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
Properties of a Sound Wave
• Pitch is complicated by the fact that sound
waves can be complex
– Made up of more than one frequency
Properties of a Sound Wave
• A 1 second, 1 Hz sound wave:
Properties of a Sound Wave
• A 1 second, 2 Hz sound wave:
Properties of a Sound Wave
• A 1 second, 3 Hz sound wave:
Properties of a Sound Wave
• What would happen if two or more sound
waves were happening at the same time?
Properties of a Sound Wave
• Sounds superpose (they add together)
Properties of a Sound Wave
• Sounds superpose (they add together)
Properties of a Sound Wave
• Another way to think of this is that any
complex wave can be decomposed into its
component frequencies
Properties of a Sound Wave
• A spectrogram shows us a picture of this
decomposition
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 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