Download The Ear - Pathway of Hearing

The Ear - Pathway of
Hearing
Auditory Phonetics
How to Handle Speech
UBielefeld
What is Sound?
“ ... a sound is any disturbance of air that
could cause a displacement of the
eardrum which, after transmission by
the bone chain, could affect the liquid in
the inner ear in such a way that the
auditory nerves are stimulated.”
(Ladefoged, Elements of Acoustic Phonetics, pp.2-3)
How Sound Travels
“The source of sound causes movements
of the air particles in its immediate
neighborhood; these movements cause
disturbances in the air a little farther
away from the source; these air
particles in their turn affect their
neighbors which are still farther away
from the source; and so the disturbance
spreads outward.”
(Ladefoged, ibid, p.3)
Just Another Sound: The Human
Voice
• voice: fluctuations of air pressure >
caused by rapid opening and closing of
vocal folds
• pressure rises to a maximum, then falls
steadily to a minimum before rising
again to repeat the cycle, all within a
small fraction of a second
Varying Air Pressure
• maxima: peaks above 0
• minima: valleys below 0
• peaks of air pressure
<=> air particles close
together
• moments of low
pressure <=> air
particles furthest apart
THINK: What about sound in a vacuum?
Hearing vs. Perception
• ear (peripheral auditory organ): hearing
> sound reception & sound
transformation
• auditory organ (includes central
nervous system; central auditory organ):
perception > interpretation of semantic
contents
Our Auditory Field
Our Auditory Field
http://home.schule.at/teaching/Hoeren_und_Hoerprobleme/vorgang/hoerfeld.htm
The Ear
• outer ear
– protection,
– collection of sound
• middle ear
– transmission of mechanic vibrations
• inner ear
– transformation of vibrations into neural
impulses
The Ear
outer ear
inner ear
middle ear
The Ear Cont‘d.
The Path of Sound
• external canal
• vibrates eardrum
• vibration moves through ossicles
– mallet (malleus), anvil (incus), stirrup
(stapes)
• stapes vibrates oval window of cochlea
• creates pressure wave in the fluid inside
The Outer Ear
• Pinna / auricle:
– protects the entrance to the ear
canal
– contributes to our ability to locate
sounds, esp. at higher frequencies
• Meatus/outer ear canal:
– pathway for the acoustic signal
– protection of the inner ear
– tube resonator, i.e. favors
transmission of high-frequency
sounds (2000-4000 Hz) and
generally between 500 and 4000 Hz
The Middle Ear
The Middle Ear Cont‘d
• Eardrum:
– is deflected by air pressure variations (= sound)
– converts air pressure variations into mechanical movement
– transmits this movement to the auditory ossicles
The Middle Ear Cont‘d
auditory ossicles: mallet, anvil, stirrup:
– pick up the mechanical movement of the eardrum and
convey it to the inner ear/oval window
– transformation of air pressure variation into equivalent
mechanical movements
– contraction between anvil and stirrup can reduce amplitude
(protection for inner ear)
The Middle Ear Cont‘d
• Eustachian tube:
– narrow tube connecting
pharynx and middle ear
– equalizes air pressure
differences between outer
and middle ear
• oval window:
– interface between middle
and inner ear
– transmits mechanical
movement
The Inner Ear
• cochlea:
– coil-like structure terminating in a window with a
flexible membrane at each end
– internally divided by two membranes, filled with
cochlear fluid
– converts mechanical movements into neural
signals
The Inner Ear Cont‘d Cochlea
The Inner Ear Cont‘d
• basilar membrane:
– stiffer at one end than at the other
> sensitive to particular frequencies at particular
points because of the different resonance which is
due to different stiffness
• high frequencies picked up at the stiff end
• lower frequencies at the more flexible end
> conducts spectral analysis of incoming sound
(see animation later!)
The Inner Ear Cont‘d
cross section of one
single turn of the cochlea
(no 5. = basilar membrane)
The Inner Ear Cont‘d
• Organ of Corti:
– attached along the basilar membrane
– hair cells which get moved and excited
– responsible to transform membrane displacement into neural
signals
1. inner hair cells
2. outer hair cells
3. tunnel of Corti
4. basilar membrane
The Inner Ear Cont‘d Hair Cells
• about 16,000 in each cochlea
• located on the basilar membrane
• do not regenerate
• inner (~ 3,500) hair cells
vs. outer hair cells (~ 20,000)
• stimulated by bending of the basilar
membrane (length a wave travels on the
basilar membrane depends on frequency
> first frequency (spectral) analysis)
• amplitude: lower & medium amplitudes
affect outer hair cells, higher amplitudes
affect inner hair cells
Anatomy of the Cochlea
Animated
http://www.hhmi.org/biointeractive/animations/c
ochlea/coc_frames.htm
The Auditory Nerve
• Auditory nerve:
– picks up the neural signals from the organ of Corti
– sends signals to the brain where the information is
processed and interpreted > analytical effects of
the human auditory system
The End