Sound and Hearing

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Sound and Hearing
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Sound travels as Longitudinal waves

• The oscillations are parallel to the direction
  of energy transfer.

Direction of energy transfer
oscillation
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Longitudinal waves
compression
rarefaction
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The Ear
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The Ear
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Outer Ear

• The sound is reflected by the sides and
  channeled into the auditory canal.

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Ear Drum

• There is a pressure difference between the two
  sides of the ear drum (the inside pressure is
  kept constant due to the Eustachian tube
  connecting with the back of the nose)
• Force on left (P ?P)A
• Force on right PA
• Unbalanced force ?PA
• This force is very small and needs to be
  amplified to move the liquid in the cochlea

P ?P
Area A
P
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The Middle Ear - Ossicles

• The small bones (ossicles) are arranged to pass
  on the vibration arranged as levers to amplify
  the FORCE (but small movement)

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The Middle Ear - Ossicles
F1
F2
d2
d1
L1
L2

• Work done by F1 F1d1
• Work done by F2 F2d2 so by conservation of
  energy F1/F2 d2/d1, and since d2ltd1, F2gtF1
• Amplification factor d1/d2 L1/L2

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The Middle Ear - Ossicles

• The small bones (ossicles) also help to dampen
  the vibrations of the ear drum.

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The Middle Ear Ossicles

• Sound travelling from air to liquid is normally
  reflected (99.5). The amplification by the
  ossicles raises the amount of sound transferred
  to 50.

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The Middle Ear Cochlea window

• A given force will result in a higher pressure as
  the area of the oval cochlea window is small
  compared to the ear drum.

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The Middle Ear Cochlea window
A1
F ?P1A1
?P2 ?P1A1 /A2
?P1

• Since A2ltA2 the pressure difference is
  amplified, amplification factor A1/A2

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The Inner Ear - Cochlea

• This is a 2cm long coiled tube containing a
  liquid. The vibrating oval window causes the
  fluid to be pushed along the tube (causing
  another round window to bulge outwards) which
  disturbs small hairs on the wall of the cochlea.
  These sends messages along the auditory nerve to
  the brain.

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Intensity of a Wave

• Remember from topic 4 that the intensity of a
  wave is the amount of energy passing through a
  unit area per unit time. It is normally measured
  in W.m-2.

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Intensity at a distance from a light source

• I P/4pd2
• where d is the distance from the light source
  (in m) and P is the power of the light source(in
  W)

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Intensity at a distance from a light source

• I P/4pd2

d
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Sound Intensity Level

• The sound intensity level attempts to quantify
  the sensation of hearing. It relates sounds to
  the lowest intensity that can normally be heard
  by a human ear (1 x 10-12 W.m-2)
• Sound intensity level (dB) 10log(I/Io)
• where Io 1 x 10-12 W.m-2.

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Note!

• Note the important difference between sound
  intensity (measured in W.m-2) and sound intensity
  level (measured in dB)
• SIL (dB) 10log(I/Io)

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Example Physics for the IB Diploma, K.A.
Tsokos, CUP

• The intensity of a sound increases from 10-10
  Wm-2 to 10-8 Wm-2. By how much does the sound
  intensity level change?

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Example Physics for the IB Diploma, K.A.
Tsokos, CUP

• The intensity of a sound increases from 10-10
  Wm-2 to 10-8 Wm-2. By how much does the sound
  intensity level change?
• Original level 10log(10-10/10-12) 10log102
  20 dB
• New level 10log(10-8/10-12) 10log104 40dB
• Increase is thus 20 dB

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The decibel scale dB
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A vacuum cleaner

• A vacuum cleaner has a SIL of 80 dB. What is its
  intensity?
• 80 dB 10log(I/Io)
• 8 log(I/Io)
• 108 I/Io I/(1 x 10-12)
• I 1 x 10-4 W.m-2

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Lets try some questions!
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Hearing Defects
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Sensory nerve deafness

• Damage to the cochlea and/or neural pathways
  (nerves)

Possibly due to tumours of the acoustic nerve or
meningitis. Acoustic trauma (injury caused by
loud noise) can damage hair cells.
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Conductive deafness

• Damage to the middle ear prevents transmission
  of sound to the cochlea

Destruction or seizing of the ossicles due to
serious infection or head injury (Otosclerosis -
an abnormal growth of bone in the middle ear).
Ear could be blocked by wax or an obstruction
Eardrum could have thickened due to repeated
infections/rupture
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Hearing tests

• http//www.wisc-online.com/Objects/ViewObject.aspx
  ?IDMBY1802

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Hearing tests

• Normally sounds of 125, 250, 500, 1000, 4000,
  8000 Hz are played through headphones.
• Sounds start very quietly and increase until the
  patient can hear them.
• Made for both ears AND with a vibrator attached
  to the bone behind the ear to send sound directly
  to the cochlea through the bone.

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Audiogram

• The hearing level in dB is then plotted against
  frequency to produce an audiogram.

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Audiogram

• Hearing can be tested using an audiogram

Bone conduction
Air conduction
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Conductive loss

• You can see that the hearing through the bone
  (straight to the cochlea) is fine, but the
  hearing through air is not. This indicates
  conductive hearing loss (cochlea is working
  fine). A hearing aid may help

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Sensory Loss
Typically exposure to loud noise over time will
see a dip at 4000 Hz.

• You can see here that the lines for air
  conduction and bone conduction follow each other
  (and lower). This is a sure sign of sensory loss.
  A cochlea implant may be useful.

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Ageing

• Aging produces a more gentle downward curve.
  This is the natural decline in hearing that many
  people experience as they get older. It's partly
  due to the loss of hair cells in the cochlea.

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Clear?

• http//www.lloydhearingaid.com/shopping/audiogram_
  start.asp