Sensory Systems: Auditory
What do we hear?
• Sound is a compression wave:
Speaker
Air Molecules
When speaker is stationary, the air is
uniformly dense
What do we hear?
• Sound is a compression wave:
Speaker
When the speaker moves, it compresses the air in front of it.
What do we hear?
• Sound is a compression wave:
Rarefaction
Compression
The speaker moves back leaving an area with less air behind called rarefaction
What do we hear?
• Sound is a compression wave:
Speaker
Compression
Rarefaction
The speaker moves forward again starting the next wave
What do we hear?
• Sound is a compression wave - it only
“looks” like a wave if we plot air pressure
against time
Time
Properties of a Sound Wave
• 1. Amplitude: difference in air pressure
between compression and rarefaction
(Sound Pressure Level)
Properties of a Sound Wave
• 1. Amplitude: difference in air pressure
between compression and rarefaction
(Sound Pressure Level)
– What is the perception that goes along with
the sensation of sound amplitude?
Properties of a Sound Wave
• 1. Amplitude: difference in air pressure
between compression and rarefaction
(Sound Pressure Level)
– What is the perception that goes along with
the sensation of sound amplitude?
LOUDNESS
Properties of a Sound Wave
• 2. Frequency: how many regions of
compression (or rarefaction) pass by a
given point per second (expressed in
Hertz)
Properties of a Sound Wave
• 2. Frequency: how many regions of
compression (or rarefaction) pass by a
given point per second (expressed in
Hertz)
– What is the perception that goes along with
the sensation of frequency?
Properties of a Sound Wave
• 2. Frequency: how many regions of
compression (or rarefaction) pass by a
given point per second (expressed in
Hertz)
– What is the perception that goes along with
the sensation of frequency?
PITCH
Sensing Vibrations
Sensing Vibrations
• Outer ear
transmits and
modifies sound
(critical for sound
localization)
Sensing Vibrations
• Middle ear turns compression waves
into mechanical motion
oval window
stapes
Sensing Vibrations
• Middle ear turns compression waves
into mechanical motion
Oval window
Ear Drum
Sensing Vibrations
• Middle ear turns compression waves
into mechanical motion
Oval window
Ear Drum
Compression Wave
Sensing Vibrations
• The cochlea, in the inner ear, is a curled
up tube filled with fluid.
Auditory
Nerve to
Brain
Sensing Vibrations
• Inside the cochlea is the basilar
membrane
• Movement of the oval window causes
ripples on the basilar membrane
Sensing Vibrations
• Basilar membrane measures the
amplitude and frequency of sound
waves
– amplitude (loudness)
–frequency (pitch)
Sensing Vibrations
• Basilar membrane measures the
amplitude and frequency of sound
waves
– amplitude (loudness) - magnitude of
displacement of the basilar membrane
–frequency (pitch)
Sensing Vibrations
• Basilar membrane measures the
amplitude and frequency of sound
waves
– amplitude (loudness) - magnitude of
displacement of the basilar membrane
–frequency (pitch) - frequency and location of
displacements of the basilar membrane
Sensing Vibrations
• Basilar membrane measures the
amplitude and frequency of sound
waves
–frequency (pitch) - frequency and location of
displacements of the basilar membrane
Sensing Vibrations
• Bundles of “hair cells” are embedded in
basilar membrane
Sensing Vibrations
• When hair cells sway
back and forth, they let
ions inside
• This flow of charges is
converted to action
potentials and sent
along the auditory
pathway
The Auditory Pathway
• The auditory pathway is
complex and involves
several “stations” along
the way to the auditory
cortex in the brain
• Lots of processing must
be done in real-time on
auditory signals!
How Can You Localize
Sound?
• Ponder this:
– Imagine digging two trenches in the sand beside a lake so
that water can flow into them. Now imagine hanging a piece
of cloth in the water in each trench. Your job is to determine
the number and location and type of every fish, duck,
person, boat, etc. simply by examining the motion of the
cloth. That’s what your auditory system does!
- Al Bregman
How do we Stay Balanced?
The Vestibular System
Vestibular System (Balance)
Vestibular System (Balance)
Vestibular System (Balance)
Vestibular System (Balance)
Head
accelerates
this way
Fluid goes
this way
Cupula gets
pushed
Vestibular System (Balance)
Fluid goes
this way
Head
accelerates
this way
Cupula gets
pushed
Vestibular System (Balance)
• movement of the cupula is detected by
hair cells
• hair cells in the vestibular system are
more sensitive than hair cells on the
basilar membrane!
Vestibular, Visual, and Proprioceptive
Systems Work Together
• Try standing on one foot with your eyes
closed!
Fun Facts about The Vestibular
System
• Seasickness arises when the vestibular
system and the visual system send conflicting
information
Fun Facts about The Vestibular
System
• Seasickness arises when the vestibular
system and the visual system send conflicting
information
• People can be knocked down by moving
walls!
Fun Facts about The Vestibular
System
• Seasickness arises when the vestibular
system and the visual system send conflicting
information
• People can be knocked down by moving
walls!
• Alcohol causes the spins by (among other
things) changing the density of the fluid in the
semicircular canals
Hearing
•
•
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Detection
Loudness
Localization
Music
Speech
Detection and Loudness
• Sound level is measured in decibels
(dB) - a measure of the amplitude of
air pressure fluctuations
Detection and Loudness
• Sound level is measured in decibels
(dB) - a measure of the amplitude of
air pressure fluctuations
• dB is a log scale - 1 dB difference =
10 times the actual air pressure
Detection and Loudness
• Sound level is measured in decibels
(dB) - a measure of the amplitude of
air pressure fluctuations
• dB is a log scale - 1 dB difference =
10 times the actual air pressure
• We have a dynamic range that is a
factor of 7.5 million!
Detection and Loudness
• minimum sound level necessary to be
heard is the detection threshold
Detection and Loudness
• detection threshold depends on
frequency of sound:
• very high and very low frequencies
must have more energy (higher dB) to
be heard
• greatest sensitivity (lowest detection
threshold) is between 1000 hz to
5000hz
Detection and Loudness
• Detection can be compromised by a
masking sound
• even masking sounds that are not
simultaneous with the target can
cause masking (forward and
backward masking)