Fundamentals of Hearing - Part 1
(Source: Boystown Research Registry)
Hearing is one of the five major senses. To help you understand hearing and hearing impairment, this article will describe how the ear works, how hearing is tested, and how to read an audiogram.
How the Ear Works
The ear has three major parts: the outer ear, the middle ear, and the inner ear. The outer ear is the part that can be seen and includes the ear canal. The middle ear includes the ear drum (also known as the tympanic membrane), the three bones (ossicles) of the middle ear, and the eustachian tube. The eustachian tube leads to the throat and helps equalize pressure in the middle ear. The inner ear includes the cochlea and the semi -circular canals. The cochlea looks like a snail shell. Inside the cochlea are sensory cells (hair cells) that respond to sound and send nerve signals to the brain. The semi-circular canals are the balance organ.
Types of Hearing Loss
There are three types of hearing loss: conductive, sensorineural, and mixed. Conductive hearing loss can occur if the structures of the outer or middle ear do not work correctly. Conductive loss is more likely to respond to medical or surgical treatment . Sensorineural hearing loss (this is sometimes called nerve deafness by non-professionals) can occur if inner ear structures do not work correctly. Sensorineural hearing loss is more likely to be permanent. Mixed hearing loss involves both conductive as well as sensorineural components.
How is Hearing Tested?
Hearing is tested using earphones or loudspeakers which send sounds to the ear. Because hearing loss can occur in either or both ears, it is important that both ears are tested. If testing reveals a hearing loss, another device, called a bone vibrator, is used to determine the type of hearing loss. The bone vibrator sends sounds to the inner ear and bypasses the ear canal and middle ear. With conductive hearing loss, sounds can be heard at softer levels when listening through the bone vibrator than through the earphone. With sensorineural hearing loss, sounds will be heard at similar levels using both devices.
What is an Audiogram and What Does it Tell Me?
An audiogram is a graph describing hearing sensitivity. The degree (or amount) of hearing loss is determined by measuring the hearing threshold. The hearing threshold is measured in decibels (dB). The hearing threshold is the amount of sound that is just barely heard. The louder that sound must be made to be barely heard represents the degree of hearing loss.
Degrees of Hearing Loss
A person can have up to 25 dB hearing level (HL) and still have "normal" hearing.
Those with a mild hearing loss (26-45 dB HL) may have difficulty hearing and understanding someone who is speaking from a distance or who has a soft voice.
They will generally hear one-on-one conversations if they can see the speaker's face and are close to the speaker. Understanding conversations in noisy backgrounds may be difficult.
Those with moderate hearing loss (46-65 dB HL) have difficulty understanding conversational levels of speech, even in quiet backgrounds. Trying to hear in noisy backgrounds is extremely difficult.
Those with severe hearing loss (66-85 dB HL) have difficulty hearing in all situations. Speech may be heard only if the speaker is talking loudly or at close range.
Those with profound hearing loss (greater than 85 dB HL) may not hear even loud speech or environmental sounds. They may not use hearing as a primary method of communicating.
How does Hearing Loss Affect Listening to Speech?
In addition to the degree of loss, the frequency (or pitch) is also important for understanding speech. Frequencies are measured in Hertz (Hz).
For example, the sounds made by a bullfrog are "low" frequency tones, between 125 and 250 Hz, whereas the sounds made by a cricket are "high" frequency tones, between 4,000 and 8,000 Hz.
An audiogram documents hearing thresholds at frequencies important for hearing speech and other environmental sounds. The degree of loss is measured by how far up-or-down the threshold lies on the graph and the frequencies are measured from lower frequencies on the left to higher frequencies on the right. The shape or pattern of the hearing loss threshold on the audiogram is called the configuration. "X´s" are used to represent the left ear and "O´s" to represent the right ear´s configuration.
An example of an audiogram from an individual with normal low frequency hearing, sloping to a severe mid- and high- frequency hearing loss is shown. This person would be able to hear the speech sounds below 1,000 Hz (l, b, m, ... , a), but would not be able to hear those above 1,000 Hz (p, k, ... , sh).
Professionals Who Evaluate and Treat Hearing
A medical doctor (physician) who specializes in treating ear and hearing problems is an otologist or otolaryngologist. These doctors can determine whether the hearing loss could be improved with medical or surgical treatment, if additional tests are nece ssary, and if other health problems are related to the hearing loss. Audiologists are professionals who specialize in evaluating hearing loss and conducting hearing tests.
By: Leisha Eiten, CCC-A Leisha Eiten is a Clinical Audiologist at the Boys Town National Research Hospital. She has a Master of Arts (M.A.) and a Certificate of Clinical Competence in Audiology (CCC-A).
Date Originally Created: Spring, 1995.
The information presented here first appeared in publications of the Boys Town National Research Register for Hereditary Hearing Loss, the National Institute on Deafness and Other Communication Disorders (NIDCD), Hereditary Hearing Impairment Resource Registry (HHIRR), or the Boys Town Research Registry for Hereditary Hearing Loss.
The Boys Town Research Registry for Hereditary Hearing Loss
The Boys Town Research Registry for Hereditary Hearing Loss (Registry) is designed to foster a partnership between families, clinicians and researchers in the area of hereditary hearing loss/deafness through three primary functions. First, the Registry disseminates information to professionals and families about clinical and research issues related to hereditary deafness/hearing loss. Second, the Registry collects information from individuals interested in supporting and participating in research projects. This information is used to support the third function of the Registry - matching families with collaborating research projects.
For more information, contact:
Research Registry for Hereditary Hearing Loss
555 N. 30th Street
Omaha, NE 68131
800 320-1171 (V/TDD)
402 498-6331 (FAX)