Medical Physics:Hearing - IB Objectives
I.1.1
Describe the basic structure of the human ear
I.1.2
State and explain how sound pressure
variations in air are changed into larger
pressure variations in the cochlear fluid
I.1.3
State the range of audible frequencies
experienced by a person with normal hearing
I.1.4
State and explain that a change in observed
loudness is the response of the ear to a
change in intensity
I.1.5
State and explain that there is a logarithmic
response of the ear to intensity
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Structure of the Ear
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Structure of the Ear
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Outer ear:
 Pinna (ear)
 Auditory canal
 Eardrum (tympanic membrane)
Middle ear:
 Ossicles (Hammer, anvil, and stirrup, or malleus,
incus, and stapes)
 Connect eardrum to cochlea
 Eustachian tube
Inner ear
 Cochlea (snail)
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Hearing – Outer Ear
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Pinna directs sound energy into auditory canal
Auditory canal directs sound energy to eardrum
(tympanic membrane)
 Length of 2.5 cm gives resonance at 3,300 Hz
 ~Peak for human speech
Eardrum vibrates at frequencies of sound
2
 Area of ~60 mm
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Hearing – Middle Ear
3 mm2
60 mm2
What is force transferred?
F2 = 1.5 F1
What is pressure transferred?
F2 = A2P2 = 1.5 F1 = 1.5 A1P1
P2 = 1.5 A1/A2 P1 = 30 P1
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Hearing – Middle Ear
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Three ossicles conduct vibration from eardrum to
cochlea
 Provide magnification of force of ~1.5
 Provide magnification of pressure ~30 to cochlea
 Cochlear oval window (fenestra ovalis) has area
of ~3 mm2
Magnification of force and pressure needed to transfer
pressure waves from air on eardrum to fluid in cochlea
Otherwise, most sound reflected back
Pressure between outer ear and middle ear equalized
by Eustachian tube
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Hearing – Inner Ear
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Hearing – Inner Ear

Cochlear has complex structure
 One tube (scala vestibuli) on other side of oval
window transmits pressure wave through
perilymph
 Pressure wave travels to helicotrema, where scala
vestibuli connects to another tube (scala tympani),
and back to round window (finestra rotunda)
 Pressure wave also induces waves in walls of
these tubes, and in the walls of a third tube
between them (scala media)
 Structures in this third tube responsible for hearing
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Hearing – Inner Ear 2
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Cochlear has complex structure
 Walls of scala media have different sizes, masses,
and tension
 Different resonant frequencies along tube
 Fluid (mesolymph) supports hair cells and organs
of corti that detect these resonances, and transmit
impulses to nerves to brain
Cochlea unrolled
Scala Vestibuli
Scala Media
Oval Window
Round Window
Scala Tympani
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Hearing – Inner Ear 3
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The hair cells and the organ of Corti detect
movements in the wall (basal membrane) of the scala
media
 Medium and high frequency sounds detected by
different regions of the cochlea
 Low frequencies (~200 - 1000 Hz) detected by
entire length of scala media
Louder noise activates
High Freq. Medium Freq.
Response
Response
more hair cells
Cochlea Unrolled
Low Freq.
Response
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Human Hearing - Active Listening
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Ear adjusts to hear anticipated sounds
 Pre-tensioning hair cells to listen for quiet sounds
 Eardrum tightness
 Support of ossicles
Ear protects itself from loud noises
 Reduces tight linkage between ossicles
 Can be too late if noise is too sudden
Ear makes its own sounds
 Ringing (tinnitis)
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Human Hearing - Frequency Limits
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“Normal” range of human hearing given as
20 Hz to 20,000 Hz
 Audible frequencies
 With age, smaller range especially at high end
Less the 20 Hz: infrasound
More than 20 kHz: ultrasound
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Sound Intensity and
Sound Intensity Level - Decibels (dB)
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Sound is longitudinal vibration in a medium
 Characterize intensity of sound by how much energy
it carries
 Per second
 Per square meter (area)
2
-1 m-2
 I (J/(s m )) or J s
Because of wide range of sound levels, use unit with
logarithmic scale: Intensity Level (IL)
IL (decibels) = 10 log (I/I0), where I0 = 1.0 x 10-12 W/m2
 I0 is the quietest sound commonly able to be heard
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Sound Intensity and
Sound Intensity Level - Examples
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What is IL of intensity I0
What is IL of intensity 1.0 W/m2
What is intensity of IL of 50 dB?
What is intensity of IL of 36 dB?
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Perceived Sound Level Frequency Dependence
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The “threshold of hearing” is not always at I0
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Perceived Sound Level 2 Loudness Dependence
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Sounds of equal intensity are “loudest” at ~3 kHz
Sounds of equal perceived loudness have same
phon values
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From Everest, Frederick Alton, The Master Handbook of Acoustics
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Perceived Loudness Loudness Dependence
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We do not hear sound loudness linearly
Sounds that are twice as loud have twice the sone
values
Perceived
loudness
(sones) show
logarithmic
behavior
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From Everest, Frederick Alton, The Master Handbook of Acoustics
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Medical Physics:Hearing - IB Objectives
I.1.6
Define intensity and also intensity level (IL).
I.1.7
State the approximate magnitude of the
intensity level at which discomfort is
experienced by a person with normal hearing.
I.1.8
Solve problems involving intensity levels.
I.1.9
Describe the effects on hearing of short-term
and long-term exposure to noise.
I.1.10
Analyze and give a simple interpretation of
graphs where IL is plotted against the
logarithm of frequency for normal and
defective hearing.
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Effect of Distance on Sound Intensity
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As a sound wave expands in space, the radius goes
from R1 to R2, Intensity goes from I1 to I2
Surface area of wavefront goes from 4R12 to 4R22
Since energy does not change, the energy/surface
area goes down
R12I1 = R22I2, or R12/R22 = I2/I1
R1
R2
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Measuring Human Hearing
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Hearing measured by audiologists
Typically, measure threshold of hearing
 Of each ear separately
 At a range of frequencies
 Report results as IL vs frequency (log)
Normal Audiogram
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Physiological Effects of Sounds
Intensity
Cause
Level (dB)
Effect
60
Conversation
90
Loud noise
Extended exposure - hearing
degraded
120
Rock concert
Discomfort, possible long term
effects
140
Jet engine at 25 m Pain, possible damage
160
Nearby rifle shot
Eardrum rupture
~180
Explosion
Death
196
Explosion
Loudest sound
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Sample Problems with
Sound Intensity Level

A jet engine creates a sound with a 120 dB sound
intensity level at 10 m.
 What is the sound intensity?
 What is the sound intensity at 65 m?
 How far do you have to be to hear the engine with
an intensity level of 60 dB?
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Hearing Problems
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Hearing problems may occur in the outer ear, middle
ear, and inner ear, or in the nerves carrying auditory
information to the brain
Commonly, hearing degrades
 With age
 With exposure to noise (usually long-term)
Cilia on hair cells in cochlea break off, and are not
replaced, especially for high-frequency sounds
(Why?)
 Increasing hearing loss over time, especially at the
high frequencies
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Noise Exposure
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Short-term effects of noise exposure can be
 Tinnitis (ringing in the ears)
 Reduced perceived loudness (muffled)
Long-term effects can be serious permanent
degradation of hearing
Long-term Noise
Exposure
Normal 65-year old
Normal Audiogram
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