Medical Physics:Hearing - IB Objectives

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Medical PhysicsHearing - 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

• 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

• 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
• Area of 60 mm2

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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

• 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

• 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

• 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 activatesmore hair cells

Cochlea Unrolled
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Human Hearing - Active Listening

• 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

• Normal range of human hearing given as20 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 andSound Intensity Level -
Decibels (dB)

• Sound is longitudinal vibration in a medium
• Characterize intensity of sound by how much
  energy it carries
• Per second
• Per square meter (area)
• I (J/(s m2)) or J s-1 m-2
• 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 andSound Intensity Level -
Examples

• 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

• The threshold of hearing is not always at I0

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Perceived Sound Level 2 -Loudness Dependence

• Sounds of equal intensity are loudest at 3 kHz
• Sounds of equal perceived loudness have same phon
  values

From Everest, Frederick Alton, The Master
Handbook of Acoustics
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Perceived Loudness -Loudness Dependence

• We do not hear sound loudness linearly
• Sounds that are twice as loud have twice the sone
  values
• Perceivedloudness(sones) showlogarithmicbehavi
  or

From Everest, Frederick Alton, The Master
Handbook of Acoustics
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Medical PhysicsHearing - 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

• 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

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Measuring Human Hearing

• 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 Level (dB) Cause 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 withSound 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

• 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

• 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