BANG! A car door slams shut. A taxi driver blasts a warning on a horn. In the middle of the street, a construction worker rat-a-tat-tats at the pavement with a jackhammer. People shout to each other over the sounds of traffic.

These are sounds you might hear at a busy city intersection. In the country, you would hear very different sounds. You could listen to leaves rustling in the breeze, or bees buzzing in flowers. You might hear a dog barking way off on the other side of a wide field.

Sound -- whether it's loud or soft -- travels through the air in waves. You can hear because your ears are built to pick up the sound waves. The waves enter your ear, where they are translated into nerve signals that your brain understands.

Look at a friend's ear. The first thing you will see is the outer ear. It gathers sound waves and passes them into the inner ear. When you look at your friend's ear, you can see a dark hole leading into the head. That's the ear canal. It leads to the middle ear, which you can't see.

A small piece of skin stretches across the opening between the ear canal and the middle ear. This is the eardrum, and it has a very important job. When sound waves from the outside strike the eardrum, it starts vibrating. The vibrations pass into a tiny bone on the other side of the eardrum. This bone, called the hammer, attaches to two other tiny bones, the anvil and the stirrup. The stirrup is the smallest bone in your body -- no larger than a grain of rice.

Let's say you whisper something to your friend. The three connected bones pass sound waves from her eardrum to her inner ear. There, a snail-shaped part of the inner ear takes over the job of translating sound waves into nerve signals. This part, called the cochlea, is carved right into the bone that forms the skull. There is liquid in the cochlea. It picks up the vibrations passed from the eardrum, through the middle ear, and into the inner ear. The liquid quivers and stimulates thousands of tiny nerve cells that line the cochlea. The cells send messages to the brain, and your friend hears your voice.

Your ears are very sensitive. They are equipped with special things to protect themselves. The wax that lines your ear canal keeps dirt from getting in. The wax in your ears is necessary for good health. You shouldn't ever try to scrape it out with a cotton swab or any other object. You could damage your delicate eardrum.

The bone that surrounds the hard-working middle and inner ear is the hardest bone in your body. It's a kind of armor to protect the ear.

Passages called eustachian tubes connect your middle ear to your throat. The tubes open and close at different times to keep the pressure inside your ears normal. When you go up or down in an airplane and your ears "pop," it's a sign that these tubes are doing their job.

But as most kids know, ears can be troublesome. Doctors say that half of all children have earaches at one time or other.

Most earaches are caused by infections in the middle ear. The infection may come from germs that travel up your eustachian tubes when you have a cold or a sore throat. The infection invades your middle ear, and makes it swollen and painful. You may start to form a thick fluid called pus, which causes your ears to feel stuffy. Pus may press on the eardrum, causing pain. Earaches are no fun. If you go to the doctor, you'll probably get medicines called antibiotics to clear up the infection fast.

Very loud noises, like explosions, can damage your ears, too. So can listening to very loud music for a long time. So turn that stereo down a little. Your ears need to last a lifetime. Tips for Parents

If your child suffers from recurrent middle ear infections, you may be facing this question: To tube or not to tube? For the last couple of decades, tympanostomy -- the insertion of a small plastic tubes through the eardrum to ventilate the middle ear -- has become a common procedure.

Dr. Gerald Leonard, otolaryngology department chairman at the University of Connecticut Medical School in Farmington, says tubes may be useful for children with repeated middle ear infections that don't completely clear up between acute episodes. Most acute cases respond to medication, however -- either antibiotic or sulfa drugs, or a combination.

Repeated infections alone, if they respond to treatment, do not mean a child needs tubes, Leonard says. "We don't just jam them in any child who's had a couple of earaches.

"But when the ears won't settle, the eardrums don't move properly, or the child develops a thick fluid -- what we call 'glue ear' for want of a better term -- then ventilating tubes may well be indicated," he says. "This is particularly true if that child is at the stage of speech acquisition. The reduction in hearing brought on by the infections can interfere with learning and speech."

The tubes are usually worn for two months to a year or more. Often, they come out by themselves.

There is some evidence that long-term use of tubes can scar the eardrums. But severe infections can do more serious damage, Leonard says. The procedure is usually done on an outpatient basis -- although it does require general anesthesia. With good preparation, the hospital stay required by tympanostomy need not be traumatic.

Tympanostomy tubes can produce dramatic results. Not only do the recurrent earaches clear up, but, says Leonard, "the child may begin to hear better. He may start picking up things his teacher or parents say that he just couldn't hear before. His behavior improves as well as his health."