NOW YOU HEAR IT, NOW YOU DON'T; NOISE, LOUD MUSIC TAKE TOLL, BUT LOSS CAN BE CORRECTED.

Byline: Liz Doup Knight-Ridder Tribune News Wire

First the eyes. Then the knees. Now the ears.

Aging plays havoc with body parts, as President Clinton knows after being fitted with two hearing aids recently. If you're on the far side of 40, you may have heard the news; several hearing-aid businesses reported a boost in business after Clinton went public.

``Our number of appointments skyrocketed the week following his announcement,'' says Dr. Paul Brown, CEO of HEARx, which has 17 stores in South Florida. ``Sales increased by 90 percent.''

Same thing happened when President Reagan, at 72, slapped on a hearing aid in 1983, says Herb Gorlin, chairman of the Florida Institute for Better Hearing, which distributes Beltone hearing aids. Industrywide, sales soared about 28 percent.

Granted, both times most customers were members of the Social Security set. But Clinton did prompt some baby boomers to admit the obvious. Among them: Lemma Daniels, 48, an instrument technician at Florida Medical Center in Lauderdale Lakes, Fla. For years, doctors and friends told her she needed a hearing aid. She balked.

``I thought, `I'm too young for this,' '' says Daniels, who was fitted for hearing aids recently. ``But hearing about Clinton convinced me to go through with it. If he can admit it and do it, then so can I.''

So what does happen to your hearing, and how do you know if you have a problem? What can you do about it? And where do you go for help?

We asked some experts.

At 51, the boomer president has what's known as presbycusis - hearing loss associated with age and noise exposure. Like a lot of people, Clinton has trouble hearing in crowded rooms and at noisy events.

To blame: too much loud saxophone music. Too much duck hunting. Take heed, because the most common cause of hearing loss today is probably plain old noise, according to the Better Hearing Institute, a nonprofit educational organization. Loud noise.

You can hear certain sounds indefinitely - such as a washing machine or vacuum cleaner - and no harm is done. Not so with exposure to high decibels over a period of time. For instance, no one wants to listen to a screaming child (90 decibels) for five minutes, but your ears can stand eight consecutive hours before incurring damage. A power mower (105 decibels): one hour. Live rock music: eight hours (90 decibels) to 18 minutes (130 decibels).

Stone deaf

So, you may pay for those Stones concerts in your past - but not right away. And you might not notice the loss at all.

Hearing loss is gradual. Experts say that Clinton is a tad young for hearing aids, but his exposure - to music and guns - isn't typical. It takes repeated exposures to loud noise over time to erode your hearing, which is why you're more likely to see hearing aids on 70-year-olds than 50-year-olds.

You hear because your ear receives sound waves and sends them through a delicately balanced system to the brain. Part of that system is a chamber in the inner ear filled with fluid and lined with thousands of tiny hair cells. The hair cells signal the auditory nerve to send electrical impulses to the brain. The brain interprets these impulses as sound. When you're exposed to loud or prolonged noise, these delicate hair cells are damaged. And you have hearing loss.

Now, understand that hearing impairments fall into two categories.

The one just described, sensorineural hearing loss or ``nerve'' deafness, involves damage to the inner ear. That's what Clinton has. It's caused not only by exposure to loud noise but also by aging, birth-related problems and heredity. Happily, hearing aids can help.

Not so conductive hearing loss, which involves the outer and middle ear and is caused by a punctured eardrum, ear wax and ear infections. Medical intervention often can help. Hearing aids generally don't.

As Clinton proved, you don't need to qualify for Social Security to experience hearing loss. About 28 million of us don't hear as well as we once did, according to the American Speech-Language-Hearing Association.

Smaller, more effective

Don't despair if you, like Daniels, think it's time to get help. Today's hearing aids aren't those clunky, whistling gizmos your grandmother wore. They're smaller and more effective.

Some are virtual mini-computers, programmed to analyze and respond to different sounds. For others, you merely push a button on a hearing aid or on a wireless remote control to switch to different listening situations. Also new: digital and digitally programmable hearing aids. They offer improved sound quality in the same way your CD player offers superior sound compared to your old stereo system.

A warning: Though hearing aids are better than a decade ago, they're far from perfect.

``A hearing aid is not a 100 percent solution and won't correct a hearing problem, but it can help,'' says Wayne Staab, author of his self-published 1991 book ``Hearing Aids: A User's Guide'' and a former Michigan State University professor of audiology.

Staab calls hearing aids ``an invention of compromise.'' Every type has its good points and bad. Your four main choices:

Completely-in-the-canal. This was Clinton's logical choice. It's the smallest and least visible, about the size of an aspirin. But you pay for the vanity. Average cost, $1,500, according to the Hearing Review, an industry publication. And this is for one hearing aid only. Note that this is an average price. Top-of-the-line hearing aids of this type run $3,000.

Clinton, incidentally, bought an aid for each ear. A good move, experts say, because you tend to lose hearing in both ears.

In-the-canal. This hearing aid is about the size of a jelly bean and easier to maneuver. Average cost: $945.

In-the-ear. This is a custom-made ear shell that fits in the outer ear and is clearly visible but offers easy adjustment and battery replacement. Average cost: $770.

Behind-the-ear. It's connected to a custom-made ear shell and offers special programming and power. Average cost: $800.

Before you buy, do your homework. A Food and Drug Administration regulation suggests a medical examination six months before buying a hearing aid to rule out medical causes. (Ear specialists are otologists; otolaryngologists are ear, nose and throat doctors.)

Keep in mind that hearing aids aren't miracle devices and can require adjustments.

Alas, you can't change the fact that blaring rock concerts were part of your past, but you can protect what hearing you have left. If you're a steady concertgoer or work around loud equipment, wear ear plugs or ear muffs, says Robin Weintraub, a Pembroke Pines, Fla., audiologist.

Warning signs of hearing loss

Is it difficult to understand conversations in a restaurant or group or whenever there's background noise?

Do you often find yourself asking people to repeat themselves?

Has your spouse or a friend ever told you that you don't hear well?

Do you turn up the radio or TV louder than others prefer?

Do you feel frustration when friends and family members seem to mumble or talk too quickly?

Do you hear some people more clearly than others?

Do you hear conversation, but have difficulty understanding the words?

Are you withdrawing more from contact with friends and family?

Do you turn one ear toward a speaker to help you hear better?

CAPTION(S):

Photo, Box

Photo: At 51, President Clinton has what's known as presbycusis - hearing loss associated with age and noise exposure. To blame: too much loud saxophone music, for one thing.

Box: Warning signs of hearing loss (See Text)