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Lecture 9

Lec9 - Hearing - Physiology and Psychoacoustics-Mar14.docx

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University of Toronto Scarborough
Matthias Neimier

PSYB51-Lec09 th Friday March 14 , 2014 Hearing: Physiology and Psychoacoustics Of boats and buoys • There are boats… but also a lot of fog • You only see the buoys and the waves… boats are too far away • Waves are caused to some extent by boats • Need to observe what is going on in the lake based of movements of buoys o Only information you have • With vision… it’s not possible to do that • Ears use vibrations to figure out what goes on in the world The Function of Hearing • The Basics: o Nature of Sound  What is sound? o Anatomy and physiology of the auditory system o How we perceive loudness and pitch o Impairments of hearing • Hearing to in the Environment o Sound localization (where?) What is Sound? • Sounds are created when objects vibrate  Sound doesn’t work in outer space o Vibrations of object cause molecules in object’s surrounding medium to vibrate as well, which causes pressure changes in medium  Molecules move towards and away from object  Causes differences in density of object  Move back and forth  Waves are being tracked o Longitudinal direction o Speed of sound depends on density of medium  Air:~340m/s; water: ~1500m/s • Much faster in water • Moving waves = vibrations o Density; increased air pressure o Confined by channel, wave only going in one direction o When looking at one point in space… air pressure increases and decreases at a certain frequency o Doesn’t matter what the frequency is, speed of waves propagating away from object stays the same o Frequency is different from speed • Acoustic monopole o Causes molecules to move towards and away from it o Moving back and forth o Waves move away from object in radio fashion Frequency and Amplitude • Vibrating object = tuning fork o Can be anything  speakers… person speaking • Have some sort of frequency … air pressure wave indicated by sine wave • When movement propagates in one direction, it does not mean the molecules are moving up and down … different from water • Everything happens in one dimension – longitudinal waves o Indicates pressure • Certain amplitude indicates different pressure levels (energy) • Waves have same difference in cycles  same frequency (pitch) • Different amplitudes  different volume of sound What is Sound? (cont’d) • Basic qualities of sound waves: • Amplitude: magnitude of displacement of a sound pressure wave o Energy • Intensity: Amount of sound energy falling on a unit area o Frequency o Measured in decibels sound pressure level (dB SPL) • Decibels: Ratio between the pressure of some sound and the pressure of a reference sound Po (~minimum). o How to express physical levels of intensity o Actual hearing has physically twice as much energy as the minimum you actually need to hear something o dB = 20*log(p/po); e.g. 20*log(2)=6  convention  difference of 6dB = something has twice as much energy than something else… relative to minimum level  doubling the intensity /energy  doubling every 6… linear increment  increment in loudness seems equal  not the same as intensity o convention chosen to make things easier • loudness: the psychological aspect of sound related to perceived intensity or magnitude o subjective experience of intensity Intensity of environmental Sounds • Humans can hear across a wide range of sound intensities o Ratio between faintest and loudest sounds is more than one to one million, 6dB = double amount of pressure • Falling leaves  20dBs – clearly above level of absolute threshold • Car  90 dBs • Jet Engine  150dBs • We perceive subjectively these increments of loudness in some sort of regular fashion, BUT in terms of energy, there is an exponential relationship doubling of energy • 90dBs to 150dBs  huge difference • Subjective experience is different from physical experience What is Sound? (cont’d) • Basic qualities of sound waves cont’d: o Pitch & sound frequency – not so clear relationship • Frequency: For sound, the number of times per second that a pattern of pressure change repeats • Measured in 1/s=Hz • Frequency is associated with pitch • Pitch: Psychological aspect of sound related to the fundamental frequency (low: tuba, high: piccolo) • (Phase) o Sine waves/ waves shifting • Interactions between amplitude and frequency • Human hearing uses a limited range of frequencies: From about 20 to 20,000Hz o Total range of which we can hear  pretty far range (for younger people) o Hearing for higher frequencies go earlier • Contrast depends on spatial frequency • Curve: Relationship between sound energy and frequency o Very complicated curve o Curve is more complex • How pressure waves are perceived as sound • Curve is irregular – reflects that certain frequencies need less energy than others o Not a straightforward relationship • In context of music… we hear it but it’s played at a different frequency • Absolute threshold – irregular blue line in bubble o Where music should be played • Speech at a certain range • Upper end – having more energy than high-risk threshold: not a sound anymore o Painful o Can damage ears o Physical energy damages ears… not how you perceive it Sine Waves and Complex Sounds • One of simplest kinds of sounds: Sine wave, or pure tone o Sine wave: Waveform for which variation as a function of time is a sine function o Most sounds in world: Complex sounds, (e.g., human voices, birds, cars, etc.)  Not just one frequency at a time o All sound waves can be described as some combination of sine waves  Fourrier  mathematical proof • Different frequencies of sine waves • Curves can be a rectangular curve o Different amplitudes (different amounts of energy) • Barely have pure tones in daily life Spectrum • Complex sounds can be described by Fourier analysis o Spectrum: A representation of the relative energy present at each frequency • Break it down to figure out components • Pure tones – one single sine wave o One vertical bar indicating pure tone has one single frequency • Can look at Fourier transformation – spectrum and identify which sine waves went in the complex function Sine Waves and Complex Sounds cont’d • Naturally we hear complex sounds all the time • Guitar string plucked: the string’s displacement moves left & right, ‘travelling waves’ o Pluck guitar strings… as you do that, based on properties of string… you get vibrations in one dimension • Waves reflected at ends; waves travel back and forth o Get several patterns of motions called modes o All these waves exist at the same time  Considered complex waves o 6 waves… occurs on one entire string • Left & right travelling components add to a standing wave o Travelling waves • Relate to cycles in time • Second mode: spatial cycle = equal to entire length of guitar string : one complete cycle • First mode: 0.5*Length of guitar string • Each wave goes up by 0.5  all related to the first mode • Has to do with guitar string and physical property of string • As long as you don’t change tuning… it’s a direct relationship to wave Complex sounds • Harmonic spectrum: Typically caused by simple vibrating source, (e.g., string of guitar). The frequencies of its components are integer multiples of lowest frequency o Tenor sax: frequency bars have somewhat regular pattern o Frequencies very regularly related to the first (lowest) frequency a.k.a fundamental frequency o Perceiving speech and enjoying music o Instruments emit the same frequency… but we hear different instruments • Fundamental frequency: lowest frequency component of a sound (=1 harmonic) • 262, 524, 786, 1048 Hz o Multiple integers of fundamental frequency o 262, 2*262=524, 3*262=786… o Travelling waves that bounce back and forth • Sounds w/same pitch and loudness may still sound different (piano vs. guitar) – timbre • Power distribution across harmonic spectrum The Function of Hearing • Anatomy and physiology of the auditory system o Outer, middle, inner ear o Two strategies to code frequency o Auditory pathway o Cortical structures Basic Structure of the Mammalian Auditory System • Outer ear: • Sounds are first collected from environment by the pinna o Ear lobe… very important to have this • Sound waves are funneled by the pinna into ear canal • Length and shape of ear canal enhance sound frequencies o Protects tympanic membrane • Main purpose of canal is to insulate structure at its end: Tympanic membrane, vibrates in response to sound o Boundary between outer and inner ear • Teach yourself about the middle ear! o Tympanic membrane in ear… comes from gills of a fish… • Step 1: What’s the problem with audition? o Identify problem o Someone talks to you… sound goes into the cochlea o Cochlea is filled with a fluid… similar to floating in water… gargled noises o Sound travel through ears… energy of sound does not go into water... it gets reflected o We need to somehow increase energy (speak louder?) • Step 2: physics refresher o Middle ear found a solution o Imagine you need to lift rock  You have a sturdy stick and a smaller stone… is that possible? o Leverage principle o Using the tip of the pen to poke your hand is more painful than pressing your hand with an open palm  Pen is more painful because energy is focused on one small surface o Same thing occurs with middle ear o Tympanic membrane shaped like a cone to enhance hearing • Step 3: solution o Tympanic membrane has a much larger surface area than the inner part of the ear bones (stapes) o Smaller surface = increased energy • Middle ear: 3 ossicles: malleus, incus, stapes; smallest bones in body  Solution o Enhance sound (lever mechanism + focusing pressure on smaller area) o Stapes transmits vibrations of sound waves to oval window  Has smaller surface area o Loud sounds: muscles • Sound vibrations transmitted to oval window • Looks like a snail (cochlea) … 3 tubes • Convey energy with higher frequency Inner Ear • Inner ear: Fine changes in sound pressure are translated into neural signals  Sensory transduction  Sound energy waves in cochlea o Cochlea: oval/round window, three canals o Vestibular organ Cochlea • Cochlear canals and membranes o Cochlea: spiral structure of the inner ear containing the organ of Corti o Cochlea is filled with watery fluids in three parallel canals  Middle canal surrounded by… • Two other canals  Vestibular canal + tympanic canal • Separated by membranes o Organ of Corti sits on top of basilar membrane, covered by tectorial membrane • Cross section of cochlea o Three canals: vestibular canal, tympanic canal and middle canal  Connected to each other (vestibular & tympanic) o Inside middle canal is the organ of corti  Physical displacements caused by sound pressure waves is conveyed into neural signals Basic Structure of the Mammalian Auditory System (cont’d) • Vibrations transmitted through tympanic membranes and middle-ear bones cause stapes to push and pull flexible oval window in and out of vestibular canal at base of cochlea o If sounds are extremely intense, any remaining pressure is transmitted through helicotrema and back to cochlear base through tympanic canal, where it is absorbed by another membrane: Round window  Sound waves wash around middle canal in the center • Organ of Corti  Vibrations stimulate this  Inside middle canal  Sits on basilar membrane that separates the middle canal from the tympanic canal • Has hair cells in it o Sound waves transformed into movements of the ossicles… o Movements of cochlear partition which then are translated into neural signals by structures in the organ of Corti; extends along top of basilar membrane o Made up of specialized neurons called hair cells, dendrites of auditory nerve fibers that terminate at base of hair cells, and scaffold of supporting cells Cochlea • Tectorial membrane: Extends atop organ of Corti; gelatinous structure o As sound pressure waves travel through cochlea, these two membranes get moved around… but they don’t move completely in parallel… move sideways o Move relatively to each other o Tectorial membrane and fluid brush over these hair cells o As hair cells get bent, it causes reactions in cells which sends out neural signals • Hair cells in each human ear: Arranged in four rows that run down length of basilar membrane o Inner hair cells: afferent auditory information  Converts sound pressure waves into neural signals that will eventually turn into the sound we hear  Many more outer hair cells than inner o Outer hair cells: efferences, feedback system  Have nothing directly to do with hearing  Function like little muscles  As they get stimulated, they will expand and/or contract  Lift or push tectorial membrane  Enhance sound (brings low level sounds to the thresholds where we can hear things • Organ of corti create their own sounds • Stereocilia of inner hair cells o Shows rows of cells • Stereocilia of outer hair cells Vibration and the Tectorial Membrane • How shearing actually works • As tectorial membrane moves sideways relative to the basilar membrane • Up and down/flopping motion • Hair cells get either depolarized/hyper-polarized depending on direction Basic Structure of the Mammalian Auditory System (cont’d) • Coding of amplitude and frequency in the cochlea • Place code: different parts of cochlea tuned to different frequencies; information about the frequency of an incoming sound is coded by place along cochlear partition, i.e. at the location with greatest mechanical displacement  Beginning is wider than the end  Therefore it responds to lower frequencies o Thickness & width of basilar membrane o One end wide and thick, one and narrow and floppy o Basilar membrane more responsive to lower frequencies • Basilar membrane determines where you have vibration • Depending on what the frequency of the sound is, different regions of the basilar membrane vibrate o Because of physical properties of basilar membrane o Different hair cells responding to different frequencies because they are organized in different parts of the
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