Mechanical Waves and Sound

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Mechanical Waves and Sound

• Ch 17

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There are Different Types of Waves

• The two basic types of waves are
• Mechanical waves examples-sound, water waves, a
  pulse traveling on a spring, earthquakes, a
  people wave in a football stadium, etc.
• Electromagnetic waves examples-visible light,
  radiowaves, infrared waves, etc.
• For now, were going to focus on Mechanical Waves

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What are mechanical waves?

• A mechanical wave is a disturbance in matter that
  carries energy from one place to another.
• The material through which a wave travels is
  called a medium.
• Mechanical waves require a medium to travel
  through. Solids, liquids, and gases all can act
  as mediums. They can not travel through a vacuum
  (like space)

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• A mechanical wave is created when a source of
  energy causes a vibration to travel through a
  medium.
• In a wave pool, the waves carry energy across the
  pool. You can see the effects of a wave's energy
  when the wave lifts people in the water.

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Types of Mechanical Waves

• There are three main types of mechanical waves
• transverse waves
• longitudinal waves
• surface waves.
• Mechanical waves are classified into these 3
  types by the way they move through a medium.

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Transverse Waves

• When you shake one end of a rope up and down, the
  vibration causes a wave.
• Vibrates in a direction perpendicular to the
  direction the wave travels.

Direction wave travels
Direction wave vibrates
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Longitudinal/Compression Waves

• A longitudinal wave is a wave in which the
  vibration of the medium is parallel to the
  direction the wave travels.

Compression
An area where the particles are spaced close
together is a compression. An area where the
particles are spread out is a rarefaction.
Direction of wave and vibration
Rarefaction
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Surface Waves

• A surface wave is a wave that travels along a
  surface separating two media.
• Ocean waves are the most familiar kind of surface
  waves.

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Properties of a Wave

• Amplitude (A) Maximum displacement of particle
  of the medium from its equilibrium point. The
  bigger the amplitude, the more energy the wave
  carries. Measured from crest/peak to rest
  position OR from rest position to trough
• Wavelength (?) For a transverse wave, wavelength
  is measured between adjacent crests or between
  adjacent troughs. For a longitudinal wave,
  wavelength is the distance between adjacent
  compressions or rarefactions.

Wavelength
Crest / Peak
Rest/equilibrium position
Amplitude
Trough
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Amplitude

• The more energy a wave has, the greater its
  amplitude.

Low amplitude low energy
High amplitude high energy
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More Wave Terms

• Period (T) Time it takes consecutive crests (or
  troughs) to pass a given point, i.e., the time
  required for one full cycle of the wave to pass
  by. Calculated by T 1 / f.
• Frequency (f ) The number of cycles passing by
  in a given time. The SI unit for frequency is
  the Hertz (Hz), which is one cycle per second.

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Frequency
Frequency 1.0 hertzOne cycle per second

• A wave vibrating at one cycle per second has a
  frequency of 1.0 Hz.
• A wave vibrating at two cycles per second has a
  frequency of 2.0 Hz.

Frequency 2.0 hertzTwo cycles per second
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• Wave speed (v) How fast the wave is moving.
  Speed depends on the medium. Calculated by v ?
  f.
• The speed of a wave can change if it enters a new
  medium, or if variables such as pressure and
  temperature change.

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Wave Terms and Concepts

• 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.

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• Refraction is the bending of a wave as it enters
  a new medium at an angle.
• Refraction occurs because one side of the wave
  moves more slowly than the other side.
• Example A lawnmower turns when it is pushed at
  an angle from the grass onto the gravel.
• The wheel on the gravel slows down, but the other
  wheel is still moving at a faster speed on the
  grass.

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• Diffraction is the 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.
• This wave diffracts, or spreads out, after it
  passes through a narrow opening.
• Diffraction also occurs when a wave encounters
  an obstacle.

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• Interference occurs when two or more waves
  overlap and combine together.
• Two types of interference are constructive
  interference and destructive interference.
• When waves collide, they can occupy the same
  region of space and then continue on.
• Constructive interference occurs when two or more
  waves combine to produce a wave with a larger
  displacement.
• Destructive interference occurs when two or more
  waves combine to produce a wave with a smaller
  displacement.

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Constructive Interference

• Two waves that are in phase (have equal
  frequencies with crests and troughs that line up)
  
• When these waves meet and overlap the result is a
  wave with an increased amplitude.

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Destructive Interference

• Two waves that are out of phase (do not have
  equal frequencies with crests and troughs that
  line up)
• When a crest meets a trough, the result is a wave
  with a reduced amplitude.

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• A standing wave is a wave that appears to stay in
  one placeit does not seem to move through the
  medium.
• A node is a point on a standing wave that has no
  displacement from the rest position. At the
  nodes, there is complete destructive interference
  between the incoming and reflected waves.
• An antinode is a point where a crest or trough
  occurs midway between two nodes.

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Sound Waves
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Sound is a longitudinal or compression wave.
Sometimes called a pressure wave.
If a sound wave is moving from left to right
through air, then particles of air will be
displaced both rightward and leftward as the
energy of the sound wave passes through it.
The motion of the particles are parallel to the
direction of the energy transport longitudinal
wave.
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Speed of Sound

• It takes time for sound to travel from place to
  place.
• In general, sound waves travel fastest in solids,
  slower in liquids, and slowest in gases.
• Particles in a solid tend to be closer together
  than particles in a liquid or a gas.

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Sound Intensity

• Intensity is the rate at which a waves energy
  flows through a given area.
• Sound intensity depends on both the waves
  amplitude and the distance from the sound source.
• The decibel (dB) is a unit that compares the
  intensity of different sounds.
• For every 10-decibel
• increase, the sound
• intensity increases tenfold.

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• Lengthy exposure to sounds more intense than 90
  decibels can cause hearing damage.

Did you know that MP3 players such as iPods can
reach the 120-decibel level when they are at
maximum volume?
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Sound Loudness

• Loudness is a physical response to the intensity
  of sound, modified by physical factors.
• The loudness depends on sound intensity.
• Loudness also depends on factors such as the
  health of your ears and how your brain interprets
  sound waves.

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Frequency Pitch

• The frequency of a sound wave depends on how fast
  the source of the sound is vibrating.
• Pitch is the frequency of a sound as you perceive
  it.
• High-frequency sounds have a high pitch, and
  low-frequency sounds have a low pitch.
• Pitch also depends on other factors such as your
  age and the health of your ears.

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The Doppler Effect

• As a source of sound approaches, an observer
  hears a higher frequency. When the sound source
  moves away, the observer hears a lower frequency.
  
• The Doppler effect is a change in sound frequency
  caused by motion of the sound source, motion of
  the listener, or both.

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