Mechanical Waves Mechanical Wave

CH 17 - MECHANICAL WAVES & SOUND
Sec. 17.2 - Mechanical Waves
Mechanical Wave - disturbance in matter that carries energy
from one place to another.
• Mechanical waves require matter called a MEDIUM to travel
through
• A medium can be solid, liquid or gas
• A mechanical wave is created when a source of energy causes a
vibration to travel through a medium.
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3 TYPES OF MECHANICAL WAVES:
1) TRANSVERSE - a wave that causes the medium to vibrate at
right angles (perpendicular) to the direction in which the wave
travels.
o
o
o
Shaking a rope up and down
Crest = highest point from rest position
Trough = lowest point below rest position
http://www.phy.hk/wiki/englishhtm/TwaveA.htm
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2) LONGITUDINAL WAVE- a wave in which the vibration of the
medium is PARALLEL to the direction the wave travels; sound
waves are this type
• Created by pushing and pulling in one direction
• Compression = area where particles are spaced close
together
• Rarefaction = area where particles are spread out
http://en.wikipedia.org/wiki/File:Longitudinalwave.ogg
• http://www.acoustics.salford.ac.uk/feschools/waves/wavetypes.htm
P waves ( primary waves ) caused by earthquakes
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3) SURFACE WAVE- a wave that travels along a surface
separating two media.
• Ocean waves
• Combination of transverse and longitudinal causes a bobber
on the surface to move in a circle in deep water
• When ocean waves enter shore they topple over themselves
because friction with the shore slows down the bottom of the
wave.
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Sec. 17.2 - PROPERTIES OF MECHANICAL WAVES
Periodic motion - any motion that repeats at regular intervals
Period - the time required for one cycle (time between 2
successive crests or compressions)
Frequency - the number of complete wave cycles in a given time
(cycles per second = hertz (Hz)
Frequency = frequency of the vibration source producing the
wave
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Wavelength- the distance between a point on one wave and the
same point on the next cycle of the wave (crest to crest or
compression to compression.)
Increasing the frequency of a wave decreases
wavelength.
its
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Amplitude - maximum displacement of the medium from its rest
position (height of wave)
o The more energy a wave has the greater its amplitude!
o In longitudinal waves the amplitude is the maximum
displacement of a point from its rest position
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Speed of wave = wavelength x frequency
V = λ f
Speed = wavelength x frequency
The speed of a wave can change if it enters a new medium or if
pressure and temperature change.
If not told otherwise, assume waves are traveling at a constant
speed. Therefore, wavelength is inversely proportional to
frequency.
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Try these:
1) The waves in a pool have a wavelength of 0.20 m and a
frequency of 2.8 Hz. What is the speed of these waves
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2) A student moves the end of a soft spring back and forth to
make waves. The waves travel at 1.8 m/s and have a wavelength of
1.2 m. What is the frequency of these waves
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Sec. 17.3 - BEHAVIOR OF WAVES
1) REFLECTION - occurs when a wave bounces off a surface that
it cannot pass through.
Reflection does not change the speed or frequency of a wave,
but the wave can be flipped upside down if the reflection occurs at
a fixed boundary.
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2) REFRACTION- the BENDING of a wave as it enters a new
medium at an angle; occurs because one side of the wave moves
more slowly than the other side.
If ocean wave fronts approach the shore at an angle they will
refract because one side of the wave moves more slowly than the
other side.
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3) DIFFRACTION - Bending of a wave as it moves around an
obstacle or passes through a narrow opening. A wave diffracts
more if its wavelength is large compared to the size of an opening
or obstacle
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4) INTERFERENCE - Occurs when two or more waves OVERLAP and
combine together
1. CONSTRUCTIVE INTERFERENCE - When two or more waves
combine to produce a wave with a larger displacement (amplitude.)
2. DESTRUCTIVE INTERFERENCE - When two or more waves
combine to produce a wave with smaller displacements (amplitude.)
http://id.mind.net/~zona/mstm/physics/waves/interference/intrfrnc.html
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Chromatic interference is seen in sea foam, which is made out of Plankton. It is an example of
the naturally occurring interference.
http://en.wikipedia.org/wiki/File:Two_sources_interference.gif
http://www.acoustics.salford.ac.uk/feschools/waves/super2.htm
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STANDING WAVES - A wave that appears to stay in one place, not
travel through the medium
v When a wave is created and its reflected wave interferes with it
“perfectly.”
v Plucking a guitar string produces a standing wave
v NODE - point on standing wave that has NO displacement (no
movement) from resting position due to complete destructive
interference.
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v ANTINODE - point on standing wave were a crest or a trough
occurs midway between two nodes. Maximum displacement due to
complete constructive interference.
A standing wave forms ONLY if half of a wavelength or a multiple
of half a wavelength fits EXACTLY into the length of a vibrating
cord.
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How many wavelengths does this standing wave have
How many nodes antinodes
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Sec. 17.4 - Sound & Hearing
Sounds waves --> longitudinal waves that travel through a medium
Properties of sound waves:
1) Speed --> 342 m/s in dry, 20 o C air; speed varies depending on
the medium.
Sound waves tend to travel fastest in solids, slower in liquids and
slowest in gases because the distance between particles is
greatest in gases. Both density and elasticity of the particles
affect speed.
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2) Intensity --> rate at which a wave‛s energy flows through a
given area; depends on both the wave‛s amplitude & distance from
the sound source.
Intensity is measured in decibels. For every 10
decibel increase, the intensity increases tenfold
Sound
Intensity level(decibels)
Human Hearing threshold 0
Whisper 15­20
Normal conversation 40­50
Street noise 60­70
Inside a bus 90­100
Operating heavy machinery 80­120
Rock concert 110­120
Threshold of pain 120
Jet plane taking off 120­160
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3) Loudness --> physical response to the intensity of sound; is
subjective as it depends on ear health and brain interpretation
4) Pitch --> how the frequency of sound is perceived; remember
that frequency is how fast the wave is vibrating.
High pitch sounds have a high frequency.
Low pitch sounds have a low frequency.
Humans typically hear between 20-20,000 hertz.
Ultrasound --> frequencies greater than 20,000 hertz; beyond
range of human hearing.
Sonar = SOund NAvigation & Ranging; uses the
speed of sound in water and the time that the
sound takes to reach an object and the bounce
back from the object (echo); also called
echolocation; uses ultrasound frequencies.
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Ex: A submarine uses SONAR to measure the distance to the bottom of the
ocean. If an ultrasound signal is sent and it takes 7 seconds to receive the echo,
how far away is the bottom of the ocean (speed of sound in water = 1546 m/s)
speed = distance x time
1546 m/s = (d)(7 s.)
d = 271 m. / 2 (because its an echo)
so: d = 136 m. to the bottom
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Doppler Effect --> a change in sound frequency caused by motion
of the sound source, motion of the listener, or both.
As a source of sound approaches, an observer
hears a higher frequency. When the sound
moves away, the observer hears a lower
freqency.
Observer B will hear a higher pitch because the waves are
bunched together while Observer A will have a lower pitch because
the waves are spread apart.
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The Human Ear --> the outer ear gathers and focuses sound into
the middle ear; the middle ear receives and amplifies the
vibrations; the inner ear uses nerve endings to sense vibrations
and send signals to the brain.
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How human hearing works:
1) The pinna (outer sound-collecting part) funnels sound waves down
the ear canal.
2) The ear canal (about 2.5 cm. long) carries the sound waves to the
tympanic membrane.
3) The tympanic membrane (a tightly stretched membrane) vibrates
at the same frequency as the sound waves striking it.
4) The tympanic membrane is touching the malleus (hammer) which
then begins to vibrate. The malleus strikes the incus (anvil) which
then begins to vibrate.
The incus then strikes the stapes (stirrup) which is set into motion.
5) These bones act to amplify the motion of the eardrum.
6) The stirrup is in contact with the cochlea, a spiral-shaped canal
filled with fluid. The inside of the cochlea is lined with thousands of
nerve cells with tiny hair-like projections. As the projections sway
back and forth, they send electrical impulses to the brain.
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How sound is reproduced:
1) Sound is recorded by converting sound waves into electronic signals
that can be processed and stored.
2) Sound is then reproduced by converting these electronic signals
back into sound waves.
No matter how sound is stored, it must be converted back into sound
waves by speakers. In a speaker, an electronic signal causes a magnet
to vibrate. The magnet is attached to a membrane. The membrane
vibrates and sends sound wavelengths through the air.
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Large-diameter speakers are better at reproducing lower frequencies
of sound. Smaller-diameter speakers are better at reproducing higher
frequencies of sound.
Singing into a microphone does the opposite - sound waves vibrate a
membrane inside the microphone, the membrane causes a magnet to
vibrate which produces an electronic signal in the microphone wires.
This electronic signal can then be processed and stored.
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Music --> most musical instruments vary pitch by changing the
frequency of standing waves.
Resonance --> the response of a standing wave to another wave of
the same frequency; can be used to amplify sound; is a form of
constructive interference; pianos use the sound board to create
resonance and increase the amplitude
Acoustics --> study of sound transmissions; very
important in designing concert halls to prevent
“dead spots” where sound waves can be cancelled
out by destructive interference.
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