Hearing Basics

How We Hear…

Hair cells in the inner ear:

How We Hear Diagram Image, Audiologist, St. Louis - Associated Hearing Professionals

Hearing depends on a series of events that change sound waves in the air into electrical signals.

Our auditory nerve then carries these signals to the brain through a complex series
of steps.

 

  • Sound waves enter the outer ear and travel through a narrow passageway called the ear canal, which leads to the eardrum.
  • The eardrum vibrates from the incoming sound waves and sends these vibrations to three tiny bones in the middle ear. These bones are called the malleus, incus, and stapes.
  • The bones in the middle ear amplify, or increase, the sound vibrations and send them to the inner ear—also called the cochlea—which is shaped like a snail and is filled with fluid. An elastic membrane runs from the beginning to the end of the cochlea, splitting it into an upper and lower part. This membrane is called the “basilar” membrane because it serves as the base, or ground floor, on which key hearing structures sit.
  • The sound vibrations cause the fluid inside the cochlea to ripple, and a traveling wave forms along the basilar membrane. Hair cells—sensory cells sitting on top of the membrane—“ride the wave.”
  • As the hair cells move up and down, their bristly structures bump up against an overlying membrane and tilt to one side. This tilting action causes pore-like channels, which are on the surface of the bristles, to open up. When that happens, certain chemicals rush in, creating an electrical signal.
  • The auditory nerve carries this electrical signal to the brain, which translates it into a “sound” that we recognize and understand.
  • Hair cells near the base of the cochlea detect higher-pitched sounds, such as a cell phone ringing. Those nearer the apex, or centermost point, detect lower-pitched sounds, such as a large dog barking.

We Need Two Ears…

Inner Ear Diagram Image, Audiologist, St. Louis - Associated Hearing ProfessionalsOur two ears act like radar antennae to register acoustic signals coming from multiple directions. The complex structures of each ear process the received signals and pass them to the brain where we interpret our acoustic environment.

Take, for example, the sound of the wind or of an approaching truck: the nearest ear receives the sound slightly earlier than the other and a little louder. Using the finely processed acoustic information from each ear, the brain has the capacity to calculate the direction of the wind or the direction of the truck’s approach and we also “know” approximately how close it is.

Advantages of Two Properly Functioning Ears:

  • Excellent Sound Localization Skills
  • Easier Speech Understanding in Noisy Situations
  • Rich Sound Quality
  • Accurate Judgement of Appropriate Volume
  • Ability to Hear All Tones of Voice, Including Children!

There are Three Major Types of Hearing Loss:

Sensorineural, Conductive & Mixed

Sensorineural hearing loss is the most common type of hearing loss and is caused by damage to the inner ear and/or the auditory nerve. Noise exposure, diseases, certain medications and aging can destroy parts of the inner ear and cause permanent hearing loss. Sensorineural hearing loss usually effects the high frequencies, which impairs a person’s ability to differentiate consonant sounds and thus the fine distinctions in words such as “fit” versus “sit”. Your audiologist can effectively treat this type of loss with hearing instruments.

Conductive hearing loss is caused by a problem in the outer or middle ear, or from a defect in the ossicular chain. Conductive hearing loss can often be medically treated. Your Audiologist will be able to diagnose this type of hearing loss and refer you to the appropriate medical professional for treatment.

When a patient has both a conductive hearing loss and a sensorineural hearing loss, it is called a “Mixed Hearing loss”. This type of hearing loss often requires both medical/surgical intervention and the use of hearing aids. Your audiologist will diagnose this type of loss and refer you to the appropriate medical professional for treatment.

Source: NIDCD, National Institue on Deafness and Other Communication Disorders, 2010.

For more information on hearing-loss, and other communication disorders, visit NIDCD.com