Understanding Hearing Loss

by CDC

Before we can understand hearing loss, we must first understand what hearing entails. When we hear sounds, we really are interpreting patterns of movement of air molecules. We can describe sounds in terms of their frequency (or pitch) and intensity (or loudness). Frequency is measured in hertz (Hz). A person who has hearing within the normal range, can hear sounds that have frequencies between 20 and 20,000 Hz. The most important sounds we hear every day are in the 250 to 6,000 Hz range. Speech includes a mix of low and high frequency sounds. Vowel sounds like "u" have low frequencies (250 to 1,000 Hz) and are usually easier to hear. Consonants like "s," "h," and "f" have higher frequencies (1,500 to 6,000 Hz) and are harder to hear. Consonants convey most of the meaning of what we say. Someone who cannot hear high-frequency sounds will have a hard time understanding speech.

Intensity, or loudness, is measured in decibels (dB). A person with hearing within the normal range can hear sounds ranging from 0 to 140 dB. A whisper is around 30 dB. Conversations are usually 45 to 50 dB. Sounds that are louder than 90 dB can be uncomfortable to hear. A loud rock concert might be as loud as 110 dB. Sounds that are 120 dB or louder can be painful and can result in temporary or permanent hearing loss.

Impairments in hearing can happen in either frequency or intensity, or both. Hearing loss severity is based on how well a person can hear the frequencies or intensities most often associated with speech. Severity can be described as mild, moderate, severe, or profound. The term "deaf" is sometimes used to describe someone who has an approximately 90 dB or greater hearing loss or who cannot use hearing to process speech and language information, even with the use of hearing aids. The term "hard of hearing" is sometimes used to describe people who have a less severe hearing loss than deafness.

Hearing loss can affect one or both ears. A loss that affects one ear is called a unilateral loss. A loss that affects both ears is called a bilateral loss.

There are four main types of hearing loss:

Conductive: Hearing loss caused by a problem in the outer ear or middle ear. Conductive losses usually affect all frequencies to the same degree. These losses are not usually severe.

Sensorineural: Hearing loss caused by a problem in the inner ear or auditory nerve. A sensorineural loss often affects a person's ability to hear some frequencies more than others. This means that sounds may be appear distorted, even with the use of a hearing aid. Sensorineural losses can range from mild to profound.

Mixed: A combination of conductive and sensorineural losses.

Central: Hearing loss caused by a problem along the pathway from the inner ear to the auditory region of the brain or in the brain itself.

Approximately 30% of children who are deaf or hard of hearing also have one or more other developmental disabilities, such as mental retardation, cerebral palsy, vision impairment, or epilepsy.

Hearing loss can affect a child's ability to learn both to speak and to understand spoken language. This is especially true if the child is born with a hearing loss or loses his or her hearing before 2 years of age. People with hearing loss may communicate using speech (sometimes called oral communication), sign language (sometimes called manual communication), or a combination of both. Oral communication focuses on speech, listening with hearing aids, and sometimes lipreading. Manual communication includes sign language.

How common is hearing loss?

In the 2002-2003 school year, nearly 72,000 children ages 6 to 21 years got special education services under the "hearing impairment" category in the United States. Another 1,600 children received services under the "deaf blind" category. The total number of children with hearing loss is likely higher, since some of them may have other disabilities and be served under other special education categories. Still others may not be counted because they receive only regular education services.

CDC tracks the number of children in a five-county area in metropolitan Atlanta, Georgia who have moderate to profound hearing loss in both ears. For this project, we define moderate to profound hearing loss as a 40 dB or greater loss in the better ear, without the use of hearing aids. This activity is part of the Metropolitan Atlanta Developmental Disabilities Surveillance Program (MADDSP). For 1991-1993, we found that, on average, about 9 in every 10,000 children ages 3 to 10 years had a moderate to profound hearing loss in both ears. Hearing loss was more common among older children than among younger children. Ninety percent of the children had a sensorineural hearing loss. Thirty percent of the children had one or more other disabilities in addition to their hearing loss.

CDC also studied hearing loss among children in metropolitan Atlanta in the mid-1980s. This project was done as part of the Metropolitan Atlanta Developmental Disabilities Study (MADDS), which studied how common certain disabilities were among 10-year-old children. The study found that 11 of every 10,000 children 10 years of age were deaf or hard of hearing. Hearing loss was slightly more common among boys than among girls. Twenty-eight percent of the children also had another disability, such as mental retardation, cerebral palsy, or epilepsy.

In the 1988 National Health Interview Survey - Child Health Supplement, parents reported that that 3.5% of children ages birth to 17 years in the United States had ever had "deafness or trouble hearing" in one or both ears.

Hearing loss is more common among older people than among children. In the 1994 National Health Interview Survey Core and Second Supplement on Aging, one third of adults ages 70 and older reported that they had trouble hearing. Seven percent reported that they were deaf in both ears and another 8% reported that they were deaf in one ear. Hearing loss was more common among men than among women.

About the Author

www.cdc.gov
The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.