How hearing works
Human ears are sound-collecting machines designed to gather noise (through the ear canal) and convert it into vibrations (in the eardrum) that are sent via the tiny bones of the middle ear (hammer, anvil, stirrup) to the coiled cochlea of the inner ear. But this is only half of what it takes for us to hear the world. The rest of the work is done in the brain.
"In the past 10 years, there has been a recognition that we don't hear just with the ears. We hear with the brain," says Barbara Cone, a professor of speech, language, and hearing sciences at the University of Arizona, in Tucson.
The brain's job is not only to receive sound but also to reduce background noise and home in on the things we most want to hear -- quite often, other people's voices. Human ears are particularly attuned to the frequency range used in conversation. And our large brains and long life spans enable us to use our experiences to differentiate an extraordinary variety of sounds.
For example, says Jan Schnupp, a research leader at the University of Oxford Auditory Neuroscience Group, in England, "I can instantly recognize and distinguish the sound of an elephant from that of an owl from that of a shopping trolley rattling over a bumpy pavement."
Our ears are about as sensitive as those of other mammals. Unlike cats, we cannot move them independently to collect sound from one direction or another (a cat has more than 20 different muscles to move each ear like a radar receiver), but we are able to sense when a sound reaches one ear before the other one or is louder in one ear than the other, and this helps us tell where sounds come from.
Structures deep inside our ears called semicircular canals also help us feel balanced and oriented in space. The movement of fluid within the canals keeps the brain aware of head motion, so that if we look up, sway from side to side, bend over, or stand on our heads, we don't (necessarily) feel dizzy or fall over.
During medical checkups at every age, we have our ears inspected. The doctor sets his otoscope in the opening of each ear and takes a quick look, just to see that things are pretty clean, that there's no puncture in the eardrum and no swelling of tissue. But problems arise now and then, from the earaches and infections of childhood to the loss of hearing that can come in late life. Here's what to look out for.
Children younger than 5 are prone to ear infections, in part because their immune systems are still developing and often are not strong enough to fight off the viruses and bacteria that reach, and inflame, the middle ear.
By adulthood, such infections are rare, but grown-ups are still prone to infections of the outer ear and ear canal, caused by swimming in not-so-clean water (swimmer's ear), for example, or by inserting objects into the ear, usually in a misguided attempt to clean out wax.
To prevent swimmer's ear, avoid polluted lakes and rivers or pools with poor chlorine and pH control, and dry your ears after swimming.
A little wax in the ears is not only normal, but healthy. It traps dust, bacteria, and other particles in the ear canal before they can reach the more sensitive middle and inner ear.
"People think earwax is dirt," says Rick Friedman, a neurotologist at the House Clinic, in Los Angeles. "It's not dirt. It's a protective coating for our ear canal."
But it's also true that in some people, the wax-producing glands make too much, and this can build up and muffle hearing. When people try to clear out the wax themselves, they often make matters worse by inadvertently pushing it deeper and compacting it.
A few drops of baby oil or mineral oil in the ear can soften wax and help it drain on its own. Flushing the ear with lukewarm water or a drugstore irrigation kit also works, but you must first make sure you have no tear in your eardrum. A doctor can help by irrigating the ear for you and suctioning the ear canal.
About one in six American adults reports some degree of hearing loss. The condition is far more prevalent in men than in women, and doctors think this is because men are generally exposed to more loud noise, in life and on the job.
People who lose their hearing can be reluctant to seek medical help. Sometimes they don't realize that they're not hearing as well as they used to. (Often, their spouses notice first.) They might also be wary of seeing a doctor because they don't want to wear a hearing aid. Unlike eyeglasses, hearing aids are negatively associated with growing old. But when people lose their hearing, they lose an important social tool.
Hearing loss is usually caused by the loss of sensory cells in the inner ear, called hair cells, that run the length of the spiral cochlea and turn sound vibrations into nerve impulses to send to the brain. Some people are genetically more susceptible to this kind of hearing loss with age, Friedman says. It is often brought on by too much exposure to loud noise, but certain antibiotics and chemotherapy drugs can also damage the hair cells, says Robert Frisina, a professor of otolaryngology at the University of Rochester Medical Center, in New York.
As if diminished hearing is not bad enough, it often comes with tinnitus, a sound of ringing, roaring, hissing, or clicking. Doctors believe this is brought on by the same loss of inner-ear hair cells that causes hearing loss. But imaging studies suggest that the ringing originates in the brain, which apparently makes up for the loss of input from hair cells by generating noise of its own.
Doctors treat tinnitus with hearing aids or electronic devices that produce masking noises, which make the ringing less noticeable. Relaxation therapy helps some people -- or at least reduces their frustration about hearing the noise.
How loud is too loud?
Ordinary conversation and most city sounds are not loud enough to damage the delicate and vital hair cells of the inner ear.
But sounds louder than 90 decibels -- such as gunshots; amplified music, including music from your MP3 player; and the revving of power tools, lawn mowers, motorcycles, and snowmobiles -- can lead to hearing loss, especially if experienced repeatedly, says Frisina. Anyone who regularly encounters this much noise should wear foam earplugs or, if the noise is especially loud, protective headphone-like devices that cover both ears completely.