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Chapter 9

PSYB51 Chapter 9

Course Code
Matthias Niemeier

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Chapter 9: Hearing: Physiology and
The Function of Hearing
What Is Sound?
-Sounds are created when objects vibrate causing molecules and objects
surrounding medium to vibrate which causes pressure changes in the medium
these pressure changes are described as waves
-Sound waves do not change as they spread out, the initial amount of wave pressure
is dispersed over a larger and larger area as the wave moves away, so the wave
becomes less prominent as it gets farther from its source
Sound waves travel faster through denser substances in error 340 m/s, and
water 1500 m/s
Basic Qualities of Sound Waves: Frequency and Amplitude
-Soundwaves that we here are fluctuations in air pressure across time
-Amplitude magnitude of displacement of a sound pressure waves or of a head
-Intensity the amount of sound energy ball in on a unit area
-We typically described sound wave patterns by noting how quickly the pressure
fluctuates; the rate of fluctuation is known as the frequency
Hertz a unit of measurement for frequency. 1 Hz equals one cycle per second
-Amplitude is associated with the perceptual quality of loudness (psychological
aspect of sound related to perceived intensity or magnitude)
-Frequency is associated with pitch (psychological aspect of sound related mainly to
the fundamental frequency)
-Relatively young people are able to detect sounds that vary from about 20 to
-To describe differences in amplitude across such a broad range, sound levels are
measured on a logarithmic scale using units called decibels (a unit of measure for
the physical intensity of sound)
-Decibels define the difference between two sounds as the ratio between two sound
pressures. Each 10:1 sound pressure ratio equals 20 dB, so a 100:1 ratio is equal to
40 dB
Equation (page 221): dB = 20 log (p / p )
If the pressure of the sound that youre measuring is equal to 0.0002 dyne/ cm²,
then dB = 20 log(1) because the log of one and zero, a sound pressure that
low would be equal to 0 dB
-Relatively small decibel changes can correspond to large physical changes for
example an increase of 6 dB doubles the amount of pressure

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Sine Waves, Complex Tones, Fourier Analysis
-Sinewave / pure tone the waveform for which variation as a function of time is a
sine function
The air pressure in a sine wave changes continuously at the same frequency
-Period the time or space required for one cycle of a repeating waveform
-Phase the relative position of two or more sine waves. For sounds, the phase of
the relative position and time
-The undulation of the sine wave overtime is described in degrees
-Sine waves are not common because few vibrations in the world are so pure
-Complex tones a sound wave consisting of more than one sinosoidial component
of different frequencies
-All sounds can be described as a combination of sine waves
Fourier analysis
-In mathematical theorem by which any sound can be divided into a set sideways.
Combining these sine waves will produce the original sound
-Spectrum a representation of the relatives energy/intensity present at each
-Sounds with harmonic spectra the spectrum of a complex sound in which energy is
at integer multiples of the fundamental frequency are typically caused by a simple
vibrating source
-Each frequency component in such a sound is called a harmonic the first
harmonic, called the fundamental frequency
-The properties of sound sources determine the spectral shapes of sounds; these
shapes can help us identify sound sources
-Timbre psychological sensation by which a listener can judge that to sound the
same loudness and pitch are dissimilar. Its quality is conveyed by harmonics and
other high frequencies
Basic Structure of the Mammalian Auditory System
Outer Ear
-Sounds are first collected from the environment by the Pinna which we usually call
the ear
Only mammals have these
-Sound waves are funneled by the pinna into and through the ear canal which
enhances sound frequencies between about 2000 and 6000Hz and whose main
purpose is to insulate the structure at its end, the Tympanic membrane (a sheet of
skin that moves in and out in response to the pressure changes of sound waves)
Middle Ear
-The pinna and the ear canal makeup the outer ear (the external gathering portion of
the ear)

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-The tympanic membrane is the border between the outer year and the middle ear,
which consists of three tiny bones called the ossicles
Malleus is connected to that in panic membrane on one side and to the second
ossicle, the incus on the other
The incus if connected to the stapes which transmits vibrations of sound waves
to the oval window, another membrane which forms the border between the
middle ear and the inner ear
-Page 226 figure 9.10
-The ossicles are the smallest bones in the human body
The joints between the bones are hinged in a way that makes them work like
levers–this lever action increases the amount of pressure change by about a
The second way these bones increase energy transmitted to the inner ear is by
concentrating energy from it larger to smaller surface area–the pressure on the
oval window is magnified 18 times relative to the pressure him into panic
-The middle ear has two muscles: the tensor tympani which is attached to the
malleus and the stapedius which is attached to the stapes
The tensor tympani and the stapedius are the smallest muscles in the body and
their main purpose is to tense and sounds are very loud, restricting the
movement of the ossicles end of the muffling pressure changes
Acoustic reflex a reflex that protects the ear from intense sounds, be a
contraction of these two musclesfollows the onset of loud sounds by about 1/5
of the second and therefore cannot protect against abrupt loud sounds
Inner Ear
-The function of the inner ear is roughly analogous to that of the retina with respect to
light waves and vision: it translates the information carried by the waves into neural
Cochlear Canals and Membranes
-The cochlea a tiny coiled structure embedded in the temporal bone of the skull that
is filled with watery fluids in three parallel canals:
the tympanic canal; the vestibular canal; the middle canal
The tympanic and vestibular canals are connected by a small opening, the
The three canals are separated by two membranes: reissners membrane,
between the vestibular Canal and the middle canal; and the basilar membrane,
between the canal and into panic now
-The basil or membrane a plate made up of fibers that have some stiffness and it
forms the base of the cochlear partition, a complex structure through which sound
waves are transduced into neural signals
-Vibrations caused the stapes to push and pull the flexible oval window in and out
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