Types of Hearing Loss

It is estimated that 25% of Americans between 65 and 75 have some level of hearing loss, and that estimate increases to 75% for those over the age of 75.  There are two main types of hearing loss, Conductive and Sensorineural.  Additionally, a person can have a mixed hearing loss, which means that they have some of both.  A conductive hearing loss means there is a problem with the ability of the outer and/or middle ear to conduct (or transfer) the sound energy to the inner ear (Cochlea).  A sensorineural hearing loss means that there is a problem with the inner ear's ability to sense the sound energy and effectively communicate it to the brain.

Other Terms as Related to Hearing Loss

  • Bilateral or Unilateral?  Bilateral means that the loss is in both ears, while unilateral means only one ear is affected.
  • Symmetrical or Asymmetrical?  Symmetrical means that both ears have very similar loss, while asymmetrical means the loss is different.
  • Progressive or Sudden?  Progressive means the loss increases gradually over time; while sudden loss happens quickly at once.  Someone with a sudden hearing loss should seek immediate medical attention.
  • Fluctuating or Stable?  Fluctuating loss means the person's hearing is repeatedly getting worse and better; while stable hearing loss means the person's hearing is not changing day to day, or month to month, but generally consistent.
  • Tinnitus.  Tinnitus is the medical term for "ringing in the ears."  It means that without any external noise, there is a perception of a ringing, whistling, hissing, or other noise in the ear.  The noise can be constant or periodic, soft or loud.  This condition can occur in a person of any age and gender.  Tinnitus can interfere with normal hearing, "masking" out external sounds.

Conductive Hearing Loss

Conductive hearing loss means there is a problem with the outer or middle ear in conducting the sound energy to the inner ear (Cochlea).  Conductive hearing loss can often be corrected.  The outer ear consists of the Pinna and the Auditory Canal.

If the Pinna is malformed or damaged, it may not be able to amplify and direct the sound energy in towards the Auditory Canal.  The amplification achieved by the Pinna can often be as high as 10 to 15 dB, depending on the Frequency and direction of the sound.

The Outer Ear
Figure 1: The outer ear funnels sound wave enery towards the middle ear.

The Auditory Canal (ear canal) also conducts and transmits sound energy towards the eardrum.  The ear canal acts like a funnel, bringing together the sound energy, and focusing it on the ear drum.  Any obstruction (or blockage) in the ear canal can cause the sound to be blocked, muffled, or cause the Auditory Canal's sound amplification ability to be ineffective.  Like the Pinna, the Auditory Canal achieves a significant sound amplification of around 10 dB, depending on the frequency of the sound.

The Tympanic Membrane (or eardrum) is between the outer and middle ear.  It vibrates with the sound energy, and causes the bones of the middle ear to oscillate.  If the eardrum is ruptured, scarred, or otherwise obstructed, it is not as effective at converting the the sound wave energy to kinetic energy (movement of the middle ear bones).

The Middle Ear
Figure 2: The middle ear bones conduct the ear drum vibrations to the inner ear.
The Ossicles are the three bones of the middle ear.  The connect Tympanic Membrane (ear drum) with the oval window of the inner ear (Cochlea).  The bones act like a lever causing the oval window membrane to be vibrated with a larger force.  Because the impediance mismatch between the air and the inner ear (air versus liquid), most of the sound energy of from the external ear would be reflected back and not enter the inner ear.  The bones of the middle ear cause the energy to be converted to so that it is effectively transmitted to the Cochlea.  The middle ear can be less effective if it is filled with fluid, or if If the ear bones are malformed or damaged.

There are several steps that the original sound waves go through before they enter the inner ear (Pinna, Auditory Canal, Tympanic Membrane, and the three bones of the Ossicles).  Each step is important in the amplification and transfferance of the sound energy to the inner ear.  If any one of those steps is blocked or diminished, the person's hearing would be partially impared.  Conductive hearing loss has often be corrected with professional medical attention, and is often due to blockage, illness or infection, or growths.

Sensorineural Hearing Loss

Sensorineural hearing loss is a decreased ability for the inner ear (Cochlea) to detect the sound energy and transmit it to the brain.  The Cochlea consists of fluid and tiny hairs that wave and vibrate with the sound energy.  The hair receptor cells sense the hair movement and send signals to the brain through the Auditory Nerve.

The hairs in the Cochlea can become broken or bent, causing them to not respond properly to the sound wave energy being transmitted from the outer and middle ear.  The nerve cells at the base of the hairs can also become damaged or degenerate.  Reducing their ability to communicate the information to the brain.

Causes of inner ear damage can be one, or a combination of the following risk factors:

  • Age.  As the body ages, the inner hairs of the inner ear can degenerate.  Just as hair on the head can thin or recede as with age, the hairs and nerve cells of the Cochlea can degenerate over time.  The level and speed of degeneration may vary greatly between people.
  • Illness.  Some illnesses, such as infections or high fevers, may cause injury to the inner ear.  Measles, Meningitis, Mumps, are some of the illnesses that may result in damage to the sensorineural hearing loss.
  • Medication.  Some drugs can adversely affect the ability of the inner ear to function.  These are classified as ototoxic drugs.  Some ototoxic drugs can cause irreversible hearing loss, like aminoglycosides; while the other can have temporary or reversible effects, like diuretics, aspirin, macrolide, and erythromycin.
  • Loud noise.  Loud noise causes wear and tear damage to the inner ear.  The damage caused by loud noise increases with the loudness and duration of the noise.  Wearing hearing protection is important for persons who are regularly in environments of loud noise.  Damage due to loud noise accululates over time.  Some damage may be temporary (like reduced hearing perception or ringing after a rock concert), but some remains and accumulates over the years.
  • Genetics.  Some hereditary factors can cause the inner ear to not form properly, or to degenerate more rapidly.  Genetics is a factor that may make some people more susceptible to the other risk factors above.

Out of the risk factors listed above, the only ones we really have control over is loud noise.  Care can be used when taking medications that they do not hurt your hearing, but doctors generally prescribe ototoxic drugs when necessary.  Nevertheless, whenever you begin a new medication, you should be alert to identify any potential side effects (like hearing loss) and talk to your doctor about them as soon as possible.

Controlling Loud Noise

Because loud noise is the main hearing loss risk factor that we can control, we should be extra careful about about the noise levels we subject our ears to.  Everyday exposure to loud noises can add up over time to a degenerative hearing loss.  Turn the radios, TVs, cell phones, and MP3 players down to safe levels.  Eliminate sources of loud noise in the home and car.  Work environments should be evaluated to ensure that they are safe.

Sometimes work environments require us to be in loud environments (gun shooting range, manufacturing facilities, construction site, airport operations, etc.)  Ear protection should be worn in these environments.  If the noise exposure is great, or prolonged, custom ear plugs can be purchased from a local audiologist or professional hearing aid dispensor.  Custom ear plugs provide better protection and comfort.

Exposure to sounds 85 dB and louder can damage your hearing.  The louder the sound, the shorter the amount of time before possible damage is done to the ears.  Below is a table showing some loudnesses, the allowed exposure, and example noises:

Example Noises and Exposure Levels
Noise Level
85 8 hours busy city traffic, idling bulldozer 
90 2.5 hours lawnmower, motorcycle, hair dryer
95 47 minutes belt sander
100 15 minutes wood shop, hand drill
105 4 minutes impact wrench
110 90 seconds chain saw, jack hammer
115 30 seconds ambulence siren

As the table above shows, some equipment and work environments can cause hearing loss with excessive exposure.  Even a hair dryer can cause hearing loss if the exposure is for more than two and a half hours.
Ear protection should also be worn when we go to rock concerts or other potentially loud events.  Even though it may not be fashionable to do so, protecting your hearing should be the primary concern.  If you are prepared with ear plugs, you can wear them if you need them.  If you don't have ear plugs, you will be left vulnerable if the situation is louder than you expected.

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