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

eardrum

the hammer (malleus)

the anvil (incus)

the stirrup (stapes)

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.

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