Download The Ear

TheEar
Audiblerange–20Hz‐20kHz
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childrentypicallyhearupto20kHz
Adultshearupto10‐14kHz
>70yearsoldusuallycan’thear>8kHz
FormandFunction
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Airpressurechangesareperceivedbythebrainthroughnervecells
i)
Outerear:pinna,andcavitytoeardrum
a) auricle/pinna:
i. collectsandfilterssound
ii. ridgeshelpfocusecholocation
‐horsescanindependentlyaimtolocalizesound
‐humanslocalizewithCNS,throughloudnessandarrivaltime
differencesbetweenears
b) Outercanal
i. helpstoamplifysoundandprotecteardrumwithwax
c) Eardrum/tympanicmembrane
ii)
MiddleEar:aircavityoninsideofeardrumventilatedbyEustachiantube
throughnose
a) 3ossicles:malleus/hammer,incus/anvil,andstapes/stirrup
i. usesleverandhydraulicphenomenatoefficientlytransmitsound
1. hammercoverslargeportionofdrumtorelayairpressure
changesthathittheeardrum
2. anvillevers
3. smallstirruppushesonovalwindowwhichmovesfluidin
cochlea/innerear
ii. protecttheinnerearfromloudnoisesandsuddenpressurechanges
byretractingthestirrupsfromwindow(triggeredat80‐95db)
iii)
InnerEar:liquidfilledcavitysurroundedbyhardbonewiththeCochlea,
Vestibular/SemicircularCanals
a) Cochlea:hearingorgan
b) Vestibular/semicircularcanals(orlabyrinth):balance(gravityandmotion)
organ
c) whenossicleskickthewindow,fluidininnerearispushedagainsthairsin
innerear,whichreleaseneurotransmitterswhenstimulating/bending
HearingLoss
Causes:
1) BirthDefects:
‐ fixedormissingossicles
‐ holesinthetympanicmembrane
2) Damage
‐ impactedearwax
‐ inflammation/infection
‐ abrasions(qtips)scarring/thickeningofeardrum,reducingsensitivity
‐ calcificationovertimewrecksbones
‐ loudsounds/highpressuredifferences(explosions/diving)
o canbursteardrum
o breakossicles
o destroyinnerearhaircells
‐ overexposuredestructionoftheouterhaircells
‐ antibiotics‐ex.tetrcyclinesdamagethehaircells
Timescaleofhearingloss:
1) Temporary
‐ earcandesensitizetolargeamountsofnoiseinashorttime
‐ physiologicalfatiguecansetin,requiring16hourstodisappear
‐ severeexposurecancausepathologicalfatigue,requiringupto3weeksforcomplete
recovery
2) Permanent
‐ Prolongedexposuretoloudnoises
‐ greatestrisksatfrequenciesabove4kHz
‐ lossof<5kHzsoundsaffectsunderstandabilityofspeechespeciallyplosives(pb)
andfricatives(s,f)
Protection
‐ softdeformableplasticcangivereductionof20‐30dB
‐ musicianshavespeciallymadeonesthatdropallfrequenciesby15dB
‐ somehaveasmallchannelincentreyields30dBreductionathighfrequencies,only
passinglowfrequencies
EXPERIMENT
speakerwithfunctiongeneratorto"testhearing"
‐ slowlysweepdownfrom20KHztoseewhenstarthearing
‐ ifbelow10kHz,seeanaudiologist!
EarandtheBrain’sSiftingofSound
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earhastheamazingcapabilityofdiscriminatingbetweenpitch,loudness,andtone
colour
Nervoussystemfunction
o assessesmodesfromeachear,andperformsaveragedmeasurement
o canmake“runningaverages”,summinginfooverashortperiodoftimeand
averagingit(tocorrectformovement)
can“makesense”outofcomplicatedsignalsbycomparinginfofrombothears,andthe
timingrelativearrivaltimes
”Precedenceeffect”:Earcombinesidenticalsoundsifthearrivewithin35msofeach
other
1sttoarrivepredominatelydetermineslocationofthesource
unlessthe2nd,3rdetc…are>3xamplitude,theywillaugment
butinitialreflectionsarrivingafter10mscanassistindetermininglocationand
intelligibilityofsound
reflectionorsecondarysourcecomeearlierthansource?
o Confusioninbrainyieldsmuddlesintelligibility
o electronicdelaysofamplifiedsoundcanbalance
o toavoid,loudspeakerstendtobeplacedbehindsource
o highceilingscandelaytheseconfusingreflections.