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Lecture 14

PSYCH 1XX3 Lecture Notes - Lecture 14: Basilar Membrane, Oval Window, Ear Canal

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Joe Kim

of 9
Lecture 14: Audition
(which ear structure cause the most amplification?)
(What ear structure protect our ear from being damaged by loud noise?)
> Ear canal : first amplification occurs
The Auditory Mechanisms of Different Species
Auditory mechanisms vary across different species according to specific needs
Sound Frequency
o Different species can hear different ranges of frequencies
o Ex. Dog whistle
o Humans can perceive sounds that lie anywhere between 20 and 20,000 Hz
o Whales, dolphins and dogs have a wider hearing range, while frogs and birds
have a narrower range
o At the lower frequency detection extreme are fish, while at the higher frequency
detection extreme are bats and rodents
o Audible frequency range is determined in part by evolution of structures of
auditory system
o One key structure is the basilar membrane, which contains the hearing receptors
o Sounds of different frequencies are processed along different areas of basilar
The Basilar Membrane
o Varies in length across species
o It is shortest in amphibians and reptiles, longer in birds and longest in mammals
o A longer basilar membrane allows processing of a wider range of frequencies
The Stimulus: Sound Waves
Sound travels in waves, although sound waves travel much slower and require some
medium to travel through
Sound waves are initiated by either a vibrating objet, like vocal cords or a guitar string,
or by forcing air past a small cavity, like a pipe organ
This causes air molecules surrounding the source of the sound to move, causing a chain
reaction of moving air particles
These alternating bands of more and less compressed air molecules interact with
eardrum to begin auditory processing
A band of compressed air molecules causes your eardrum to get pushed slightly inwards
whereas a band of less dense air particles causes the eardrum to move outwards
Changes in air pressure over time that make up a sound wave can be graphed as a sine
The three physical characteristics of the wave, amplitude, wavelength and purity, when
applied to sound waves, translate into the three psychological properties of loudness,
pitch and timbre
Amplitude: Measure of Loudness
o Variations in amplitude (can be measured) or height of sound wave affect the
perception of loudness
o Since waves of greater amplitude correspond to vibrations of greater intensity,
higher waves correspond to louder sounds
o Loudness is measured using a logarithmic scale of decibels (db)
o In this scale, the perceived loudness of a sound doubles for every 10 dB increase
o A whisper is at around 27 dB, a normal conversation at 60 dB and the front row
of a rock concert at around 120 dB
Frequency: Measure of Pitch
o Sound waves also vary in the distance between successive peaks, called
wavelength or frequency of sound and this property affects the perception of
o Pitch is the internal representation on how we present frequency.
o Pitch is measured in hertz (Hz), which represents the number of cycles per
second or the number of times in a second that a sound wave makes one full
cycle from one peak to the next
o So if many wave peaks are condensed into one second, then this sound will be of
a high frequency and result in a perception of a high pitched sound
o Similar to light, the audible zone of frequencies that humans can detect
represents only a portion of the possible frequencies that can be produced
Timbre: Measure of Complexity/Purity
o Most sounds we hear everyday are complex sounds that are composed of
multiple sound waves that vary in frequency
o Timbre refers to the complexity of a sound
o Ex. When you pluck a guitar, it vibrates as a whole (fundamental), but also
vibrates at shorter segments along the string (overtones)
The final sound you hear is a mixture of the fundamental tones and all
the overtones and this combination is timbre
o So a piccolo and a bassoon may both play the same note, but because each
instrument produces a unique combination of fundamental frequency and
overtones, they still sound different to us even though each instrument is
producing same frequency and amplitude
The Ear
Structure of Ear
o Can be divided into the external, middle and inner ear and each area conducts
sound in a different way
o Incoming changes in air pressure are channelled through the external ear, onto
the middle ear and amplified so that it can be detected as changes in fluid
pressure by inner ear
o These changes in fluid pressure are then finally converted to auditory neural
The External Ear
o Made up of the pinna, ear canal and eardrum
o The pinna is the folded cone that collects sound waves in the environment and
directs them along the ear canal
o Since the ear canal narrows as it moves towards the eardrum, it functions to
amplify the incoming sound waves, much like a horn
o The eardrum is a thin membrane vibrating at a frequency of the incoming sound
wave and forms the back wall of the ear canal
The Middle Ear
o Begins on the other side of the eardrum, which connects the ossicles, the three
smallest bones in the body
o The ossicles consist of the hammer, the anvil and the stirrup
o The amplification of vibrating waves continues here in the middle ear
o Vibrating ossicles are about 20 times larger than the area of the oval window to
which they connect to create a lever system that amplifies the vibrations even
o The additional amplification is necessary because the changes in air pressure
originally detected by the external ear are about to be converted to waves in the
fluid-filled inner ear
The Inner Ear
o Vibrating oval window connects to the cochlea of inner ear
o Cochlea is a fluid-filled tube about 35 mm long, coiled like a snail shell
o Cochlea contains the neural tissue that is necessary to transfer the changes in
fluid to neural impulses of audition
The Cochlea
o The oval window is actually a small opening in the side of the cochlea and when
it is made to vibrate, it causes the fluid inside the cochlea to become displaced
o The round window, located at the other end of the cochlea, accommodates for
the movement of the fluid by bulging in and out accordingly
The Basilar Membrane
o Inside the cochlea is a flexible membrane, called the basilar membrane that runs
the length of the cochlea like a carpet
o So when the basilar membrane is pushed downwards, the fluid inside the
cochlea causes the round window to bulge out and when the basilar membrane
is forced upwards, the round window bulges inwards
o Although the cochlea itself gets narrower toward the end, the basilar membrane
gets wider
o Because the length of the membrane varies in both flexibility and width, sounds
of different frequencies cause different regions of the membrane to vibrate
o Higher frequency sounds cause the end nearest the oval window to vibrate
whereas lower frequency sounds cause the end nearest the round window to
Hair Cells