SHORT NOTES ON BASIC CLINICAL SCIENCES–EAR AND HEARING MECHANISM-

Sound is Produced by Vibrations in Matter (Air, Liquids, Solids)

• Anything which causes matter to vibrate will produce sound
• Vocal cords vibrate and push on the air flowing through the larynx, causing the air to vibrate
• Sound cannot travel through a vacuum
• Sound velocities:
  • Air: 344 meters/sec (770 miles/hr)
  • Water: 1500 meters/sec (3360 miles/hr)
  • Solids: about 5000 meters/sec (11,200 miles/hr)

Pitch is Determined by the Frequency of Vibration

• We perceive fast vibrations as high pitches, slow vibrations as low pitches
• When we are young we can hear vibrations from about 20 hertz to about 20,000 hertz
• A hertz = cycle/second
• Middle C on the music scale is 256 hertz

Sound Intensities Are Measured in Decibels

• The stronger the vibration (the higher the sound pressure) the louder the noise
• The scale used for sound intensity is logarithmic because of the very wide range of sound intensities that we encounter
• The ear is sensitive to sound intensities over a range of a million to one
• Sound intensities are given in decibels (dB)
  • Decibel = 20 X log P/Po
  • P = sound pressure;
  • Po = reference sound pressure = pressure at threshold of hearing for 4000 herz tone
• The human ear can respond to a range of sound intensities of about a million to one
• For an intensity a million times Po the number of decibels is:
  • Decibels = 20 X log (1,000,000/1) = 120
  • Sounds louder than this will be painful and will damage the ear
• A few examples of sound intensities:
  • Threshold of hearing (4000 herz) = 0 dB
  • Soft whisper = 20 dB
  • Conversation = 60 dB
  • Busy traffic = 70 dB
  • Rock band = 120 dB
  • Pain threshold = 130 dB

Basic Ear Anatomy

The Ear Has 3 Essential Parts

• mechanism for collecting and amplifying sound
• a transducer to convert sound vibrations into action potentials
• nerves to deliver the action potential signals to the brain for interpretation

The Outer Ear Collects Sound Waves

• The pinna (auricle) and ear canal funnel the waves inward to the eardrum (tympanic membrane)
• The waves cause the eardrum to vibrate

Bones of the Middle Ear (Malleus, Incus, Stapes) Amplify the Sound Waves

• The eardrum attaches to the malleus (hammer)
• The malleus attaches to the incus (anvil), which in turn attaches to the stapes (stirrup)
• The stapes is attached to the oval window of the cochlea
• Because of the way the bones are attached together the vibrations in the oval window are 20X larger than those in the eardrun (amplification)
• If the sound is too loud small muscles attached to the ear bones contract and dampen the vibrations

This diagram is modified from one in The Sourcebook of Medical Illustration, edited by Peter Cull (Park Ridge, NJ: Parthenon, 1989).

The Cochlea Converts Sound Vibrations to Action Potentials in the Auditory Nerve

• Vibration of the oval window causes cochlear fluid (perilymph & endolymph) to vibrate
• The fluid vibrations in turn cause the basilar membrane to vibrate, producing traveling waves
• The basilar membrane vibrations cause hair cells to bend -> generator potential
• If generator potentials are large enough they will stimulate fibers of the auditory nerve to produce action potentials
• Different pitches are detected in different parts of the cochlea:
  • High pitches produce traveling waves at the base of the cochlea (near the oval window)
  • Low pitches produce traveling waves at the apex

The Ear is More Sensitive to Some Frequencies than to Others

• The ear is most sensitive to tones best in the range of 500 to 4000 hertz (this is the range of normal speech)
• An audiogram measures the hearing threshold at different frequencies: note that the threshold is lowest at middle frequencies:

• At about 120 decibels we start to feel the sound as a tickling sensation
• The ear is useful in the decibel range between the hearing threshold and the feeling threshold
• At about 130-140 decibels sound starts to produce pain
• As we age we loose our ability to hear high pitched sounds (see red curve on graph)
  • Later loss occurs in the speech range

The Auditory Nerve Delivers the Action Potential Signals to the Temporal Cortex

• After synapsing in the thalamus auditory impulses are sent to the temporal cortex
• Different tones are sent to different parts of the cortex
• If the sounds have special meaning (speech, music) other parts of the cortex are activated
• Auditory reflexes are controlled by the inferior colliculus

Deafness Can Be Conduction, Sensorineural or Central

• Conduction deafness: mechanical problems in conducting sound to oval window- often caused by problems with ear bones (middle ear infections)
• Sensorineural deafness: cochlear or auditory nerve damage- often caused by hair cell loss
• Central deafness: damage to auditory pathways or centers in the central nervous system– sometimes caused by strokes

More Information

Click to see QuickTime animations of the ear in action (John Brugge, Thomas Pasic, Bill Rhode and Tom Yin , University of Wisconsin).

If you are interested in music you may enjoy David Worrall’s notes on the Physics and Psychophysics of music.

The Center for Hearing Loss in Children in Omaha, NE, has a site that shows how hearing loss affects speech perception.