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Chapter 5

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PSYC 212
Evan Balaban

PSYC 212 | Chapter 5 – The Auditory System: Sound and the Ear Audition/auditory system: - Used so that chemosensory information can be used for communication between animals - Is a distant sense  stimulus can be detected from far away - Fastest among all sensory systems A. The Physics of Sound 3 requirements for humans to hear sound: - Something that creates the sound - Sound must propagate through medium from its source - Mechanism to translate sound energy into a biological signal that ultimately produces hearing (aka our perceptual experience) Sound is caused by transmission of vibrational waves through a medium: - Proposed by Galileo - Boyle’s bell jar experiment: sound produced by bell is less audible if in a jar in which air is slowly pumped out. Becomes silent when no air left in jar. A1. The Creation of Sound Sound has 2 definitions: - Psychology definition: physical event that produced perceptual experience (hearing). Described by a set of qualities (loudness, pitch, etc…) since they are based on our perception. - Physics definition: vibrational disturbance of a medium (medium can be solid, liquid, or gas); can be associated with physical qualities. Its existence is independent of perception - “If a tree falls and no one hears it, does it make a sound?” – Psychologist: yes – Physicist: no Vibrational properties of objects - To produce sound, an object must possess two opposing forces (inertia and elasticity) to able to vibrate (to impart a vibrational disturbance to air) - Inertia: - “things like to keep doing what they’re doing” - Since objects have mass  If object is at rest, it will resist being moved  If object is in motion, it will resist coming to rest - Object will have tendency to resist becoming deformed - But once deformation starts, it will continue until opposing force comes - Elasticity: - Tendency to return object to its original state; ability to resist deformation - Ensures that object’s deformation does not continue beyond certain extent The tuning fork as a sound source & Impact of a sound source on the medium The opposing forces of inertia and elasticity causes the back and forth motion of a tuning fork  Simple harmonic motion  sinusoidal function - Tuning fork’s SHM makes a vibrational disturbance in air - Since air also possesses inertia and elasticity, will have similar effects - Compression (increase in air pressure) immediately surrounding tuning fork’s prongs caused but its outward movement - Rarefaction (decrease in air pressure) caused when prong moves in opposite direction - Air molecules in immediate surrounding of prongs alternate between states of compression and rarefaction as they vibrate - Each air molecule acts as one vibrator, colliding with neighboring particles, which causes them to vibrate. - Momentary compressions and rarefactions passing from region to region cause waves of outward vibrational changes in air pressure (vibrational energy). This is what generates the sound waves. - Sinusoidal SMH causes sinusoidal profile of pressure change  Results in pure tone: single sinusoidal function of air pressure change over time A2. The Properties of Sound Sound: longitudinal travelling wave of pressure disturbance in a medium. Auditory system detects the pressure changes, causing “hearing”. Amplitude and its relationship to sound intensity - Amplitude: pressure change from baseline to peak of sinusoidal function (for pure tone).  The louder the sound, the greater the pressure changes, the greater the amplitude - Unit of pressure for sound: micropascal (μPa) 2 - Intensity: square of sound pressure  I = P - For sound, intensity in relation to reference level is used, not absolute intensity. 2 2 - We use ratios  P /Ps r - Ps: peak pressure (amplitude) of sound we want to measure - Pr: peak pressure of reference sound (usually minimum audible sound level: 20 μPa) - Intensity depends on vibrational amplitude Representing sound intensities Since scale of sound level ratios is so large, we use decibels (one tenth of a bel): dB = 10log(I /Is)r= 10log(P /P )s r2 When dB values are shown as dB SPL(where SPL stands for sound pressure level), infers that minimum audible reference value was used for I . r - Since decibels are a logarithmic scale, two 40 dB SPLdoes not equal 80 dB SPL Sound frequency Frequency: number of cycles (complete vibration containing one instance and compression and one of rarefaction) in a second (measured in Hz) - Frequency depends on the vibrational (physical) properties of a sound source. This is why any tune fork (has one vibrational property) can produce only one pure tone of only one frequency. - Resonant (natural) frequency: vibrational frequency resulting from the sound sources’ mass and stiffness (elasticity). The greater the stiffness, the greater the vibrational frequency; the greater the mass (or length), the lower the frequency. Speed of sound - Depends only on the medium, not the source. - Characteristics that affect sound speed: inertia and elasticity - Inertial property: expressed by medium’s density. The greater the density, the lower the sound. - Temperature affects this relationship (especially for gas and liquids) - as temperature increases, medium expands and decreases in density  so speed of sound increases - Elasticity: the greater the elasticity of a medium, the greater the speed of sound. - Speed of sound in steel is much greater than in air. Although steel is much more dense than air, it is also much more elastic. A3. Complex Sounds Most sounds do not display SHM, as their vibrational sequence is more complex. Complex periodic sounds Period sounds are produced when the pattern of pressure change repeats itself at regular intervals over time. Simple periodic wave: produces a wave with a sinusoidal profile (ex: pure tone) Complex periodic waves: repeating wave that does not have sinusoidal profile. - Contain discrete patterns that are exactly duplicated in a cyclic manner - Created by combining multiple sinusoidal waves that differ in frequency - For any point in time, simply add the respective amplitudes of each wave to produce the compound wave of the complex periodic sound. Notes on a musical instrument have periodic profile characterized by the summation of series of sine waves (harmonic series). Strings on a guitar vibrate as a whole at a fundamental frequency, but parts of the string also vibrate separately, but simultaneously, to create overtone harmonics. Overtone frequencies are whole number multiples of the fundamental frequency (2 , 3 ,…) harmonic. (Discussed further in Chapter 7) Complex aperiodic sounds Noise: sounds produced by aperiodic vibration - Vibrations are random, impossible to find any time intervals where pressure change is identical. - Can be created by combining many sound frequencies with random, varying amplitudes White noise: noise pattern composed of all the frequencies within a particular range (ex: range of human hearing) (same amplitude) - Ex: traffic, noise from a large crowd. Can also be soothing: waterfall. Fourier analysis Any function, no matter how complex, can be decomposed intro a series of sine-wave functions, without prior knowledge of what those constituent patterns are. Fourier spectrum: sine wave functions derived from Fourier analysis in terms of amplitude and frequency. X- axis = frequency, y-axis (height) = amplitude. Ohm’s law of hearing Complex sound waves can be subjected to Fourier analysis and this operation occurs in hearing. A4. Sound Transmission Sound intensity becomes dissipated with distance. Sound waves must interact with objects along their path, which can interfere with, causing it to enhance or dampen. The inverse square law  Intensity = 1/r (only true if there are no obstacles in path) Sound from a point source radiate in all direction, creating a moving spherical wave front. Sound intensity dissipates with distance because the energy contained in sound must be distributed over a progressively larger area of the sphere as sound propagates outward. The area of a sphere grows according to the square of this distance, so sound intensity decreases correspondingly. For example: doubling the distance would cause sound intensity to be decreased to ¼ of its original value. Interaction of sound with objects Sound can interact with obstacles in its path in 3 ways: reflection, absorption, or diffraction Reflection: As with light, a boundary can serve as a reflective surface if the second medium has a greater resistance to sound transmission. Examples: Hard surfaces and air-water interface. Acoustic impedance: resistance to sound. The greater the impedance difference at a surface, the greater the amount of sound reflection. Echoes/reverberations: reflected sounds. Reverberant room: room with highly reflective walls. Absorption: Interface (border) between different surface is absorbed and could be transmitted though the second. Absorption coefficient: the proportion of sound energy absorbed to that contained in the incident wave. Denotes the magnitude of absorption. Examples: water has low absorption coefficient, while rubber and Styrofoam can absorb up to 1/3 of the incident sound. Anechoic room: room with walls made with high sound absorbance materials Diffraction: sound waves bend around objects when the object is too small for reflection or absorption to occur. Sound waves of lower frequency span a larger space, so they diffract more easily. Sound waves of high frequency have more compressed undulations (ripples) of pressure change, and do not diffract easily. Architectural acoustics Properties of absorption and reflection need to be considered for auditoriums/concert halls Wallace Sabine: important factor in acoustic sustainability is sound reflection, quantified by reverberation time. Fullness: when rooms have high reverberation Clarity: when rooms have less reverberation B. Auditory Processing of Sound – Physical Characteristics B1. Anatomical Components of the Human Ear Sound waves are transmitted though series of structures in the ear. These structures must retain the frequency characteristics of the original sound stimulus and transmit it without causing (or minimally) disruption in amplitude. The outer ear  3 parts: pinna, external auditory cannal, and tympanic membrane (eardrum) Pinna: - made of cartilage, has bumps and grooves that enhance sound transmission - The sound funnel - Some animals have pinna muscles that can move in the direction of incoming sound - Humans do not have this muscle, so human pinnas are passive receptors of sound External auditory canal: - pinna funnels sound into here - Is S-shaped, ~25mm in length, 5-7mm in diameter - Outer half lined by cartilage, inner part covered by skin lining the temporal bone. - Canal is lined with wax secreting glands that protect eardrum from small foreign objects Eardrum: - interior boundary of the external ear. - Thin elastic membrane, ~10 mm in diameter - Attached to skin of EAC’s innermost end, and seals it completely - Incoming sound waves set off vibrational pattern on it, which then pass to middle ear - Protects middle ear against foreign objects. The middle ear – air filled space within temporal bone; immediately adjacent to eardrum - Rectangular chamber; eardrum forms wall, other walls formed by thin plates of bone. Except: front wall opens into narrow Eustachian tube, which connects middle ear to back of throat. This allows equalization of any pressure differences across eardrum, so eardrum can work under different atmospheric pressures. (Ears popping on plane) - Increased/decreases outside pressure makes the eardrum inward/outward, respectively. These two cases create tension in the eardrum and reduce its vibrational response to incoming sounds waves = reduced hearing. Sound waves transmitted from eardrum to middle ear happens in 1 of 3 ways: 1) Transmitted through the bony matrix of the head 2) Conducted through air hat is present in the middle ear space These two play a role in sound transmission, but their effect is negligible due to dampening of sound stimulus. Third mechanism evolved to physically conduct sound vibrations through series of bony levers (auditory ossicles) to the inner ear. These bones from the outside in: mallus, incus, and stapes. Ossicles are suspended in middle air space by ligaments, connected to form ossicular chain. Suspension allows them to vibrate freely and provide efficient transmission of the original acoustic pattern. Malleus: made of long bony protrusions (called “handle”) which are attached to eardrum - Head is tightly connected to body of incus. Incus: - its shaft (the “long process”) forms a joint with head of the stapes. Stapes: lies horizontally at right angle to incus, has oval-shaped footplate that makes a tight connection with oval window of inner ear. Vibration of eardrum conducts through this ossicular chain, and then transfers to inner eat at oval window. Inner ear Temperal bone  cavity = bony labyrinth  vesticular system and cochlea Cochlea: coiled fluid-filled bony structure, contains sensory transduction apparatus for hearing - Where physical energy in sound waves cause electrochemical signal transmitted to the brain - Begins at its basal, makes almost 2 ½ turns and ends at apex. - ~35 mm in length, ~9 mm at diameter at base (but less at apex) - Divided into 3 fluid-filled channels (in order): scala vestibuli, cochlear duct, scala tympani - scalae vestibuli (largest channel) and tympani are connected by helicotrema (part of apex) so contain same fluid Cochlear duct: smallest channel (7% of cochlea) - Reisner (thin & delicate)/basilar separates it from scala vestibuli/tympani, respectively. - Self-contained chamber filled with different fluid (although similar consistency, but different ionic concentrations, does not come in contact with the other scalae. Sound transmission through the ear Funneled by pina  transmitted through external auditory canal eardrum (vibrational pattern set off, conducted by ossicles, goes through oval windown to)  cochlea and its fluids  stapes does back and forth motion on oval window, initiates compressional sound wave in scala vestibulli fluid  Sound now characterized by movement of liquid molecules, not air (so faster). - Moving stapes inward on the oval window creates momentary pressure increase, which then distributes in the cochlear fluids. Increased pressure in the scala vestibuli goes ascorss Reissner’s membrane, through the cochlear duct, and across basilar membrane into scala tympani. - The round window (elastic membrane at basal end of scala tympani) relieves the increased fluid pressure in the tympani by its outward distension. - This whole process is in opposite direction when stapes moves outward. - Pressure fluctuations in cochlear fluids need to cause movement of membranes that separate the 3 channels. - Neural transduction apparatus rests on the basilar membrane, so the membrane’s up and down movement is critical. B2. Amplitude Preservation 4 physical features make up for potentially large vibrational amplitude loss that occurs with sounds reaching the cochlear interface on their own (directly through air). The lever effect Transmission of acoustic energy thought bones of middle ear produces ~2 dB of amplification. Difference in size between the malleus and incus create a small increment in mechanical force. The condensation effect – occurs from the design of the auditory components - Sounds channels from eardrum to footplate of stapes (20x smaller than eardrum). This results in vibrational pressures condensing from large to small area, which produces an amplification corresponding to the surface area difference. - Amplification is ~25dB The resonance effect For sound frequencies from 2500-5000 Hz, can gain 10-15 dB due to this effect. Resonant properties of some outer ear components (pinna, external auditory canal, …) amplify sounds at their according resonant frequency [range]. The directional effect - Ossicular chain of middle ear channels all incoming sound energy upon only the oval window. - Sound energy at this window causes reciprocal movement of round window. - Result: vibrational amplitudes in cochlea would be reduced by as much as 60 dB. - This occurs in diseases of middle ear where ossicles are impaired. B3. Frequency Representation The resonance theory – Hermann von Helmholtz: basilar membrane is composed of a series of fibers tuned to different frequencies. - Sound of a certain frequency would set an appropriate fiber in motion that would stimulate a certain nerve fiber projecting to the brain - Abandoned since cannot account for full range of frequencies audible to humans The frequency theory – Ernest Rutherford - Basilar membrane is capable of vibrating within the full range of frequencies audible to humans - Auditory never fibers are stimulated by basilar membrane; their firing rate conveys sounds frequency information to the brain. - Neural firing rate mirrors sound frequency, neural firing amplitude mirrors intensity. Problems: - Neural firing is all-or-none, so they cannot encode sound intensity. - Firing rate is constrained by time required to complete action potential, so cannot encode frequency either. - In this theory, basilar membrane would have to have uniform width, so that it can oscillate at same frequency throughout. However, its width is uneven. The place theory – George von Bekesy - Sounds of different frequencies produce a vibrational pattern, where its maximum amplitude occurs at different place along the basilar membrane - The fact that basilar membrane was uneven and the tension on this membrane is higher at the basal end, showed Bekesy that the membrane cannot show the same vibrational response to sounds of different frequencies. Resonance frequency along its length is not the same. - Bekesy proposed that vibrational disturbance in cochlear fluids set up a travelling wave within the basilar membrane whereby a pure-tone of a certain frequency produces maximum displacement only at the point where it matches with the resonant frequency of the basilar membrane. - The smaller width and greater tension at basal end produce higher resonant frequency, but as you go towards the apex, width increases and resonant frequency decreases. - As a result, high/low frequency stimulus makes maximum vibration at basal end/apical end of the basilar membrane, respectively Tonotopic organization Tonotopic map: the way frequency is represented The basilar membrane changes systematically to include nearly the full frequency range audible to humans. The closer you are to the base, the higher the frequency. Frequency analysis of complex sounds The auditory system filtration process produces a frequency decomposition similar to that of Fourier analysis. When the ear is exposed to complex sound, vibrational disturbance is created in cochlear fluid and follows the complex waveform of the sound stimulus exactly. Since there are vibrational restrictions on the basilar membrane (due to its physical characteristics), the sine-wave components of the complex sound produce the vibrational disturbance (but they only occur where the frequencies of the sound’s spectrum match the basilar membrane’s resonant frequency). - The basilar membrane acts as a frequency analyzer; the distribution of vibrational disturbances across it is then analyzed by neural components of the auditory system. C. Auditory Processing of Sound – Biological Mechanisms Sound must be transduced into a neural signal, which the brain can interpret to produce hearing C1. Auditory Transduction Organ of Corti: group of cells and structures that span across the cochlea in its entirety - Since it’s so close to the basilar membrane, it can impart its vibrational disturbances, which produce the initial bioelectric response through a certain process, the first event of neural processing of sound. Organ of Corti – structural features Is in the cochlear duct, so is
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