This presentation briefly describe how the sound is conducted from external environment to the cochlea via the consideration of physics point of view. it also gives basic understanding how the physics plays an important role in sense of hearing.

1. PHYSICS OF AUDITION
PRADEEP YADAV MSC PART II
PAPER 301 PHYSIO - BIOPHYSICS
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IA SEM III

2. Topics at a glance…
• Structure of Ear
• Sound Conduction Mechanism
• Sound Transduction Mechanism
• Threshold and Range of hearing.
• References
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3. Structure of Ear…
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4. Sound Conducting Mechanism…
• The sound conducting mechanism is divided into two parts, an outer
and the middle ear, the outer part pinna and auditory canal which
catches sound and the middle ear which is an impedance matching
device.
• Auditory canal serves to increase sensitivity of ear in region of 3000
to 4000 Hz.
• Length of canal = 2.5 cm, diameter is the size of a pencil.
• Since it closed at one end thus behaves as organ pipe closed at one
end (length = λ/4) wit resonating frequency of 3300 (λ = 10 cm).
• Ear drum, thickness = 0.1 mm and Area = 65 mm2 , couples
vibration of air to bones.
• The threshold of hearing at 3k Hz = 10-9 cm and at lowest threshold
frequency 20 Hz = 10-5 cm (motion of ear drum).
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5. Middle Ear
• Motion of stapes = 0.7 X motion of
malleus at TM
• Lever action amplifies force by 1.3
fold.
• The ratio of surface area of TM (55
mm2) to surface area of stapes (3.2
mm2) is 17 fold difference.
• Total force = 1.3 x 17 = 22.1 dyne.
• Lever action causes 22 times as
much as force on liquid system of
cochlea by tympanic membrane.
• Impedance matching is 50 to 75 %
perfect for frequencies between 300
and 3000 cycles/sec.
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6. Sound Transduction Mechanism
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8. •Basillar membrane = 20 – 30 k fibers
•These fibers are stiff, elastic and reed
like which may or may not be fixed on
one end.
•Length of these fibers increases from
oval window to apex of cochlea (0.04
mm to 0.5 mm)
•Diameter also tends to decrease from
oval window to helicotrema.
•Short fibers vibrate at higher
frequency and long fibers vibrate at
lower frequency.
•High frequency resonance occurs near
bas of basillar membrane.
•Low frequency resonance occurs near
helicotrema.
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The dashed curves of Figure 53-6A show the
position of a sound wave on the basilar membrane
when the stapes (a) is all the way inward, (b) has
moved back to the neutral point, (c) is all the way
outward, and (d) has moved back again to the
neutral point but is moving
inward. The shaded area around these different
waves shows the extent of vibration of the basilar
membrane during a complete vibratory cycle. This is
the amplitude pattern of vibration of the basilar
membrane for this particular sound frequency.
Figure 53-6B shows the amplitude patterns of
vibration for different frequencies, demonstrating
that the maximum amplitude for sound at 8000
cycles/sec occurs near the base of the cochlea,
whereas that for frequencies less than 200 cycles/sec
is all the way at the tip of the basilar membrane near
the helicotrema, where the scala vestibuli opens into
the scala tympani.

11. Threshold and Range of Hearing
• Frequency Range of Hearing -
a young person can hear
frequency are between 20 and
20,000 cycles/sec.
• In old age, this frequency range
is usually shortened to 50 to
8000 cycles/sec or less.
• The sound range depends to a
great extent on loudness.
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12. Refernces…
• Guyton and Hall textbook of medical physiology –
thirteenth edition: John E. Hall.
• Lecture notes on sense of hearing by Dr. Sandhya
Kulkarni.
• Physics of Ear and Hearing – Dr. Suha Adel Al-Ani.
• The anatomy and physiology of the ear and hearing
Peter W. Alberti professor em. Of Otolaryngology.
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