Download 23-Audition

Audition
December 3, 2008
The Rest of the Way
• Production Exercise #4 due at 5 pm today
• Friday: review + practice spectrogram reading
• Production Exam:
• posted as soon as I grade PE #4
• due on Friday (5 pm) of finals week
• Final Exam Reminder: Friday, December 12th
• 12 - 2 pm
• SS 541
How Do We Hear?
• The ear is the organ of hearing. It converts sound waves
into electrical signals in the brain.
• the process of “audition”
• The ear has three parts:
• The Outer Ear
• sound is represented acoustically (in the air)
• The Middle Ear
• sound is represented mechanically (in solid bone)
• The Inner Ear
• sound is represented in a liquid
The Ear
Outer Ear Fun Facts
• The pinna, or auricle, is a bit more receptive to sounds
from the front than sounds from the back.
• …but basically functions as an “earring holder”
• Sound travels down the ear canal, or auditory meatus.
• Sounds between  3000-4000 Hz resonate in the ear
canal
• The tragus protects the opening to the ear canal.
• Optionally provides loudness protection.
• The outer ear dead ends at the eardrum, or tympanic
membrane.
The Middle Ear
the anvil
(incus)
the hammer
(malleus)
the stirrup
(stapes)
eardrum
The Middle Ear
• The bones of the middle ear act as an amplifier
• the “ossicles”
• increase sound pressure by about 20-25 dB
• Works by focusing sound vibrations into a smaller area
• area of eardrum = .85 cm2
• area of footplate of stapes = .03 cm2
• Leverage also factors in…
• Like a crowbar.
The Attenuation Reflex
• For loud sounds
(> 85-90 dB), a reflex
kicks in to attenuate
the vibrations of the
middle ear.
• This helps prevent
damage to the inner
ear…
tensor
tympani
stapedius
The Attenuation Reflex
• Requires 50-100
msec of reaction time.
• Poorly attenuates
sudden loud noises
• Muscles fatigue after
15 minutes or so
• Also triggered by
speaking
tensor
tympani
stapedius
The Inner Ear
• The action of the stirrup at
the oval window shoves
fluid around in the inner ear,
including the cochlea
• The fluid is electrically
charged
• Inside the cochlea is the
basilar membrane
• Different parts of the
basilar membrane are
maximally displaced by
sounds of different
frequencies.
How does it work?
• On top of the basilar
membrane are
thousands of tiny hair
cells.
• Upward motion of the
basilar membrane
pushes these hairs into
the tectorial membrane.
• The upward deflection of the hairs opens up channels in
the hair cells.
• ...allowing the electrically charged fluid of the inner ear
to flow in.
• This sends a neurochemical signal to the brain.
Auditory Frequency Analysis
• Individual hair cells in
the cochlea respond
best to particular
frequencies.
• General limits:
20 Hz - 20,000 Hz
• Cells at the base
respond to high
frequencies;
tonotopic organization of the
cochlea
• Cells at the apex
respond to low.
Frequency Perception
• There are more hair cells that respond to lower
frequencies…
• so we can distinguish those from each other more
easily.
• The Mel scale test.
• Match this tone:
• To the tone that is twice its frequency:
• Now try it for a high frequency tone:
The Mel Scale
• Perceived pitch
is expressed in
units called mels.
• Note: 1000 Hz =
1000 mels
• Twice the
number of mels =
twice as high of a
perceived pitch.
Loudness
• The perceived loudness of a sound is measured in units
called sones.
• The sone scale also exhibits a non-linear relationship
with respect to absolute pressure values.
Equal Loudness Curves
• Perceived loudness also depends on frequency.
Audiograms
• When an audiologist tests your hearing, they determine
your hearing threshold at several different frequencies.
• They then chart how much your hearing threshold differs
from that of a “normal” listener at those frequencies in an
audiogram.
• Noise-induced
hearing loss tends
to affect higher
frequencies first.
• (especially
around 4000 Hz)
Deafness
• Deafness results when the hair cells of the cochlea die,
or do not work properly.
• Presbycusis is a natural loss of auditory sensitivity to
high frequencies due to age
• = loss of hair cells at the base of the cochlea
• Note: the “teen buzz”
• A hearing aid simply amplifies sounds entering the ear.
• (sometimes at particular frequencies)
• For those who are profoundly deaf, a device known as a
cochlear implant can restore hearing.
Cochlear Implants
A Cochlear Implant artificially stimulates the nerves
which are connected to the cochlea.
Nuts and Bolts
•
The cochlear implant chain of events:
1. Microphone
2. Speech processor
3. Electrical stimulation
•
What the CI user hears is entirely determined by the
code in the speech processor
•
Number of electrodes stimulating the cochlea ranges
between 8 to 22.
•
•
 poorer frequency resolution
Also: cochlear implants cannot stimulate the low
frequency regions of the auditory nerve
Nuts and Bolts
• The speech processor divides up the frequency scale into
8 (or 22) bands and stimulates each electrode according to
the average intensity in each band.
This results in what sounds (to us) like a highly degraded
version of natural speech.
What CIs Sound Like
• Check out some nursery rhymes which have been
processed through a CI simulator:
Mitigating Factors
• The amount of success with Cochlear Implants is highly
variable.
• Works best for those who had hearing before they
became deaf.
• Depends a lot on the person
• Possibly because of reorganization of the brain
• Works best for (in order):
• Environmental Sounds
• Speech
• Speaking on the telephone (bad)
• Music (really bad)
Critical Period?
• For congentially deaf users, the Cochlear Implant
provides an unusual test of the “forbidden experiment”.
• The “critical period” is extremely early-• They perform best, the earlier they receive the implant
(12 months old is the lower limit)
• Steady drop-off in performance thereafter
• Difficult to achieve natural levels of fluency in speech.
• Depends on how much they use the implant.
• Partially due to early sensory deprivation.
• Also due to degraded auditory signal.
Practical Considerations
• It is largely unknown how well anyone will perform with a
cochlear implant before they receive it.
• Possible predictors:
• lipreading ability
• rapid cues for place are largely obscured by the
noise vocoding process
• fMRI scans of brain activity during presentation of
auditory stimuli