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How Do We Hear?
Hearing is a series of events in which the ear converts sound waves into electrical signals that are sent to the brain and interpreted as sound. The ear has three main parts: the outer, middle, and inner ear. Sound waves enter through the outer ear and reach the middle ear where they cause the eardrum to vibrate.
Drawing of the internal structure of the ear. Image courtesy of the FDA.
The vibrations are transmitted through three tiny bones in the middle ear, called the ossicles. These three bones are named the malleus, incus, and stapes (and are also known as the hammer, anvil, and stirrup). The eardrum and ossicles amplify the vibrations and carry them to the inner ear. The stirrup transmits the amplified vibrations through the oval window and into the fluid that fills the inner ear. The vibrations move through fluid in the snail-shaped hearing part of the inner ear (cochlea) that contains the hair cells. The fluid in the cochlea moves the top portion of the hair cells, called the hair bundle, which initiates the changes that lead to the production of nerve impulses. These nerve impulses are carried to the brain, where they are interpreted as sound. Different sounds move the hair bundles in different ways, thus allowing the brain to distinguish one sound from another, such as vowels from consonants.
Noise-Induced Hearing Loss
Every day we experience sound in our environment such as the television, radio, washing machine, automobiles, buses, and trucks. But when an individual is exposed to harmful sounds--sounds that are too loud or loud sounds over a long time--sensitive structures of the inner ear can be damaged, causing noise-induced hearing loss (NIHL).
NIHL can be caused by a one-time exposure to loud sound as well as by repeated exposure to sounds at various loudness levels over an extended period of time. Exposure to harmful sounds (over 80 decibels) causes damage to the sensitive hair cells of the inner ear as well as the hearing nerve. These structures can be injured by two kinds of noise: loud impulse noise, such as an explosion, or loud continuous noise, such as that generated in a woodworking shop.
Impulse sound can result in immediate hearing loss that may be permanent. The structures of the inner ear may be severely damaged. This kind of hearing loss may be accompanied by tinnitus, a ringing, buzzing, or roaring in the ears or head, which may subside over time. Hearing loss and tinnitus may be experienced in one or both ears, and tinnitus may continue constantly or occasionally throughout a lifetime.
Continuous exposure to loud noise also can damage the structure of the hair cells, resulting in hearing loss and tinnitus. Exposure to impulse and continuous noise may cause only a temporary hearing loss. If the hearing recovers, the temporary hearing loss is called a temporary threshold shift. The temporary threshold shift largely disappears 16 to 48 hours after exposure to loud noise.
Both forms of NIHL can be prevented by the regular use of hearing protectors such as earplugs or earmuffs.
The symptoms of NIHL increase gradually over a period of continuous exposure. Sounds may become distorted or muffled, and it may be difficult for the person to understand speech. The individual may not be aware of the loss, but it can be detected with a hearing test.
NIHL is preventable. All individuals should understand the hazards of noise and how to practice good health in everyday life.
- Know which noises can cause damage (those above 85 decibels).
- Wear earplugs or other hearing protective devices when involved in a loud activity (special earplugs and earmuffs are available at hardware stores and sporting good stores).
- Be alert to hazardous noise in the environment.
- Protect children who are too young to protect themselves.
- Make family, friends, and colleagues aware of the hazards of noise.
- Have a medical examination by an otolaryngologist, a physician who specializes in diseases of the ears, nose, throat, head, and neck, and a hearing test by an audiologist, a health professional trained to identify and measure hearing loss and to rehabilitate persons with hearing impairments.
Hearing Loss and Older Adults
Hearing loss is one of the most common conditions affecting older adults. One in three people older than 60 and half of those older than 85 have hearing loss. Hearing problems can make it hard to understand and follow a doctor’s advice, to respond to warnings, and to hear doorbells and alarms. They can also make it hard to enjoy talking with friends and family. All of this can be frustrating, embarrassing, and even dangerous.
Hearing loss happens for many reasons. Some people lose their hearing slowly as they age. This condition is known as presbycusis (prez-buh-KYOO-sis). Doctors do not know why presbycusis happens, but it seems to run in families. Another reason for hearing loss may be exposure to too much loud noise. This condition is known as noise-induced hearing loss. Many construction workers, farmers, musicians, airport workers, tree cutters, and people in the armed forces have hearing problems because of too much exposure to loud noise. Sometimes loud noise can cause a ringing, hissing, or roaring sound in the ears, called tinnitus (tin-NY-tus). Hearing loss can also be caused by a virus or bacteria, heart conditions or stroke, head injuries, tumors, and certain medicines.
Your treatment will depend on your hearing problem, so some treatments will work better for you than others. Here are the most common ones:
- Hearing aids » tiny instruments you wear in or behind your ear. They make sounds louder. Things sound different when you wear a hearing aid, but an audiologist can help you get used to it. To find the hearing aid that works best for you, you may have to try more than one. Ask your audiologist whether you can have a trial period with a few different hearing aids. You and your audiologist can work together until you are comfortable.
- Personal listening systems » help you hear what you want to hear while eliminating or lowering other noises around you. Some, called auditory training systems and loop systems, make it easier for you to hear someone in a crowded room or group setting. Others, such as FM systems and personal amplifiers, are better for one-on-one conversations.
- TV listening systems » help you listen to the television or the radio without being bothered by other noises around you. These systems can be used with or without hearing aids and do not require you to use a very high volume.
- Direct audio input hearing aids » hearing aids that can be plugged into TVs, stereos, microphones, auditory trainers, and personal FM systems to help you hear better.
- Telephone amplifying devices » some telephones are made to work with certain hearing aids. If your hearing aid has a "T" switch, you can ask your telephone company about getting a phone with an amplifying coil (T-coil). If your hearing aid is in the "T" position, this coil is activated when you pick up the phone. It allows you to listen at a comfortable volume and helps lessen background noise. You can also buy a special type of telephone receiver and other devices to make sounds louder on the phone.
- Mobile phone amplifying devices » to help people who use a T-coil hear better on mobile phones, an amplifying device called a loopset is available. The wire loop goes around your neck and connects to the mobile phone. The loop transmits speech from the phone to the hearing aid in your ear. It also helps get rid of background noise to make it easier to talk in a noisy environment.
- Auditorium-type assistive listening systems » many auditoriums, movie theaters, churches, synagogues, and other public places are equipped with special sound systems for people with hearing loss. These systems send sounds directly to your ears to help you hear better. Some can be used with a hearing aid and others without.
- Cochlear (COKE-lee-ur) implants » have three parts: a headpiece, a speech processor, and a receiver. The headpiece includes a microphone and a transmitter. It is worn just behind the ear where it picks up sound and sends it to the speech processor, a beeper-sized device that can fit in your pocket or on a belt. The speech processor converts the sound into a special signal that is sent to the receiver. The receiver, a small round disc about the size of a quarter that a surgeon places under the skin behind one ear, sends a sound signal to the brain. Cochlear implants are most often used with young children born with hearing loss. However, older adults with profound or severe hearing loss are beginning to receive these implants more often.
This illustration compares the mammalian cochlea to a new artificial cochlea implant developed by Karl Grosh and Robert White. Image courtesy of Zina Deretsy, National Science Foundation.
- Lip reading or speech reading » another option. People who do this pay close attention to others when they talk. They watch how the mouth and the body move when someone is talking. Special trainers can help you learn how to lip read or speech read.
When a Newborn Doesn't Pass the Hearing Screening
Each year, approximately 12,000 babies are born with hearing loss in the United States. If hearing loss is discovered during the "critical period" for children to develop their speech and language skills -- from birth to the first two to three years of life -- they will have a much better chance of keeping up with their peers in learning language and speech skills. If they miss this critical period of development, they can fall far behind their peers in these areas. This delay, in turn, can affect a child's ability to learn and interact socially.
Medical and allied health professionals across the United States have played a vital role in helping identify hearing loss during a child's first months of life. As of July 2003, mandatory newborn hearing screening programs have been implemented in 38 states and the District of Columbia. As a result, roughly 86.5 percent of all infants are now screened for hearing loss, usually before they leave the hospital.
But screening is only the beginning of a successful path for infants who are deaf or hard-of-hearing. Newborns who don't pass the screening should receive an audiometric evaluation and medical diagnosis before the child is three months of age. An audiologist is a health professional who conducts a series of tests to determine whether the child has a hearing problem and, if so, the type and severity of that problem. An otolaryngologist, or ear, nose, and throat doctor, will try to find out the reason behind a hearing loss and offer treatment options. These professionals will help direct parents or other caregivers to resources that can help them.
In the year 2000, only approximately half of the children who were referred for a follow-up examination were brought back for one. Parents of deaf or hard-of-hearing children in this group were much less likely to take advantage of the many resources that could give their children the best start possible and the best long-term outcome.
Source: National Institute on Deafness and Other Communication Disorders
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