HEARING SCIENCE SPHSC 461
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Date Created: 09/09/15
EE516 C ompllmr Speech Processing Winmr 2005 Lecture 4 Jan 12 2005 Lecturer Prof J Bilme ltbi lineseee Washington edugt University oi Washington Dept oi Electrical Engineering Scribe ScotrPhilip 41 Anatomy and Physiology of the Ear The eat can be divided into thtee gtoss sections shown in Figute 41 o Outet Eat a Middle Eat a Innet Eat impulse signals that ate used by the brain Outer Mldt s nnal 1 Semtmcular t canals lncus t t l l l l l l Audltu ry nelve rs Cuclllca l Hmund Ovalwumui wmaoh under outplate at stapes Figute 41 Closerection of the Human Eat W04 42 The Outer Ear ale Pinna This is the extemal ap of cattilage sunounding the enttance to the eaL The shape of the pinna causes a Iesonance effect that will altei the amplitude of the piessuie wave at diffeient fiequenciesl Since this spectIum 39 39 i aonL a 39 r39 39 quot lnt aliatinn Auditory Canal 39 quotmi 39 10 p p r r Speci cally this Iesonatoi ampli es the specttum between 2kHz and SkHzl This is an impoitant Iange fox speech Iecognitionl Figuie 42 shows the oveiall tIansfeI function fox both the pinna and auditoiy canal The auditon canal combined with the pinnais known as the meatus Tympanic Membianeeadrum The function of the tympanic membianeis to collect aiI vibiations at the end of the auditoiy canal and convert m of mole than 100dBl In othei woids the maximum sound piessuie level spl the eaidium can detect is mote than 10000000000 times the minimum Amplituddda 5 Frequency um Figu1e4l2 Tiansfei Function ofthe Extemal Eat W04 43 The Middle Ear he middle eat is a 2 cm space between the tympanic membiane and the cochlea It consists of thee bones the malleus incus and stapes sunounded by Bill This is shown in Figuie 43 Togethei these bones ale known as the ossiclesl The function of L 39dd39 39 39 39 e aiI in the outei eat and the uid in the innei eaL It do this b r 39 focusing it on the Ielatively small oval window that sits at the base of the stapes Like any medium the middle eat will also slightly altei the spectrum Its ttansfei function is shown in Figuie 4A connect AUI T r 39 f L 39 39 39 L r 39 themiddle eaL To peifoimoptimally the t L L L i a we have an eustachian tube that connects the innei eat to the back of the mouth allowing the piessuie in the middle eat to equalize with the ambient piessuiel This is what happens when youI eats pop when changing elevationl Anothei thing that can deciease the ttansfei of eneigy in the middle eat ate the muscles sunounding the ossiclesl When 39 the r 39 39 39s can happen both consciously and unconsciously to prevent intense sounds from damaging the inner ear This often happens after a loud sound since loud sounds often follow loud sounds The military ofter takes advantage of this when ring large cannonsi They will set off an initial precharge to prep peoples middle ears for the following larger sound Malleus Imus I Tympanic membrane eardrum Auditory canal 4 A A4 Figure 43 The Middle Ear W04 Prawns Gain dB rmm Eamrum to MICE o A lo 100 1000 moou Frequency Hz Figure 44 Transfer Function of the Middle Ear P88 44 The Inner Ear o Cochlea The most important part of the inner ear is the cochlea shown in Figure 45 It is a coiled tube about 35mm long The tube is lled with a lymphatic uid and is divided lengthwise by the basilar membrane and organ of cortii The stapes connects to the cochlea at its base through the oval windowi Vibrations from the stapes travel through the lymphatic uid to the apex of the cochlea and then back down the other side of the partition to the round window This induces movement of the partition A lengthwise crosssection of the cochlea is shown in Figure 46 The manner in which the partition moves plays an important pait in how the sound is encoded into neural impulses semicircular auditory incus amp nerve malieus Figure 45 The Inner Ear W04 Scam vestibull I it Scale medl window Round window Hehcotremu Scale Tympani Figure 46 The Cochlea Unrolled P88 Organ of Corti Figure 47 shows a cross section of the cochlea Along the left side there are a series of nerves that are fed into the middle of the cochlea This area is known as the organ of corti and is where transduction conversion of physical movement to neural impulses occurs Figure 48 show a closer view of the organ of corti It sits atop the basilar membrane and below the tectorial membrane As the basilar membrane moves up and down the tectorial membrane sheers across the organ of corti This causes the cilia that sit above the hair cells to bend This results in the ring of the nerves attached to the hair cells It is important to note that at different points along the cochlea the organ of corti may be registering different levels of vibration All told there are about 30000 sensory hair cells measuring the exact movement of the cochlea Innr lml Relitul l39 ll I39Hin I TIP Inn MI quot Cells of Hansen Inner radial fihrn Ea Heran 1 membrane Dow soiqu bril arfereml Supporting Balers Hells Figure 48 Organ of Corti P88 45 Encoding of Sound It is very dif cult to obtain a precise measurement of have the cochlea moves It is surrounded by bone as a result any instrument use for measurement is bound to affect the normal operation of the ear Therefore there is no consensus as to exactly how the inner ear decomposes acoustical waves Two common theories are the place theory and the timing theory Both believe the ear contains a series of band pass lters that perform something similar to a short time fourier transform These lters are thought to have a constant Q where Q is de ned as the center frequency divided by its bandwidth 0 Place Theory As sound travels down the cochlea a traveling wave is induced along the basilar membrane Measurements have shown that the peak of the traveling wave occurs at different positions along the basilar membrane depending upon the frequency of the sound For high frequencies the peak is near the base of the cochlea and for low frequencies it is near the apex This effect is shown in Figures 49 and 410 The characteristic frequency CF for a position is the frequency that causes the most movement at that position The place theory states that each position acts like a bandpass lter only recording sounds around its CF This is how we distinguish between frequencies The problem with this theory is that it is weak at encoding information at less that lkHz when in fact the ear is very good a encoding this information This discrepancy is due to the fact that the traveling wave envelope is very wide at low frequencies 60 H1 sine wave 300 Hz slne wave 2000 Hz slne wave Figure 49 Traveling wave in the cochlea YOO I u a 13 F r y7 li quotI 20 r UIsmnre dd 30 mm stage 55m 1 3mm 28 mm 24 mm 20 mm l7 mm l3mm from 3 Lo stone E 0 05 n 3 g o t l i t t J 5 20 30 50 IOO 200 300 500 IOOO 2000 5000 Frequency cos Figure 410 The upper plot show the traveling wave envelope for different frequencies The lower plot show the lter responses for different positions W04 0 Time Theory To compensate for problems in the place theory the time theory was introduced The time theory states that the traveling wave in the cochlea moves at the frequency of the sound being played This motion causes the neurons in the cochlea to re at that frequency The result being that the time waveform is encoded as neural ring The issue with this theory is the relatively slow neural recovery time A neuron needs about 1ms of recovery time after it has red before it can re again This means that any single neuron cannot encode frequencies above kHz In order to increase this number the brain is thought to sum the responses of a number of neurons Now en some neurons are in recovery others will be ring and higher frequencies can now be encoded This is shown in Figure 411 Pure Tone Signal WWW Neural Firing Poiiems Neumn 1 Composite newan Figure 411 Summed response of neural rings W04 The current consensus is that the brain uses both the time and place encodeing in sound perception Under 1kHz timing is probably used because it is most ef cient in that range between 1 and SkHz the brain may use a combination and above SkHz place is used because it is most ef cient it that range The loss of timing around SkHz could explain why we can t hear musical tones above that frequency The Meliscale frequency warping has been developed to simulate place and time encoding This warping simulates a constant Q lter bank and has helped improved speech recognition References G02 BE GO LDSTEIN Sensation and perception Wadswoi thiThomson Learning 2002 L00 D O SHAUGHNESSY Speech communications human and machine Institute ofElectrical and Eleci tronics Engineers 2000 M03 BC MOORE An introduction to the psychology of hearing Academic Press 2003 P88 10 PICKLES An introduction to the physiology of hearing Academic Press 1988 CY95 J CLARK and C YALLO P An introduction to phonetics and phonology Blackwell 1995 Y00 WtAt YOST Fundamentals of healing Academic Press 2000 W04 Lt WERNER Lecture Notes from Introduction to Healing Science SPHSC 461UW 2004
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