Resonance, Sound Waves
and The Ear
http://www.phys.unsw.edu.au/jw/hearing.html
Natural Frequency--Forced Vibration-Resonance
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Forced Vibration: a vibrating object in contact with
another object causes it to vibrate at the same
frequency.
Natural frequency: the frequency (or frequencies) at
which an object naturally vibrates when receiving a
disturbance.
Resonance: Condition occurring when the frequency
of a vibration of one object matches the natural
frequency on another object and causes it to
dramatically increase in amplitude
Resonance can occur whenever successive impulses
are applied to a vibrating object in rhythm with its
natural frequency. (pushing someone on a swing)
http://www.youtube.com/watch?v=WDZmjzxaxhs
What does the natural frequency
depend upon?
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The natural frequency depends on many
factors, such as the tightness, length, or
weight of a string.
We can change the natural frequency of a
system by changing any of the factors that
affect the size, inertia, or forces in the system.
For example, tuning a guitar changes the
natural frequency of a string by changing its
tension.
Resonance
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You can think of resonance as having the natural
frequency of the system exactly in tune with your
force. Each cycle of your force exactly matches each
cycle of the system.
As a result, each push adds to the next one and the
amplitude of the oscillation grows.
Two tuning forks with the same natural frequency—
one vibrating nearby will cause the other to
vibrate—the forks are connected by air molecules.
26.6 Natural Frequency vs. Forced Vibration
When any object composed of an elastic material is
disturbed, it vibrates at its own special set of frequencies,
which together form its special sound.
We speak of an object’s natural frequency,
the frequency at which an object vibrates
when it is disturbed.
A forced vibration occurs when an object
is made to vibrate by another vibrating
object that is nearby.
A natural frequency is one at which minimum energy is
required to produce forced vibrations and the least amount of
energy is required to continue this vibration.
26.8 Resonance
An object resonates when there is a force to pull it back to
its starting position and enough energy to keep it
vibrating.
It is the frequency or frequencies at which an object will
most easily vibrate.
Examples of Resonance
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Sympathetic vibrations
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speakers buzzing
cilia in cochlea of inner ear vibrating
Building up amplitude
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girl pushed on swing gains height
one tuning fork causes another to vibrate without
direct physical contact
Tacoma Narrows Bridge Collapse
26.8 Resonance
If the frequency of a forced vibration
matches an object’s natural frequency,
resonance dramatically increases the
amplitude.
You pump a swing in rhythm with the
swing’s natural frequency.
Timing is more important than the
force with which you pump.
Even small pumps or pushes in rhythm
with the natural frequency of the
swinging motion produce large
amplitudes.
26.8 Resonance
a.
The first compression gives the fork a tiny push.
26.8 Resonance
a.
b.
The first compression gives the fork a tiny push.
The fork bends.
26.8 Resonance
a.
b.
c.
The first compression gives the fork a tiny push.
The fork bends.
The fork returns to its initial position.
26.8 Resonance
a.
b.
c.
d.
The first compression gives the fork a tiny push.
The fork bends.
The fork returns to its initial position.
It keeps moving and overshoots in the opposite direction.
26.8 Resonance
a.
b.
c.
d.
e.
The first compression gives the fork a tiny push.
The fork bends.
The fork returns to its initial position.
It keeps moving and overshoots in the opposite direction.
When it returns to its initial position, the next compression arrives to repeat
the cycle.
Tacoma Narrows Bridge
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http://www.youtube.com/watch?v=3mclp9Q
mCGs
now
Then (July 1940)
Sound waves and the Ear
What is the audible range of
frequencies for a human?
20 -20,000 Hz
What type of
wave is a sound
wave?
A Compressional
wave
The frequency of
a sound wave is
called pitch.
Like all mechanical waves,
sound waves can only travel
through matter
Relative intensity of sound
wave is volume and is
measured in decibels (dB)
BASIC FUNCTION OF THE EAR
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The ear converts changes in air pressure due
to sound waves to nerve impulses that signal
Eardrum
the brain compression
vibrates at same
frequency as
tuning fork and
with a certain
intensity
256 Hz
256 Hz
rarefaction
The Ear
3 tiny bones
(hammer, anvil
and stirrup)
Semicircular canals
(for balance)
cochlea
Pinna
Cochlea of the Human Ear—cilia and nerves in different regions in the cochlea
resonate to specific frequencies of sound waves. The region that resonates at 256 Hz
“lights up” and signals your brain via the nerves.
256 Hz
Hearing Problems
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conductive hearing loss (interferes with the
transfer of sound vibrations)
sensory hearing loss (affects the cochlea’s
ability to resonate from 20 - 20,000 Hz.)
neural hearing loss (affects the connection
between the cochlea and the brain.)
Hearing Corrections
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Repairs to the conductive parts of the ear
Cochlear implants (addresses frequency
deficiencies)
Hearing aids (increase amplification)
Main Parts of the Ear
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Inner Ear
Middle Ear
Outer Ear
outer
middle
inner
Outer Ear
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Structures: the pinna, ear canal and eardrum.
Purpose: to receive, focus (or amplify) and
transmit sound vibrations to the middle ear.
Eardrum
vibrates at same
frequency as
tuning fork
256 Hz
256 Hz
Middle Ear
Structure: Ear bones (hammer, anvil and
stirrup are the three tiniest bones in the human
body)
 Purpose: To transmit sound
vibrations from the eardrum
to the inner ear.
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Inner Ear
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Structure: Cochlea
Purpose: The fluid in the cochlea receives sound
vibrations from the stirrup, causing tiny hairs inside
the cochlea to vibrate, which stimulates auditory
nerves connected
to the brain.
256 Hz
http://www.cedengineering.com/upload/Ethical
%20Issues%20Tacoma%20Narrows.pdf
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http://www.thinkreliability.com/CMTacoma.aspx