantation because they may eventually require several operations. Additionally, better devices may be available soon.