Audition
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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

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Audition

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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 fluidof 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.


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:

    • Microphone

    • Speech processor

    • 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


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