Sound

• Sound


  Waves of compression and rarefaction in which the human ear is sensitive
• Transverse waves


  • Vibrations are at right angles to the direction the wave moves, can travel in vacuum and all media, speed is very high
  • Example: Light wave
• Longitudinal waves


  • Vibrations are parallel to the direction the wave moves, cannot travel in vacuum, need a medium to travel, speed is less
  • Example: Sound wave
• Compression


  Molecules moving closer together in a sound wave (high density region)
• Rarefaction


  Molecules moving far apart from each other in a sound wave (low density region)
• In sound waves molecules move back and forth
• Speed of sound in air= 330m/s
• Speed of sound in water = 1500m/s
• Speed of sound in steel = 5000m/s
• Sound


  Produced when a vibration causes pressure variations in the medium
• Propagation of sound


  Medium - Material that sound waves can travel through, can travel through solids, liquids and gases but not through a vacuum
• Loudness or intensity


  Description of how high or low the sound seems to a person, determined mainly by the amplitude of the sound wave
• Decibel (dB)


  Unit used to measure sound intensity or loudness
• The human ear relates amplitude to loudness and frequency to pitch
• Speed of Sound


  • Depends on type of medium (travels better through liquids and solids and gas, can't travel through a vacuum)
  • Depends on temperature of medium (travels faster at higher temperature)
• Echo


  Reflected sound, what you heard shortly after the original
• Diffraction of Sound


  Sound waves bend or diffract around corners or barriers like doors and walls
• Noise is unwanted sound and unpleasant sound which will not let us concentrate, unit of loudness is decibels
• Humans can hear sounds between about 20 Hz and 20 000 Hz in frequency (although this range decreases with age)
• Ultrasound can be used

  • To construct images of a foetus in the womb
  • In industry, to check for cracks inside metal objects
  • In industry, to generate images beneath surfaces
• Sonar system on a boat

  1. Sends an ultrasound pulse towards the seabed

  2. Pulse is reflected

  3. Pulse is detected 0.1 s later by the system
• Depth of water


  Speed of sound in water is 1,480 m/s, time taken for pulse to travel to seabed and back is 0.1 s, depth = (speed x time)/2 = 74 m
• Ultrasonic device to measure room length

  1. Device shows distance from one wall to opposite wall is 8.25 m

  2. Speed of ultrasound in air is 330 m/s

  3. Time taken for ultrasound to travel to far wall and back = distance/speed = 0.050 s
• Boy blows whistle


  1. Boy stands 83 m in front of high wall

  2. Time interval when he hears echo = 2 x distance/speed of sound = 0.5 s