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PSYB51H3 (301)
Chapter 9

Chapter 9 Notes

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University of Toronto Scarborough
Matthias Niemeier

Chapter 9 What is Sound? - Sounds are created when objects vibrate- the vibrations of an object cause molecules in the object’s surrounding medium to vibrate as well and this vibration in turn causes pressure changes in the medium - Sound waves travel at a particular speed depending on the medium, moving faster through denser substances - Sound is faster in water than it is in air Basic Qualities of Sound Waves: Frequency and Amplitude - the magnitude of the pressure change in a sound wave- the difference between the highest pressure area and the lowest pressure area is called the amplitude or intensity of the wave • pressure fluctuations may be very close together or spread apart over longer periods - although sound waves have wavelengths we typically describe the pattern as the frequency of the wave • sound wave frequencies are measured in hertz - the amplitude and frequency of sound waves are highly correlated with auditory characteristics • amplitude is associated with the perceptual quality of loudness, the more intense a sound wave is, the louder it’ll sound • frequency is associated with pitch; low-frequency sounds correspond to low pitches, and high-frequency sounds correspond to high pitches - to describe differences in amplitude across such a broad range, sound levels are measured on a logarithmic scale using units called decibels (dB)- decibels define the difference between two sounds in terms of the ratio between sound pressures - the range of human hearing extends from 0 to over 120 dB SPL - sounds with amplitudes even smaller than p0 have negative decibel levels, just as substances colder than freezing point of water have negative centigrade temperatures Sine Waves and Complex Sounds - sine wave is often called a pure tone - complex sounds are best described in a spectrum that displays how much energy or amplitude is present at multiple frequencies - sounds with harmonic spectra are typically caused by a simple vibrating source such as a guitar or the reed of a saxophone • each frequency component in such a sound is called a “harmonic” • the first harmonic called the fundamental frequency is the lowest- frequency component of the sound- all the other harmonics have frequencies that are integer multiples of that fundamental - Timbre is a term used to describe the quality of a sound that depends in part, on the relative energy levels of harmonic components Basic Structure of the Mammalian Auditory System Outer Ear - Sounds are first collected from the environment by the pinna, the curly structure on the side of the head that we call an ear • Only mammals have it - Sounds waves are funneled by the pinna into and through the ear canal which extends about 23 millimeters into the head - The main purpose of the canal is to protect the structure at it’s end, the tympanic membrane from damage • Is a thin sheet of skn that moves in and out in response to the pressure changes of sound waves Middle Ear - The pinna and ear canal make up a division of the auditory system called the outer ear - The tympanic membrane is the border between the outer ear and the middle ear which consists of three tiny bones; the ossicles (amplifies sound waves) • First ossicle is the malleus- connected to the tympanic membrane on one side and to the second ossicle • Incus- the second ossicle which is connected to the third ossicle • Stapes- third ossicle which transmits the vibrations of sound waves to the oval window another membrane which forms the border between the middle ear and the inner ear - Ossicles are the smallest bones and they amplify sound vibrations in two ways • Joints between the bones are hinged in a way that makes them work like levers; a modest amount of energy on one side of the joint becomes larger on the other- this increased the amount of pressure change by a third • The second way is by concentrating energy from a larger to a smaller surface area; the tympanic membrane which moves the malleus, is about 18 times as large as the oval window which is moved by the stapes o Pressure on the oval window is them magnified 18 times relative to the pressure on the tympanic membrane - Amplification provided by the ossicles is essential to our ability to hear faint sounds because the inner ear is made up of a collection of fluid-filled chambers - The middle ear has two muscles; the tensor tympani (attached to the malleus) and the stapedius (attached to the stapes) • Their main purpose is to tense when sounds are very loud, restricting the movement of the ossicles and thus muffling pressure changes that might be large enough to damage the delicate structures in the inner ear o This acoustic reflex follows the onset of loud sounds by about one- fifth of a sound- can’t protect against loud sounds like the firing of a gun Inner Ear - Function of the inner ear is to translate the information carried by the waves into neural signals Cochlear Canals and Membranes - Major structure of the inner ear is the cochlea a tiny coiled structure embedded in the temporal bone of the skull • Is filled with watery fluids in three parallel canals o The tympanic canal o Vestibular canal o Middle canal o The tympanic and vestibular canals are connected by a small opening the helicotrema and these two canals are effectively wrapped around the middle canal - The three canals of the cochlea are separated by two membranes o Reissner’s membrane between the vestibular canal and the middle canal o Basilar membrane between the middle canal and the tympanic canal  Is not really a membrane but it’s a plate made up of fibers that have some stiffness  Basilar membrane forms the base of the cochlear partition a complex structure through which sound waves are transduced into neural signals - Vibrations transmitted through the tympanic membrane and middle-ear bones cause the stapes to push and pull the flexible oval window in and out of the vestibular canal at the base of the cochlea- this movement of the oval window causes waves of pressure change called “travelling waves” to flow through the fluid in the vestibular canal • If sounds are extremely intense, any pressure that remains is transmitted through the helicotrema and back to the cochlear base through the tympanic canal where it’s absorbed by stretching yet another membrane called the round window - the movement of the oval window causes pressure bulges to move down the length of the vestibular canal and these bulges in the vestibular canal displace the middle canal up and down The Origin of Corti - Movements of the cochlear partitions are translated into neural signals by structures in the organ of corti which extends along the top of the basilar membrane o Made up of specialized neurons called hair cells, dendrites of auditory nerve fibers that terminate at the base of hair cells, and a scaffold of supporting cells - Inner and outer hair cells provide the foundations for miniscule hair-like bristles called stereocilia o on an inner hair cell it’s arranged as if posing for a group photo o on an outer cell it stands in rows that form the shape of a V or W - the tectorial membrane extends atop the organ of Corti • is a gelatinous flap that’s attached on one end and floats above the outer hair cells on the other • because the tectorial membrane is attached on only one end, it shears across the width of the cochlear partition whenever the partition moves up and down- this shearing motion in turn called the sterocilia of both inner and outer hair cells to bend back and forth Inner and outer Hair Cells - hair cells are specialized neurons that transduce one kind of energy into another form of energy - the cochlea has about 14,000 hair cells and the stereocilia of hair cells blow away the competition when it comes to speed and sensitivity - hair cells aren’t only extremely fast but also extremely sensitive - each stereocilium is connected to its taller neighbour by a tiny filament called a tip link so the sterocilia connected by tip links bend together as a set when deflected by the shearing motion of the tectorial membrane • when a sterocilium deflects, the tip link pulls on the taller stereocilium in a way that opens up an ion pore somewhat like opening a gate for a fraction of a second- this action permits potassium ions to flow rapidly into the hair cell; causing rapid depolarization o depolarization then leads to a rapid influx of calcium ions and initiation of the release of neurotransmitters from the base of the hair cell to stimulate dendrites of the auditory nerves - the opening of the ion pores that results from the direct connection between sterocilia via tip links is the only known example of mechanoelectrical transduction (MET) which is responsible for both the extreme speed and
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