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University of Toronto Scarborough
Matthias Niemeier

Chapter 9 Hearing Physiology and PsychoacousticsWhat is Sound Sound created when objects vibrateVibration causes pressure changes in medium pressure change describes as waveSound wave Travel at particular speed depends on mediumMove faster through denser substance Pattern of sound waves will not change as they spread out Initial amount of pressure change dispersed over a larger and larger area as waves moves awayBasic Qualities of Sound Waves Frequency and AmplitudeAmplitude or Intensity magnitude of pressure changes in sound wave The difference between highest and lowest pressure area Perceived as loudness pressure wave is represented as sine waveFrequency Rate of fluctuation perceived as pitch The number of times per second which pattern of pressure change repeats Sound wave frequency is measured in hertz Hz 1 Hz1 cycles per second Able to detect sound from 2020000 Hz in humanDecibelsdB Unit of measure for physical intensity of sound Difference between 2 sounds in terms of ration between sound pressures dB20 log pp 0P corresponds to pressure intensityP Reference pressure levels of DBSPL 0 P Defined as 00002 dyne per square centimeter common measure to many system 0Range of human hearing extends from 0 to over 120 dB SPL Decibel is a logarithmic scales relatively small decibel changeslarge physical changesSounds with amplitudes smaller than Phave negative decibel levels 0 Sine Waves and Complex SoundsSine wave Or pure tone waveform for which variation as function of time All sounds are combination of sine wavesSpectrum Representation of relative energy intensity present at each frequency Show how much energyamplitude is present at multiple frequencies Display the amplitude for each frequency present in sound waveShape of spectrum enables to distinguish different sounds Properties of sound sources determines spectral shapes of sounds Shape of spectraPattern of amplitudes for each harmonicHarmonic spectra Caused by simple vibrating source eg string of guitar Spectrum of complex sound which energy is at integer multiples of fundamental frequency Harmonic each frequency component in sound Fundamental frequency First harmonic frequency lowestfrequency component of sound All other harmonics have frequencies that are integer multiple of fundamental frequencyTimbre Describes quality of sound depends on relative energy levels of harmonic components Able to judge 2 sounds with same loudness and dissimilarity of pitches Conveyed by harmonics and other high frequenciesBasic Structure of Mammalian Auditory System Outer EarOuter ear External soundgathering portion consisting of pinna and ear canalPinna Curly structure on side of heard typically call an ear Outer funnellike part of the ear Where sounds are first collected from environment role in localize sound sourcesEar canal Canal which conducts sound from pinna to tympanic membrane Prevents damage to tympanic membrane enhance sound frequenciesTympanic membrane Eardrum thin sheet of skin at the end of outer ear canal Vibrates in response to pressure change of sound waves by moving in and out Border between outer ear and middle ear Damaged tympanic membrane is able to heal itself Middle EarRole Conveys and amplifies vibration from tympanic membrane to oval windowStructures Airfilled chamber containing middle bones or OssiclesConsist of 3 tiny ossicles 1 Malleus Connected to tympanic membrane and second ossicleReceives vibration from tympanic membrane is attached to incus 2 Incus Middle of 3 ossicles connecting malleus and stapes 3 Stapes Connected to incus on one endTransmit vibrations of sound waves to oval windowPresses against the oval window of cochlea on the other endOval window Flexible opening to cochlea staples transmits vibration to fluid inside Forms border between middle ear and innerOssicle Smallest bones in human body amplify sounds in 2 ways 1 Joints between bones are hinged which work like leversEnergy on one side of fulcrum joints become larger on anotherLever action increases amount of pressure change by about a third 2Ossicles amplify sounds by concentrate energy from larger to smaller surface area Tympanic membraneMalleusStapesOval windowPressure on oval window is magnified relative to pressure on tympanic membraneConsist of 2 muscles Tenor tympani and stapedius are smallest muscles in body Tense when sounds are very loud restrict movement of ossicles and muffle pressure changes 1Tenor tympani Muscle attached to malleusTensing the tensor tympani will decrease vibration 2 Stapedius Muscle attached to stapesTensing the stapedius will decrease vibrationAcoustic reflex Reflex protects ear from intense sounds via contraction of stapedius and tensor tympani muscles Cannot protect against abrupt loud sounds eg fire of a gunInner Ear Hollow cavity in temporal bone of skull consists of cochlea and semicircular canals of vestibular sys Made up of collection fluidfilled chambers Changes in sound pressure are translated into neural signalssimilar to retina Cochlea Major structure of inner ear spiral structure contains organ of CortiFunction as acoustic prism its sensitivity spreads across different sound frequency along length Structure of Cochlea Coiled structure embedded in temporal bone of skull size of baby pea Filled with watery fluids in 3 parallel cannels1 Tympanic canal also known as scala tympani Extends from round window at base of cochlear to helicotrema at the apex 2Vestibular canal also known as scala vestibule Extends from oval window at base of cochlea to helicotrema at the apex Will bulge out and put pressure on middle canal 3Middle canal also known as scala media Sandwiched between tympanic and vestibular canalsContains cochlear partition3 canals are separated by 2 membrances 1 Reissners membrane Between vestibular canal and middle canal 2Basilar membrane Between middle canal and tympanic canal A plate made up of fibers which have some stiffness not a real membrane Forms the base of cochlear partitionCochlear partition Combined basilar membrane tectorial membrane and organ of CortiResponsible for transduction of sound waves into neural signals Helicotrema Small opening connects tympanic and vestibular canalsBoth tympanic and vestibular canals wrap around middle canal Process of Vibration transmission1 Vibration transmitted through tympanic membrane and middleear bones2 Causes stapes to push and pull oval window inout of vestibular canal at base of cochlear3 Movement of oval window cause traveling waveswaves of pressure changes4 Pressure change flows through fluids in vestibular canalBase of cochlearApex Excessive intensive sound pressure remain transmitted through helicotremaBack to cochlear base through tympanic canal absorbed by stretching round windowRound window Soft area of tissue at base of tympanic canal Releases excess pressure remaining from extremely intense sounds Organ of Corti Structure on basilar membrane of cochlea
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