audition december 4, 2009 the rest of the way production exercise #4 due at 5 pm today monday:...
TRANSCRIPT
The Rest of the Way• Production Exercise #4 due at 5 pm today
• Monday: review + practice spectrogram reading
• Production Exam:
• posted as soon as I finish grading Production Exercise #4
• due on Friday the 18th (at 5 pm) of finals week
• Final Exam Reminder: Friday, December 11th
• 3:30 - 5:30 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
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 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;
• Cells at the apex respond to low.tonotopic organization of the
cochlea
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.
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