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Chapter 9

PSYB51H3 Chapter Notes - Chapter 9: Incus, Ossicles, Retina

Course Code
Matthias Niemeier

of 10
Ch. 9- Hearing: Physiology & Psychoacoustics
fundamental human ability= communication through speech
fundamental sound qualities: loudness + pitch
sound created when object vibrates--> vibrations cause molecules in the object's medium to
vibrate= pressure changes (waves) in the medium=wave becomes less prominent as it moves
farther away
sound waves travel at a particular speed depending on the medium= move faster through denser
substances (340 m/sec in air vs. 1500 m/sec in water)
light waves moves through the air almost a million times faster than sound= lag time between
seeing and hearing thunder
Basic Qualities of Sound Waves
sound waves we hear= fluctuations in air pressure across time
amplitude/intensity= magnitude of displacement (increased/decrease) of a sound pressure
wave= difference between highest and lowest pressure area
perceived as LOUDNESS in hearing vs. BRIGHTNESS in vision
height of the wave
inc. Amplitude= inc. Loudness
measured in dB (decibels)= a unit of measure for the physical intensity of sound= defines
the difference between 2 sounds as the ratio between 2 sound pressures = 20 log (p/p0)
where variable p is the pressure/intensity of the sound being described and p0 is a constant
relatively small decibel changes can correspond to large physical changes
frequency= the number of times per second that a pattern of pressure change repeats (number
of wavelengths/cycles per second)
measured in Hz (hertz)= a unit of measure for frequency, one hertz is one cycle per second
perceived as PITCH
wavelengths in light= colour
lower freq = lower pitch (tuba)
human hearing= limited range of frequency (20 to 20,000 Hz)
Sine Waves and Complex sounds
sine wave/pure tone= simplest kind of sound, a waveform for which variation as a function of
time is a sine function
all sounds are composed of sine waves
complex sounds are best described in a spectrum (representation of the relative
energy/intensity present at each frequency)
harmonic spectra= spectrum of a complex sound in which energy is at integer multiples of
the fundamental frequency
typically caused by a single vibrating source (string of guitar/reed of a sax)
each frequency component= a “harmonic”
first harmonic= fundamental frequency= the lowest freq. Component--> all other
harmonics have frequencies that are integer mulitples of this harmonic
Shape of a spectrum= important quality that distinguishes different soundes
can help identify sound sources--> spectra from different instruments will have diff
shapes (different patterns of amplitudes for each harmonic)
timbre= psychological sensation by which a listener can judge two sounds with the
same loudness and pitch dissimilar. Timbre quality is conveyed by harmonics and other
high frequencies. Quality of sound that depends, in part, on the relative energy levels of
harmonic components
Basic Structure of the Mammalian Auditory System
Outer Ear
outer ear= external, sound-gathering portion of the ear, consisting of the pinna and the ear canal
pinna= the outer, funnel-like part of the ear
sounds first collected from the environment here= typically called the ear
only mammals have pinna (vary in shape and size between species, less within species)
pinna--> ear canal= canal that conducts sound vibrations from the pinna to the
tympanic membrane and prevents damage to the tympanic membrane = extends 25mm
into the head
pinna--> ear canal--> tympanic membrane= ear drum= a thin sheet of skin at the end
of the outer ear canal= vibrates in response to sound (pressure change of sound waves)
painful to rupture tympanic membrane, but often it will heal itself
border between outer and inner ear
Middle Ear
middle ear= an air-filled chamber containing the 3 tiny middle bones/ossicles= conveys and
amplifies vibrations from the tympanic membrane to the oval window
ossicles= malleus, incus, stapes= 3 bones of the inner ear that amplify sound waves=
smallest bones in the human body= amplify sound vibrations in 2 ways
1. work like levers= a modest amount of energy one side becomes larger on the
other= increases amount of pressure by about a third
2. ossicles increase energy transmitted to inner ear, by concentrating energy
from a larger surface to a smaller surface= oval window is about 18X smaller
than the tympanic membrance = 18X magnification of pressure
**important because takes more energy to move liquid than it does to move air= sound would not be
able to move the fluid-filled chambers of the inner ear without this magnification= would just bounce
back of the oval window
malleus= connected to the tympanic membrane and the incus= incus= connects the
malleus and the stapes= stapes= third ossicle, pressed against the oval window,
transmits the vibrations of the sound waves to the oval window
pinna--> ear canal--> tympanic membrane--> malleus--> incus--> stapes--> oval
window= flexible opening to the cochlea through which the stapes transmits vibrations
to the fluid inside
oval window= border between middle ear and the inner ear
**ossicles play an important role in LOUD sounds too= middle ear has 2 muscles (smallest muscles):
1. tensor tympani= attached to the malleus= tensing this muscle decreases vibration
2. stapedius= attached to the stapes= tensing this muscle decreases vibration
** main purpose= ACOUSTIC RELEX= 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= follows onset of loud sounds by 1/5th of a second= helps in sustained loud
sounds but not abrupt ones (ex. Firing of a gun)= also tense during swallowing, talking, general body
movement helping to keep auditory system from being overwhelmed by sounds generated by our own
Inner Ear
here fine changes in sound pressure available in the environment are translated into neural
signals that inform the listener
function of inner ear= analogous to that of retina with respect to light waves= translates the
info carried by waves into neural signals
major structure= cochlea= a spiral structure of the inner ear containing the organ of Corti
cochlea is filled with water fluids in three parallel canals
tympanic canal= one of three fluid-filled passages in the cochlea. Extends from the
round window at the base of cochlea to the helicotrema at the apex
vestibular canal= extends from oval window at the base of the cochlea to the
helicotrema at the apex
middle canal= sandwiched between the tympanic and vestibular canals= contains
the cochlear partition
**tympanic canal and vestibular canal are effectively wrapped around the middle canal
**helicotrema= opening that connects the tympanic and vestibular canals at the apex of the cochlea
3 canals of the cochlea are separated by two membranes
Reissner's membrane= between the vestibular canal and the middle canal
basilar membrane= between the middle canal and tympanic canal= not really a
membrane (unlike tympanic membrane, oval window or Reissner's membrane) b/c its a
plate made up of fibres that have some stiffness
basilar membrane= forms base of the cochlear partition
cochlear partition= a complex structure through which sound waves are transduced
into neural signals= made up of the basilar membrane, tectorial membrane and organ
of Corti
**vibrations transmitted through the tympanic membrane and middle-ear bones (malleus, incus,
stapes), cause the stapes to push and pull the flexible oval window in and out of the vestibular canal at
the base of the cochlea--> movement of the oval window causes waves of pressure changes, called
“travelling waves” to flow through the fluid into the vestibular canal--> “ bulge” forms in the vestibular
canal because its a closed system--> “ bulge travels from the base of the cochlea down to the apex-->
by this time the traveling wave's displacement has mostly dissipated
**IF sounds are extremely INTENSE= any pressure that remains is transmitted through the helicotrema
(opening that connects the tympanic and vestibular canals at the apex of the cochlea) and back to the
cochlear base through the tympanic canal---> absorbed by stretching the round window= soft area of
tissue at the base of the tympanic canal that released excess pressure remaining from extremely intense
** vestibular and tympanic canals are wrapped tightly around the middle canal--> when the vestibular
canal bulges out, it puts pressure on the middle canal which DISPLACES the cochlear partition lying at
the bottom of the middle canal--> the moves the partition down as the vestibular bulge comes through
and back up as the bulge passes by
Organ of Corti- structure on the basilar membrane (btwn the middle and tympanic canals,
forming the base of cochlear partition) of the cochlea that is composed of specialized neurons
called hair cells and dendrites of auditory nerve fibres that terminate at the base of the hair cells
and a scaffold of supporting cells= translates movements of the cochlear partition into neural
hair cell= any cell that has stereocilia for transducing mechanical movement in the inner ear