Sound Waves Part II

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Sound Waves Part II
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Compressional Waves

• Compressional waves do not have crests and
  troughs as the waves dont go up and down
• The crests are similar to the compressions of
  sound waves
• The troughs are similar to the rarefactions of
  sound waves

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• A wavelength in a compressional wave includes one
  compression and one rarefaction

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Compression

• In the compression, the medium particles (air for
  us) get pushed together
• This makes them closer together and this push
  moves forward through the air and to our ears

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Rarefaction

• In the rarefaction, the particles of the medium
  are pulled further apart, creating a short hole
  in the medium
• This hole follows along behind the compression

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Hearing

• The sequence of compression and rarefaction
  carries the sound vibrations through a medium
• Repeating series of waves make the sounds we hear
• Demo

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Interference

• Waves can interfere with each other, creating a
  new wave pattern where they overlap
• If you watch the wake from your boat on a wavy
  day on the lake, you can see interference

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• This interference can be
• constructive the two waves add together to make
  a larger wave
• destructive the two waves cancel each other, in
  part or whole, and create a shorter wave
• Interference Demo

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Sound Interference

• It is easy to see transverse waves interfering
• In the case of sound waves, both constructive and
  destructive interference also take place
• Constructive interference will make a sound
  louder
• Destructive interference will make a sound quieter

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Seismic Waves

• When portions of the earths crust slip along or
  push against each other, there will be an
  earthquake
• The location that the earthquake happens, usually
  deep in the crust, is the focus
• The location on the surface above the focus is
  known as the epicenter
• Waves travel out from the focus carrying the
  vibrations we feel in an earthquake
• video

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Body Waves

• The two waves that travel through the body of
  the earth are P and S waves
• The P waves are compressional waves with a speed
  of 5-7 km/s
• The S waves are transverse waves with a speed of
  3-4 km/s
• Measuring the time between the P and S waves
  helps determine how far away the earthquake was
  (like watching lightning and listening for
  thunder)

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• P waves can travel through solids, liquids, and
  gases, so they can travel throughout the earth
• S waves can not travel through liquids, so oceans
  will stop their travel, as will the liquid outer
  core of the earth

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Surface Waves

• Surface waves do almost all of the damage caused
  by an earthquake
• One form of surface wave shakes the surface
  side-to-side, while another shakes the surface up
  and down (like shaking a rug)
• Surface waves travel at 2-4 km/s

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Additional Threats

• If an earthquake happens underwater, and it
  causes part of the crust to either lift or drop
  suddenly, the earthquake can cause a tsunami
• These tidal waves can travel at speeds of 500
  mph in the open ocean and reach heights of 100 m
  when reaching shallow water of a shoreline