Module 20 Notes
Hearing
Audition = the sense or act of hearing
 We hear a wide range of sounds but mostly those with similar frequency to the human
voice
 Can differentiate between thousands of different voices instantly
Act of hearing:
Soundwaves force are molecules to move
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Amplitude of waves determines loudness
Frequency determines pitch (# of times a complete wave passes a given point)
o Pitch = tone – high or low
Sound measures in decibels (0 = absolute threshold….every 10 = 10 fold increase in intensity
 Whisper = 20, Conversation = 20, Train = 100 (1 Billion times more intense)
The Ear:
Sound waves are funneled from outer ear (visible) through auditory canal to the eardrum.
Eardrum vibrates, 3 bones (Hammer, anvil, stirrup) in middle ear (between eardrum and
cochlea) transmit vibrations to cochlea . Cochlea (snail shaped in inner ear) – full of liquid that
vibrates causing ripples and bending the hair cells that line its surface. Hair cell movement
triggers impulses in nearby nerve cells. Axons converge and carry those neural impulses
through the auditory nerve, through the thalamus, and into the auditory cortex in the
temporal lobe.
Cochlea has 16,000 hair cells (compared to 130 million rods and cones dedicated to vision)
Sensorineural hearing loss – damage to the cochlea’s receptor cells or auditory nerve (nerve
deafness)
Conduction hearing loss – damage to the mechanical system that conducts sound waves to the
cochlea
Hair cells are like carpet – footsteps will quickly disappear, heavy furniture may leave
permanent damage.
Hearing loss is now 1 in 5 for teens (especially boys)
Cochlear Implant – only way to offset hearing lose. Converts sounds into electrical signals,
uses electrodes threaded into the cochlea to stimulate auditory nerve.
Loudness is determined by the number of hair cells stimulated (individual hair cells are all or
none in what they communicate to the brain)
Perceiving Pitch
Place Theory – Hermann Helmholtz = pitch is determined by the specific location of the hair
cells that were stimulated in the cochlea. (Like a shaken bed sheet)
 Explains how we hear high pitch sounds but not low pitch sounds
Frequency Theory – Pitch is determined by the rate (frequency) of the neural impulses
traveling to the brain. Frequency of the impulse matches the frequency of the sound tone
Locating Sounds
Sound travels 750 miles per hour
By having two ears, 1 ear will detect a sound slightly faster than the other which allows us to
determine locations.
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Brain is well adjusted to the rate of sound thus computing distance.