Vibrations

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Vibrations Waves Sound

• a periodic motion of the particles of an
  elastic body or medium in alternately opposite
  directions from the position of equilibrium when
  that equilibrium has been disturbed
• When things vibrate, they move the air and create
  sound waves
• Music, talking, sirens, etc.

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Vibrations Waves
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Vibrations Waves

• Wavelength - Distance between peaks
• Amplitude - Max Height above resting spot
• Frequency - Number of Bounces/second
• Period - Time of a complete vibration
• Period 1 / Frequency
• Frequency 1 / Period

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Vibrations Waves

• Water Waves - move stick up and down in a pond
• Sound Waves - move air back and forth
• Electromagnetic Waves - turn electric and
  magnetic fields on and off
• Remember, changing magnetic field produces
  electric field and vice versa

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Wave Motion

• Consider our stick moving up and down in the
  water
• Create circular waves that move out from the
  disturbance
• Water moves up and down, the wave moves outward
  away from the disturbance
• Waves carry energy of the moving stick
• A bug in the water will move and and down

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Wave Speed
Wavelength is distance between peaks Frequency is
number of peaks per second going past a point in
the water Frequency depends on wavelength and how
fast the wave moves!!!
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Longitudinal or Transverse?
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Sound Waves
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Sound Waves
Molecules in the air vibrate about some average
position creating the compressions and
rarefactions. We call the frequency of sound the
pitch.
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Sound Waves
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Speed of Sound

• Depends on the material of the vibrating medium
• Sound can vibrate water, wood (speaker
  enclosures, pianos), metal, plastic, etc.
• Sound speed in dry air is 330 meters/second at 0o
  C
• Faster in warm air, slower in cold
• Water 4 times faster, steel 15 times faster

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Wave Reflection

• When a sound wave reflects from a surface we
  generate an echo
• Wave reflection from surfaces depends on the
  characteristics of the surface
• Smooth hard surfaces reflect best
• Rough soft surfaces reflect poorly
• Energy not reflected is absorbed or transmitted
  through the material

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Wave Reflection

• Think of arrows pointing in the direction of the
  wave motion
• We can trace the path of these arrows

Angles Equal
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Wave Reflection
Acoustics of room design is very interesting.
Need some reflections to liven the room. Too
many reflections and the sound gets mushy. Look
in a concert hall or auditorium to see the
different sound treatments
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Wave Refraction

• If there is a change in the characteristics of a
  medium, waves are bent
• This occurs because different parts of the wave
  front travel at different speeds
• Think of a marching around a curved track
• The inside people have to move more slowly than
  the outside people to keep the lines straight

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Wave Refraction
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Wave Reflection Refraction

• The combination of reflection and refraction
  enables imaging
• Ultrasonic medical imaging
• Naval SONAR for detecting submarines
• Bats catch mosquitoes
• Catch discussion of dolphins on p. 261

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Natural Frequencies

• Objects have natural frequencies based on their
  size and structure
• Guitar strings are an example
• Timpani heads
• Air columns

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Forced Vibrations

• Can externally impose a vibration on an object
• Guitars and violins and pianos
• Set the wood into motion at the frequency of the
  string
• This provides a larger surface to interact with
  the air
• Harp vs. Piano

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Resonance

• When the forced vibration matches a natural
  frequency we get a resonance condition
• Think about a swing on a playground
• You go high when you pump the swing at its
  natural vibration frequency
• Sympathetic vibrations in tuning forks
• Famous Tacoma Narrows bridge collapse