Fundamentals of Hearing - Part 2
(Source: Boystown Research Registry)
This article is the second in a series developed to help you understand hearing and hearing impairment. In an effort to expand on the first article in this series titled "Fundamentals of Hearing", tests used to check the function of the middle ear will be covered here, as well as how the ability to understand speech is assessed.
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 eardrum (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.
What Does the Eardrum Do and How Is It Tested?
The eardrum is a thin membrane located at the end of the ear canal. It is attached to the wall of the ear canal, as well as to the first ossicle. The eardrum and ossicles work together, to send sound entering the ear canal through the air, to the cochlea of the inner ear which is fluid-filled. If the eardrum does not work effectively, less sound will be transmitted to the cochlea.
Eardrum movement can be assessed in different ways. Doctors often use a Pneumatic Otoscope (ear light), which is an instrument used to look at the ear canal and eardrum.
This type of otoscope can blow a puff of air against the eardrum to show how well the eardrum moves or vibrates back and forth.
In order for sound to be transmitted effectively through the eardrum, it must have a certain amount of mobility. Another test of eardrum movement is called Tympanometry. This test measures how well the eardrum responds to changes in pressure and results are shown on a graph called a tympanogram. To obtain a tympanogram, a machine called a tympanometer is used. This machine is attached to a small, soft tip which fits snugly into the ear canal. A soft noise may be heard while the tympanometer records eardrum movement.
What Does a Tympanogram Look Like?
A tympanogram shows the amount of eardrum movement (compliance), as well as the pressure at which the eardrum moves best. Because the middle ear is typically an air-filled space, the pressure in a normal middle ear should be close to room pressure (0 daPa). The shape of the tympanogram suggests how the eardrum is functioning. Normal eardrum movement is indicated on the tympanogram by a well-formed peak which occurs near room pressure.
The height of the peak indicates the amount of eardrum compliance. Pressure is measured in decaPascals (daPa) on the horizontal axis.
If the tympanogram shows a flat line, this indicates little or no eardrum movement. This type of tympanogram is commonly seen when fluid is present behind the eardrum, not allowing it to move well. It also can be consistent with a hole or perforation in the eardrum.
A tympanogram that shows a well-formed peak ocurring to the left of the range of normal pressure indicates adequate eardrum movement, with negative pressure behind the eardrum.
This type of tympanogram can be seen when cold symptoms persist or when an ear is starting the process of producing fluid or recovering from fluid. Neither of these tests measure hearing loss. However, hearing loss may be present when concern for eardrum movement exists.
When Hearing Loss Exists How Well Can Speech Be Understood?
As described in "Fundamentals of Hearing," hearing loss affects a person´s ability to hear and understand speech. For some people with hearing loss, making speech loud enough does not always make it clearer or easier to understand. As part of a comprehensive hearing assessment, testing how well speech is understood should be completed.
A Speech Reception Threshold (SRT) is the softest level at which spondee words can be accurately identified. A spondee word has two syllables with equal emphasis on both syllables (i.e., baseball, railroad). The SRT is obtained before a word recognition test is given.
A word recognition test assesses a person´s ability to understand words. It requires the listener to repeat a list of single syllable words presented at a constant loudness level. The loudness level at which the words are presented is chosen by considering the shape of the audiogram, the SRT and the listener´s loudness comfort level. It is important that the presentation level is loud enough so that speech information is as audible as possible. This test is commonly scored by the percentage of words accurately identified. For example, a score of 90 - 100% is considered excellent, 80 - 89% is rated as good, 70 - 79% is fair, 60 - 69% is poor, <50% is considered very poor. </p />
To obtain a more complete view of hearing and ear function, a listener´s word recognition ability should be considered along with other tests, such as the audiogram and tympanogram. Each piece of information fits together to help those who specialize in ear and hearing problems determine the most appropriate method of treatment for each individual.
Date Originally Created: Spring, 1996.
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)