Topic 4: Waves

• Frequency (Hertz/Hz) is the number of waves passing a point per second
• Time Period (s) is the time it takes the wave to travel one wavelength
• Transverse - oscillations are perpendicular to the direction of energy transfer (light)
• Longitudinal - oscillations are parallel to the direction of energy transfer (sound)
• When a wave meets a boundary it may:

  • reflect
  • refract
  • be absorbed
  • be transmitted
• Ultrasound is a sound that vibrates at frequencies above 20,000Hz and it can be partially reflected at a boundary (sonar, pregnancy scans, vibrate placc/kidney stones to break apart)
• An ultrasound machine for pregnancy scans sends ultrasound waves through a device that transmits and receives:

  • check if foetus is healthy
  • timings and distributions of ultrasound echoes are processed to generate an image
• Infrasound is a sound that vibrates at frequencies below 20Hz (investigating Earth's internal structure, monitoring earthquakes and volcanoes)

  Two types of waves infrasound detects in Earth are Primary Waves (P-Waves) and Secondary Waves (S-Waves)
• S-Waves in Earth:

  • transverse
  • slower than P-waves
  • travel through solids
  • outer core is a liquid so the shadow zone is about half the world

  
• P-Waves in Earth:

  • longitudinal
  • travel fast
  • travel through solids and liquids
  • waves refract a lot in the mantle as density varies
  • shadow zones on opposite sides and identical in length as any vibrations that reach the outer core are refracted in a way to make it impossible to reach the inner core

  
• When sound is absorbed, reflected, transmitted or refracted the frequency doesn't change when passing through one medium to another:

  • sound is quicker in solids and the vibrations from the molecules are passed on faster/easier
  • human ears can detect sound between 20-20,000Hz
  • certain parts of the ear membrane vibrate best at different frequencies because of the liquid and thickness inside the cochlea
• The Ear:

  1. Sound waves enter the ear canal
  2. Vibrations cause eardrum (thin membrane) to vibrate
  3. 3 tiny bones (malleus, incus, stapes) amplify vibrations
  4. Cochlea has coiled up membrane with liquid to pass on vibrations (base detects high frequencies, apex detects low frequencies)
  5. Tiny hairs in cochlea create electrical signals (impulses) based on vibrations
  6. Neurons in auditory nerve transfer impulses to the brain
• Wavelength is shorter after crossing a medium and the frequency stays the same.

  The velocity changing causes refraction
• Investigating Waves in a Solid Practical:

  1. Suspend metal rod using rubber bands horizontally and hit one end with a hammer
  2. Hold a frequency detector on the opposite end and note down the peak frequency
  3. Measure the length of rod as wavelength will be double the measurement
  4. Repeat experiment with another metal type to compare results
• Investigating Waves in a Liquid Practical:

  1. Fasten ruler to one of the long sides to measure wavelength
  2. Vary current until there are at least 2 waves always (wavelength half the tank)
  3. Count the amount of waves in 10 seconds
  4. Use markings on the ruler to estimate wavelength and use a camera for accuracy - note down findings
  5. Mark two points and use a stopwatch to time how long it takes for a wave to travel from one point to another