auditory system
DESCRIPTION
working of human hearing systemTRANSCRIPT
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Speech Processing
mohitgoel4u.net(Mr. Feb)
Human Auditory System
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Why Do We Have Two E
• Sound localization – spatially locate sou
sources in 3-dimensional sound fields, batwo-ear processing, loudness differencestwo ears, delay to each ear
• Sound cancellation – focus attention on‘selected’ sound source in an array of sousources – ‘cocktail party effect’, BinauralMasking Level Differences (BMLDs)
• Effect of listening over headphones =>
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Overview of Auditory Mecha
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The Human Ear
Outer ear : pinna and external canal
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Ear and Hearing
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Human Ear • Outer ear: funnels sound into ear canal
• Middle ear: sound impinges on tympanicmembrane; this causes motion – middle ear is a mechanical transducer, consisti
hammer, anvil and stirrup; it converts acousticawave to mechanical vibrations along the inner e
• Inner ear: the cochlea is a fluid-filled champartitioned by the basilar membrane – the auditory nerve is connected to the basilar m
via inner hair cells
– mechanical vibrations at the entrance to the coc
create standing waves (of fluid inside the cochlecausing basilar membrane to vibrate at frequen
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The Outer Ear
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The Middle Ear The Hammer (M
Anvil (Incus) an
(Stapes) are thebones in the bod
they form the cou
between the vibr
eardrum and the
exerted on the ovthe inner ear.
These bones can
of as a compoun
achieves a multipforce—by a facto
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O u t e r e a r g a i n ( d B )
0.2 0.3 0.5 1.00.7 2 3 5 107
Frequency (KHz)
20
10
0
-10
Transfer Functions at the Perip
M i d d l e
e a r g a i n ( d B )
20
10
0-5
20
0
-20
-40 R e s p o n s e G a i n ( d B )
Combined respo(outer+middle ear)
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The CochleaMalleus
Incus
Stapes
Ossicles
(Middle Ear Bo
Oval Windo
Vestibule
Auditory nerv
Tympanic
Membrane
Round Window
Cochlea
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The Inner Ear The inner ear ca
of as two organs
semicircular can
serve as the bod
organ and the c
serves as the bo
microphone, con
sound pressure
the outer ear int
impulses which
on to the brain v
auditory nerve.
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The Auditory Nerve
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Auditory Nerves
IHC IHCInner
Hair Cells
Basilar Membran
Cochlear
(Implic
Vestibular System
Oval Window
Perilymph
Malleus
Incus Stapes
RoundWindow
Eustachian Tube
Middle Ear Cavity
Tympanic Membrance
Middle and Inner Ear
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Schematic Representation o
Ear
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Stretched Cochlea & Basilar Memb1600 Hz
800 Hz
400 Hz
200 Hz
100
50 Hz
0 Distanc
Basilar
Membrane
Scala
Vestibuli
C
Cochlear Base
(high frequency)
Unrolled
Cochlea
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Basilar Membrane Mecha
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Basilar Membrane Mecha• characterized by a set of frequency responses at differe
along the membrane
• mechanical realization of a bank of f ilters• filters are roughly constant Q (center frequency/bandwidt
logarithmically decreasing bandwidth
• distributed along the Basilar Membrane is a set of about 3sensors, called Inner Hair Cells (IHC), which act as mech
motion-to-neural activity converters• mechanical motion along the BM is sensed by local IHC c
firing activity at nerve fibers that innervate bottom of eac
• each IHC connected to about 10 nerve fibers, each of difdiameter => thin fibers fire at high motion levels, thick fibe
lower motion levels• 30,000 nerve fibers link IHC to auditory nerve
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Human Auditory System
mohitgoel4u.net(Mr. Feb)
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Human Auditory System
• The pinna (or informally the ear) is the surface
surrounding the canal in which sound is funneled.
• Sound waves are guided by the canal toward the
eardrum—a membrane that acts as an acoustic-to-mechanic transducer. The sound waves are then
translated into mechanical vibrations that are passed
to the cochlea through a series of bones known as
the ossicles.
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Human Auditory System
• Presence of the ossicles improves sound propagation by
reducing the amount of reflection and is accomplished by
the principle of impedance matching.
• The cochlea is a rigid snail-shaped organ filled with
fluid. Mechanical oscillations impinging on the ossiclescause an internal membrane, known as the basilar
membrane, to vibrate at various frequencies.
• The basilar membrane is characterized by a set of
frequency responses at different points along the
membrane.
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Human Auditory System
•Motion along the basilar membrane is sensed by theinner hair cells and causes neural activities that are
transmitted to the brain through the auditory nerve.
• Due to this arrangement, the human auditory system
behaves very much like a frequency analyzer.
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Absolute Threshold
•The absolute threshold of a sound is the minimumdetectable level of that sound in the absence of any other
external sounds.
The horizontal axis is frequency measured in hertz (Hz);
while the vertical axis is the absolute threshold in decibels
(dB), related to a reference intensity of watts per
square meter—a standard quantity for sound intensity
measurement.
1210
−
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Absolute Threshold
• As we can see, human beings tend to be more
sensitive toward frequencies in the range of 1 to 4
kHz, while thresholds increase rapidly at very high
and very low frequencies. It is commonly acceptedthat below 20 Hz and above 20 kHz, the auditory
system is essentially dysfunctional.
•These characteristics are due to the structures of thehuman auditory system.
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Absolute Threshold
We can take advantage of the absolute threshold curvein speech coder design. Some approaches are the
following:
• Any signal with an intensity below the absolutethreshold need not be considered, since it does not
have any impact on the final quality of the coder.
• More resources should be allocated for the
representation of signals within the most sensitive
frequency range, roughly from 1 to 4 kHz, since
distortions in this range are more noticeable.
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Masking
• Masking refers to the phenomenon where one sound is
rendered inaudible because of the presence of other
sounds.
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Masking
• The presence of a single tone, for instance, can mask
the neighboring signals—with the masking capability
inversely proportional to the absolute difference in
frequency.
• Masking capability increases with the intensity of the
reference signal, or the single tone in this case.
mohitgoel4u.net(Mr. Feb)