Download Ch09 Lecture Part I

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• The Basics:
– Nature of sound
– Anatomy and physiology of the auditory system
Hearing: Physiology and
Psychoacoustics
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The Function of Hearing
What Is Sound?
– How we perceive loudness and pitch
– Impairments of hearing
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Sound Wave and Air Pressure
• Sounds are created when objects vibrate
– Vibrations of object cause molecules in object’s
surrounding medium to vibrate as well, which
causes pressure changes in medium
– Waves pressure changes
• Compression – increased pressure
• Rarefaction – decreased pressure
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What Is Sound? (cont’d)
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• Sound waves travel at a particular speed
• Basic qualities of sound waves
– Depends on medium
– Frequency: For sound, the number of times per
second that a pattern of pressure change repeats
– Example:
– Amplitude: Magnitude of displacement of a sound
pressure wave
• Speed of sound through air is about 340
meters/second
– Waveform: The shape of the soundwave
• Speed of sound through water is 1500
meters/second
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What Is Sound? (cont’d)
What Is Sound? (cont’d)
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Frequency and Amplitude
• Frequency is associated with pitch
– Low-frequency sounds correspond to low pitches,
(e.g., low notes played by a tuba)
– High-frequency sounds correspond to high
pitches, (e.g., high notes from a piccolo)
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Frequency
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• Human hearing uses a limited range of frequencies:
From about 20 to 20,000 Hz
• One cycle: rarefaction and compression
• Frequency: # of cycles per unit time
• Distinction:
– Physical stimulus -- frequency
– Psychological experience -- pitch
• Unit: Herts (hz)
– #cycles / second
– 500 Hz -- 500 cycles/second
– 2000 Hz -- 2000 cycles/second
• Human range: 20 - 20,000 hz
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What Is Sound? (cont’d)
• Humans can hear across a wide range of sound
intensities
– Ratio of pressure changes between faintest and
loudest sounds is more than one to one million
• Faintest: .0002 dynes/cm 2
• High Risk: >200 dynes/cm 2
– In order to describe differences in amplitude, sound
levels are measured on a logarithmic scale, in units
called decibels (dB)
– Relatively small decibel changes can correspond to
large physical changes (e.g., increase of 6 dB
corresponds to a doubling of the amount of pressure)
What Is Sound? (cont’d)
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Decibels
• dB = 20 log (p1/p0)
– P1 = pressure of interest
– P0 = standard pressure (threshold = .0002 dynes/cm 2)
• Absolute threshold example
– dB = 20 log (.0002 / .0002)
– dB = 20 log (1)
– dB = 0
• High Risk example
– dB = 20 log (200 / .0002)
– dB = 20 log (1,000,000)
– dB = 120
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Intensity of Environmental Sounds
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What Is Sound? (cont’d)
•One of simplest kinds of sounds: Sine wave, or pure
tone
–Sine wave: Waveform for which variation as a
function of time is a sine function
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What Is Sound? (cont’d)
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Sine
• Sine waves: Not common everyday sounds because
not many vibrations in the world are so pure
– Most sounds in world: Complex sounds, (e.g.,
human voices, birds, cars, etc.)
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Example waveforms
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Human speech (long e)
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Complex Waveforms
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Complex Sound Waves
• Sound waves -- pressure changes
– Will summate
– Point by point addition of pressure fluctuations
– Example
• Consequently
– Can think about “assembling” complex wave
– Can think about “disassembling” complex wave
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What Is Sound? (cont’d)
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Wave Form and Spectrum (Part 1)
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Harmonic Sounds with the Same Fundamental
• Complex sounds can be described by Fourier
analysis
– A mathematical theorem by which any sound can
be divided into a set of sine waves. Combining
these sine waves will reproduce the original sound
– Results can be summarized by a spectrum
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What Is Sound? (cont’d)
• Harmonic spectra: Typically caused by simple
vibrating source, (e.g., string of guitar, or reed of
saxophone)
– Relative intensities of different frequency
components
– Waveform of sound
– Timbre: Psychological sensation by which listener
can judge that two sounds that have same
loudness and pitch are dissimilar
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Basic Structure of the Mammalian Auditory System (cont’d)
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Outer Ear
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Basic Structure of the Mammalian Auditory System (cont’d)
• Outer ear:
– Sounds are first collected from environment by the
pinnae
– Sound waves are funneled by the pinnae into ear
canal
– Length and shape of ear canal enhance sound
frequencies
– Main purpose of canal is to insulate structure at its
end: Tympanic membrane
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Mammalian Pinnae
•Tympanic membrane: Eardrum; a thin sheet of skin at
end of outer ear canal; it vibrates in response to sound
–Increased pressure – moves in
–Decreased pressure – moves out
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Basic Structure of the Mammalian Auditory System (cont’d)
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Structure of the Human Ear (Part 1)
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Basic Structure of the Mammalian Auditory System (cont’d)
• Middle ear:
– Air Filled pocket behind tympanic membrane
– Three tiny bones: Ossicles
• Malleus, Incus, Stapes
• (aka: Hammer, Anvil, Stirrup)
• Role: Amplify sounds
– Stapes transmits vibrations of sound waves to
oval window, another membrane which represents
border between middle ear and inner ear
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Basic Structure of the Mammalian Auditory System (cont’d)
• Amplification provided by ossicles is essential to
ability to hear faint sounds
– Inner ear is made up of collection of fluid-filled
chambers
– Need to amplify pressure to create pressure
waves in cochlear fluid
– Amplification (magnify pressure 30x)
• Lever principle
• Ossicles also important for loud sounds
• Middle ear: Two muscles-tensor tympani and
stapedius
– Purpose: To tense when sounds are very loud,
muffling pressure changes
– However, acoustic reflex follows onset of loud
sounds by about one-fifth of second, so cannot
protect against abrupt sounds, (e.g., gun shot)
• Funnel energy from larger tympanic membrane
to smaller foot plate of stapes
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Basic Structure of the Mammalian Auditory System (cont’d)
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• Inner ear: Changes in sound pressure are translated
into neural signals
Basic Structure of the Mammalian Auditory System (cont’d)
• Cochlear canals and membranes
– Cochlea: Spiral structure of the inner ear
containing the organ of Corti
– Function is roughly analogous to that of retina
– Cochlea is filled with watery fluids in three parallel
canals
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The Cochlea (Part 1)
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The Cochlea (Part 2)
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The Cochlea (Part 3)
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The Cochlea (Part 4)
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Basic Structure of the Mammalian Auditory System (cont’d)
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Basic Structure of the Mammalian Auditory System (cont’d)
• The three canals of the cochlea
– Tympanic canal
– Vestibular canal
– Middle canal
• Vibrations transmitted through tympanic membranes
and middle-ear bones cause stapes to push and pull
flexible oval window in and out of vestibular canal at
base of cochlea
– If sounds are extremely intense, any remaining
pressure is transmitted through helicotrema and
back to cochlear base through tympanic canal,
where it is absorbed by another membrane:
Round window
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Basic Structure of the Mammalian Auditory System (cont’d)
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• Organ of Corti
Basic Structure of the Mammalian Auditory System (cont’d)
• Hair cells in each human ear: Arranged in four rows
that run down length of basilar membrane
– Movements of cochlear partition are translated
into neural signals by structures in the organ of
Corti; extends along top of basilar membrane
– Made up of specialized neurons called hair cells,
dendrites of auditory nerve fibers that terminate at
base of hair cells, and scaffold of supporting cells
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Vibration and the Tectorial Membrane
• Tectorial membrane: Extends atop organ of Corti ;
gelatinous structure
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Review of Neural Functioning
• - charge inside / + charge outside
• Negative membrane potential
• Ion channels -- change membrane potential
• Transduction -- modify ion channels
– Modify potential of neuron
– Neural signal
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Transduction in Audition
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Transduction
• Hair cells -- resting potential (-50 to -70 mv)
• Movement of BM - movement of cilia (back and forth)
• Cilia movement
– One direction
• Opens ion channels
• Depolarize hair cell (more +)
• Increase release of NT to auditory nerve fibers
– Other direction
• Closes ion channels
• Hyperpolarize hair cell (more -)
• Decrease release of NT to auditory nerve fibers
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Auditory Nerve Fibers
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