Transcript Presentation

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
Period - amount of time for one cycle
Frequency = number
of cycles per second
(1/Period)
Air Pressure
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
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 charges
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 realtime 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
Hearing
•
•
•
•
•
•
Detection
Loudness
Localization
Scene Analysis
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 - small increases in dB
mean large increases in sound energy
Detection and Loudness
• Sound level is measured in decibels (dB) a measure of the amplitude of air pressure
fluctuations
• dB is a log scale - small increases in dB
mean large increases in sound energy
• 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)
Detection and Loudness
• Loudness is the subjective impression of
sound level (and not identical to it!)
Detection and Loudness
• For example,
tones of different
frequencies that
are judged to be
equally loud have
different SPLs
(dB)
Detection and Loudness
• Hearing loss due to exposure to high-intensity sounds
(greater than 100 dB) can last many hours
Detection and Loudness
•
•
•
Incidence of noise-related hearing loss is increasing dramatically
iPods and other “earbud” music players are thought to be partly responsible
How loud is an iPod?
– maximum volume is approximate but is somewhere between 100 dB (hearing
damage in about 2 hours) to 115 dB (hearing damage in about 15 minutes)
•
•
Consequences: difficulty understanding speech, tinnitus, deafness
Your perception of loudness adapts so it’s hard to tell how loud your iPod is LOCK THE VOLUME ON YOUR iPOD!
Localization
• recall the lake analogy: task is to localize the
positions of the boats on a lake using the
pattern of ripples at two points on the shore
Localization
• All you have is a pair of instruments (basilar
membranes) that measure air pressure
fluctuations over time
Localization
• There are several clues you could use:
Localization
Left Ear
Right Ear
Compression
Waves
Localization
•
There are several clues you could use:
1
arrival time - sound arrives first at ear
closest to source
Localization
Left Ear
Right Ear
Compression
Waves
Localization
•
1.
2.
There are several clues you could use:
arrival time
phase lag (waves are out of sync) - wave at
ear farthest from sound source lags wave at
ear nearest to source
Localization
Left Ear
Right Ear
Compression
Waves
Localization
•
1.
2.
3.
There are several clues you could use:
arrival time
phase lag (waves are out of sync)
sound shadow (intensity difference)- sound
is louder at ear closer to sound source
Localization
•
What are some problems or limitations?
Localization
•
Low frequency sounds aren’t attenuated
by head shadow
Sound is the same
SPL at both ears
Left Ear
Right Ear
Compression
Waves
Localization
•
Left Ear
Right Ear
High frequency sounds have ambiguous
phase lag
Left Ear
Right Ear
Two locations, same phase information!