Normal Hearing & Hearing Loss

Normal Hearing & Hearing Loss

1. Ear Canal
2. Eardrum
3-5. Ossicles
6. Oval Window
7. Canal Leading to the Nose
8.Cochlea
9. Auditory Nerve

Normal Hearing

The ear is made up of three main sections:

The Outer Ear
The Middle Ear
The Inner Ear

Sounds from the outside world are picked up by the outer ear, which is made up of the pinna and the ear canal. As the sound waves enter the ear, the ear canal (1) serves to increase the loudness of those pitches that make it easier to understand speech. At the same time the ear canal protects another important part of the ear: the eardrum (2) - a flexible, circular membrane which vibrates when touched by sound waves.

The sound vibrations continue their journey into the middle ear, which contains three tiny bones called the ossicles (3-5), which are also known as the hammer, anvil and stirrup. These bones form the bridge from the eardrum into the inner ear. They increase and amplify the sound vibrations even more, before safely transmitting them on to the inner ear via the oval window.

The Inner Ear, or cochlea (8), resembles the circular shell of a snail, and houses a system of tubes which are filled with a watery fluid. As the sound waves pass through the oval window (6) the fluid begins to move, setting tiny hair cells in motion. In turn, these hairs transform the vibrations into electrical impulses that travel along the auditory nerve (9) to the brain itself. Exactly how the brain actually translates these nerve impulses remains a mystery.

The following video from MED-EL (www.medel.com) will explain this process in further detail.
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Degree of Hearing Loss

Results of the audiometric evaluation are plotted on a chart called an audiogram. Loudness is plotted from top to bottom. Frequency, from low to high, is plotted from left to right. Hearing loss (HL) is measured in decibels (dB) and is described in general categories. Hearing loss is not measured in percentages. The general hearing loss categories used by most hearing professionals are as follows:

Normal hearing (0 to 25 dB HL)
Mild hearing loss (26 to 40 dB HL)
Moderate hearing loss (41 to 70 dB HL)
Severe hearing loss (71 to 90 dB HL)
Profound hearing loss (Greater than 91 dB HL)

Types of Hearing Loss

The external and the middle ear conduct and transform sound; the inner ear receives it. When there is a problem in the external or middle ear, a conductive hearing impairment occurs. When the problem is in the inner ear, a sensorineural or hair cell loss is the result. Difficulty in both the middle and inner ear results in a mixed hearing impairment (i.e. conductive and a sensorineural impairment). Central hearing loss has more to do with the brain than the ear, and will be discussed only briefly.

Conductive hearing loss occurs when sound is not conducted efficiently through the ear canal, eardrum, or tiny bones of the middle ear, resulting in a reduction of the loudness of sound that is heard. Conductive losses may result from earwax blocking the ear canal, fluid in the middle ear, middle ear infection, obstructions in the ear canal, perforations (hole) in the eardrum membrane, or disease of any of the three middle ear bones.

A person with a conductive hearing loss may notice that their ears may seem to be full or plugged. This person may speak softly because they hear their own voice quite loudly. Crunchy foods, such as celery or carrots, sound very loud and this person may have to stop chewing to hear what is being said. All conductive hearing losses should be evaluated by a physician to explore medical and surgical options.

Conductive hearing loss is further explained by this video from MED-EL.
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Sensorineural hearing loss is the most common type of hearing loss. More than 90 percent of all hearing aid wearers have sensorineural hearing loss. The most common causes of sensorineural hearing loss are age related changes and noise exposure. A sensorineural hearing loss may also result from disturbance of inner ear circulation, increased inner fluid pressure or from disturbances of nerve transmission. Sensorineural hearing loss is also called "cochlear loss," an "inner ear loss" and is also commonly called "nerve loss." Years ago, many professionals said there was nothing that could be done for sensorineural hearing loss - that is totally incorrect today. There are many excellent options for the patient with sensorineural hearing loss.

A person with a sensorineural hearing loss may report that they can hear people talking, but they can't understand what they are saying. An increase in the loudness of speech may only add to their confusion. This person will usually hear better in quiet places and may have difficulty understanding what is said over the telephone.

Sensorineural hearing loss is explained in more detail in the following video from MED-EL.
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Central hearing impairment occurs when auditory centers of the brain are affected by injury, disease, tumor, hereditary, or unknown causes. Loudness of sound is not necessarily affected, although understanding of speech, also thought of as the "clarity" of speech may be affected. Certainly both loudness and clarity may be affected too.
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