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A wiggle in time is a vibration.
A wiggle in space and time is a
wave.
A vibration can exist in one location,
a wave cannot. A wave must travel
from one place to another.
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A pendulum swings back and forth with
regularity.
The time a pendulum takes to swing
back and forth depends on its length and
the acceleration of gravity.
The time it takes for one back and forth
swing is called a period.
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The longer the pendulum is, the
longer the period will be.
Our legs act like pendulums with
the help of gravity.
Long legs have a slower, larger gait
and short legs have a faster, shorter
gait.
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The back and forth vibratory motion
of a pendulum is called simple
harmonic motion.
This means that a sine curve is
formed when the acceleration is
proportional to the distance from
equilibrium and is directed toward
that equilibrium position.
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A sine curve is a curve whose
shape represents the crests and
troughs of a wave.
The high points of a wave are
called crests.
 The low points are called
troughs.
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Measuring from the rest or home
position is called the amplitude.
This measurement is the maximum
displacement from equilibrium.
Wavelength is the distance between
identical parts of the wave, but
usually we measure crest to crest.
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How often a vibration occurs is
called its frequency.
Frequency is measured in hertz
(Hz).
One cycle per second is one hertz;
two per second is 2 hertz, and so on.
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High frequencies are measured in
kilohertz, megahertz, or gigahertz.
AM radio is an example of kilohertz,
FM radio is in megahertz, and radar
and microwave ovens operate in
gigahertz.
980 kHz on the AM radio causes 960,000
vibrations per second. 101MHz on FM
causes 101,000,000 vibrations.
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The source of all waves is something
that vibrates.
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When energy is transferred by a wave
from a vibrating source to a distance
receiver, there is no transfer of matter
between the two points.
The energy transferred from a vibrating
source to a receiver is carried by a
disturbance in a medium, not by matter
moving.
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Dropping a stone in a pond
creates a wave, but the water is
still there after the wave has
passed and the water is calmed.
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The speed of a wave depends on the
medium through which the wave
moves.
The speed of sound varies
depending on the medium. It is
slowest through gasses, then liquids,
and fastest through solids.
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Wave speed, wavelength and
frequency are all related.
Consider a rock concert; you do not
hear the high notes before the low
notes that are played in a chord.
Wave speed = wavelength x
frequency
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When the motion of the medium is
at right angles to the direction of the
wave movement, the wave is a
transverse wave.
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Waves in the stretched strings of
musical instruments and water
waves are transverse.
Electromagnetic waves are also
transverse.
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Sometimes the particles of a substance
move in the same direction of a wave.
The particles move along the wave.
This type of wave is longitudinal.
Sound waves are an example of
longitudinal waves.
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Two or more waves present in
the same space create
interference pattern.
Within the pattern, waves can be
increased, decreased or
neutralized.
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When two crests overlap, they
add together to increase the
amplitude.
This is called constructive
interference.
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When one crest and one trough
interact, their individual effects
are reduced, or cancelled.
This is called destructive
interference.
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Interference occurs clearly in
water, but occurs in all types of
waves including sound, and
light waves.
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A standing wave occurs when the
nodes of a wave remain permanent.
The wave appears not to move.
Standing waves occur when
interference occurs between a
incident wave and a reflected wave.
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Standing waves have nodes and
antinodes.
Antinodes are the positions of greatest
amplitude.
Nodes are areas of destructive
interference.
Standing waves occur in both
longitudinal and transverse waves.
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The change in frequency due to the
motion of the source or of the
receiver causes the Doppler effect.
The greater the speed of the source,
the greater the Doppler effect will
be.
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The frequency is not actually
changing; it is an apparent change
that is sensed.
This effect is apparent when you
hear the changing pitch of a car horn
as the car passes you.
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Police use this to clock your
speed on the highway, except
they use radar waves.
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This effect also works with light.
An increase in frequency is called
blue shift, and a decrease is called
red shift.
This is often used in astronomy to
determine the speed at which
galaxies and stars are moving away
or toward us.
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We can also tell how fast stars
are spinning, because the side
spinning away shows a red shift
and the side spinning toward us
shows a blue shift.
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When the speed of the source in a
medium is as great as the speed of
the wave it produces, the waves pile
up.
This is what happens when jets
travel at the speed of sound.
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Supersonic planes are faster than
sound.
There is no such thing as a sound
barrier. What actually happens is
the plane gets harder to control as it
nears the speed of sound.
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A jet flying at the speed of sound flies
in smooth, undisturbed air, because
there are no sound waves in front of it.
The crests begin to overlap at the front
edges, and the pattern made by the
overlapping crests is a V-shape called a
bow wave.
Bow waves are also formed when a
speedboat knifes through the water
faster than the wave speed.
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The above speedboat generates a
two-dimensional bow wave.
A supersonic aircraft generates a
3-D shock wave.
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A shock wave is formed from
overlapping spheres that form a
cone.
This cone spreads until it reaches the
ground.
When it reaches the ground and
passes you, you will hear the sonic
boom.
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Sonic booms occur the entire time a
jet is traveling faster than sound.
Once an object is moving faster than
sound, it will make sound after that,
whether it was making sound before
or not.
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All sounds are produced by the
vibrations of material objects.
This vibrating material then sends a
disturbance through a surrounding
medium in the form of longitudinal
waves.
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Under ordinary circumstances, the
frequency of the vibrating source
equals the frequency of sound
waves produced.
The subjective frequency of sound
is described by the word pitch.
A high pitch has a high vibration
frequency and a low pitch has a
low frequency.
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A young person can usually hear
pitches with 20 to 20,000 hertz.
Sound waves below 20 hertz are
called infrasonic, and those that
are above 20,000 are called
ultrasonic. We cannot hear these
pitches.
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When sound moves through air as a
longitudinal wave, it compresses and
rarefacts the air.
A pulse of compressed air is called a
compression.
In between the pulses of compressed air are
regions of air particles that are allowed to
spread out. This disturbance is called a
rarefaction.
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In all wave motion, it is not the
medium that moves across the
room, it is the pulse, or energy
that travels.
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Most sounds we hear are
transmitted through air, but they
can travel through liquids and
solids also.
Solids and liquids are good
conductors of sound
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Sound cannot travel in a vacuum.
Sound requires a medium to travel.
If there is nothing to compress and
expand, there can be no sound.
There can be vibration of an object,
but no sound.
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Sound is much slower than light.
The speed of sound in dry air at 0oC
is about 330m/s. Water vapor in the
air increases the speed.
Increased temperature increases
speed also.
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The intensity of a sound is proportional
to the square of the amplitude of a
wave.
An oscilloscope measures sound
intensity.
Loudness is a physiological sensation
sensed by the brain. It is different in
various people.
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The unit for intensity of sound is the
decibel. (dB)
Sound decibels increase
logarithmically. Meaning, that 10dB
is 10 times as intense as 0. 20 dB is
10x as intense as 10dB.
Because of this logarithmic pattern,
human hearing is also logarithmic.
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A forced vibration occurs when an
object is forced to vibrate because of
its proximity to a vibrating object.
The items that are forced to vibrate
are called sounding boards.
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Examples of sounding boards
are guitar bodies, washtubs, and
tuning fork boxes.
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When any object composed of an
elastic material is disturbed, it
vibrates at its own special set of
frequencies. This forms its own
special sound.
This is called its natural frequency.
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A natural frequency is one at which
minimum energy is required to
produce forced vibrations.
It also requires the least amount of
energy to keep the vibration going.
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When the frequency of a forced
vibration on an object matches the
objects natural frequency, an increase in
amplitude occurs.
This is called resonance.
Resonance means resound, or sound
again.
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An example of resonance is when
you pump on a swing, or when
someone pushes you at the right
time. Your amplitude increases.
The Tacoma Narrows Bridge in WA.
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Sound waves can be made to
interfere.
Constructive interference makes
the sounds louder.
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If you move two speakers so that
you receive one compression and
one rarefaction at the same time,
it can form a dead spot. You will
still hear the sounds, but it will
not sound good.
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Dead spots can form in
amphitheaters and gymnasiums.
Destructive noise interference is
being used in anti-noise technology.
They have created noise-canceling
earphones for pilots, and are
working on using this technology
for electronic car mufflers.
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Interference occurs when two tones of
slightly different frequency are sounded
together.
A fluctuation of the loudness of the
sounds occurs; loud then faint, loud
then faint.
This is called beats.
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This is similar to walking next to
someone who has a different
stride than you. Sometimes you
are in step, and sometimes you
are not.
Beats can be displayed on an
oscilloscope.
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To tune a piano, a tuner will use a
tuning fork that matches the note
correctly. When the beats disappear,
he knows that the piano is in tune.
Bands can tune themselves the same
way. They only need to listen to a
standard note played by one
instrument.