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Auditory System:
Sound
The Stimulus Input: Sound Waves
•
Sound, like light, comes in waves, which are characterized by their frequency (pitch)
and amplitude (decibels). The height of the wave gives us the amplitude of the
sound. The frequency of the wave gives us the pitch of the sound.
•
Sound is vibration. The vibrational energy of vibrating objects, such as guitar
strings, transfer the surrounding medium-air as the vibrating objects push the
molecules of the medium back and forth. In space, there is no air, so the sound
wave would have no medium to push. Any explosion, for example, would be eerily
without sound.
High pitched sounds
QuickTime™ and a
decompressor
are needed to see this picture.
Frequency --> number of cycles completed
by a wave in a given amount
of time determines pitch
**Pitch --> sound’s highness or lowness
measured as hertz (Hz)
Amplitude --> physical strength of a
wave--the “volume” of a
sound; measured in
decibels
Low pitched sounds
The Stimulus Input: Sound Waves
Locating where sound
is originating from
requires two cues:
--Which ear hears
the sound first?
--Which ear hears
the louder sound?
The Stimulus Input: Sound Waves
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
4,186 Hz
1,000 Hz
100 Hz
27 Hz
Highest note on a piano
Highest pitch of human voice
Lowest pitch of human voice
Lowest note on a piano
The Stimulus Input: Sound Waves
• Above are examples of Frequency & Amplitude/decibels
• Timbre – distinctive quality of a sound determined by the complexity of
the wave and its different combinations of frequencies. (Figure C is more
complex than Figures A or B)
Distinguishing Pitch
• Frequency theory -- basilar membrane vibrates at the same frequency as the
sound wave
– The higher the frequency wave, the faster the firing of hair cells
– Theory used to explain how you hear low frequencies
• Place theory -- different frequencies cause larger vibrations at different
locations along the basilar membrane
– Different pitches stimulate different areas on the basilar membrane
– The brain receives these messages and interprets them as different
pitches
– Theory used to explain how you hear high frequencies
• Use both theories when you listen to sounds with both high and low
frequencies
Auditory Masking
• Low frequency
sounds effectively
mask high
frequency sounds
Vibration
amplitude
of basilar
membrane
Bassoon, loud
Piccolo, soft
Distance along basilar membrane
(a)
Effect of bassoon on basilar membrane
• High frequency
sounds cannot
effectively mask low
frequency sounds
Vibration
amplitude
of basilar
membrane
Piccolo, loud
Bassoon, soft
Distance along basilar membrane
(b)
Effect of piccolo on basilar membrane
Visual Capture
The most accessible example is a movie theater.
Imagine being in a movie theater. Where does the
sound originate? The sides of the room we are in,
because we can picture in their minds the big
speakers hanging on the sides of the movie
theater. When we are watching a movie, we
believe the sound comes from the front of the
room. When the actors are talking on-screen, the
sound appears to come from their mouths because
of visual capture - the eyes overrule the ears.
The Process Of Hearing (How
Sound Waves Become
Auditory Sensations)
Auditory perception occurs when sound waves
interact with the structures of the ear
Audition (sense of hearing) results in sound waves
being collected in the outer ear, amplified in the middle
ear, and converted to neural messages in the inner ear.
•
The middle ear transmits the eardrum’s vibrations
through a “piston” made of 3 small bones (the hammer,
anvil and stirrup) to the cochlea (snail shaped tube).
•
The incoming vibrations cause the cochlea’s membrane
(oval window) to vibrate, moving the fluid that fills the
tube. This motion causes ripples in the basilar
membrane (hair cells).
•
The movement of cells triggers impulses in the adjacent
nerve fibers which form the auditory nerve that connects
via the thalamus to the temporal lobe.
Parts Of The Ear
Parts Of The Ear
Revisited
Anatomy Of The Cochlea: Another
View
Outer ear Middle ear
Inner ear
Cochlea,
partially
uncoiled
Tectorial
membrane
Hair cells
Hammer Anvil
Basilar
membrane
Stirrup
Oval window
Sound
waves
Auditory
canal
Eardrum
Round window
A sound causes
the basilar
membrane to wave
up and down
Parts Of The Ear
Pinna --> Sounds waves enter it and are funneled to the auditory canal
Auditory Canal --> Connects the outer ear with the eardrum (tympanic membrane)
Ear Drum --> Tissue barrier that transfers sound vibrations to the ossicles; can be damaged by
objects in the ear or exceptionally loud noises
Ossicles (hammer, anvil, stirrup) --> Three tiny bones that transfer sound waves from the ear drum
to the cochlea; in old age, they may become brittle or damaged,
resulting in conduction deafness
Oval window --> Point on the surface of the cochlea which receives the sound vibration from the
ossicles; as the oval window vibrates, the fluid in the cochlea vibrates, moving hair
cells along the basilar membrane
Cochlea --> A hearing organ where sound waves are changed into neural impulses
Hair cells --> Receptor cells for hearing in the cochlea that change sound vibrations into neural
impulses. When they move, they trigger action potential in the base of the hair cell
(transduction).
--Similar to the rods and cones within the eye
--If these are damaged (due to prolonged loud noises), then you have nerve deafness
which cannot be helped by a hearing aid.
Auditory Nerve --> Stimulated by the hair cells in the basilar membrane of the cochlea, it carries
sound information from the ears to the thalamus then to the auditory cortex in the
temporal lobes of the brain
Position And Movement
• Vestibular sense: The sense of body orientation with
respect to gravity
– The receptors for this information are tiny hairs in the
semicircular canal of the inner ear
Types Of Hearing Loss (Deafness)
Conduction Hearing Loss --> Hearing loss caused by damage to
the mechanical system that conducts sound waves to the cochlea.
 Surgery
Sensorineural Hearing Loss --> Hearing loss caused by damage
to the cochlea’s receptor cells (i.e. hair cells) or to the auditory
nerve, also called nerve deafness.
 Hearing aid to amplify sound
 Cochlear Implant