Sound

• Sound longitudinal wave


  Produced by vibrating objects in air/liquids/solids
• Longitudinal


  Parallel to vibrating particles (direction)
• Compressions


  Squashed particles (higher pressure)
• Rarefactions


  Spread out particles (lower pressure)
• Through air a sound wave consists of a series of compressions and rarefactions
• Compression


  A region of slightly higher pressure where the air molecules are closer together than usual
• Rarefaction


  The opposite of compression
• Wavelength of the sound wave


  Equal to the distance between the centres of two successive compressions
• Sound travels faster in solids because particles are closer together
• In a vacuum sound cannot travel because there are no particles to vibrate
• The bell jar experiment shows that sound needs a material medium for transmission
• As the air pressure inside the bell jar is reduced


  The loudness of the sound heard outside decreases
• The bell can still be seen to be working normally
• Measuring the speed of sound (Method 1)

  1. Time how long the echo takes to come back

  2. 2x distance to wall

  3. Time for echo
• Speed of sound in air = 340 m/s
• Measuring the speed of sound in a laboratory (Method 1)

  1. Use echoes

  2. Time how long the echo takes

  3. Calculate speed
• The speed of sound in air is 130 m/s
• The problem with measuring the speed in a laboratory is that it shows how the signal is processed
• Two microphones are placed short distances apart
• The oscilloscope can measure the time it takes for the sound to reach each microphone
• When calculating the speed of sound by reflection, remember that the sound travels there and back
• Measuring the speed of sound outside (Method 1)

  1. Stand 40m in front of a tall building

  2. Clap hands or bang blocks

  3. Time how long it takes to hear echoes
• The distance traveled is 80m when sound travels there and back
• Humans can hear frequencies from 20Hz to 20,000Hz
• Animals that can hear higher frequencies


  • Dogs (40,000Hz - 60,000Hz)
  • Bats and Dolphins (up to 100,000Hz)
• Age and damage reduce the upper limit of hearing
• Ultrasound


  Frequency above 20,000Hz, too high to be heard by humans
• Infrasound


  Frequency below 20,000Hz, too low to be heard by humans
• Loudness


  Increases with the amplitude of the wave
• Pitch


  Increases with the frequency
• A low pitch corresponds to a low frequency and a high pitch corresponds to a high frequency
• Measuring frequency using an oscilloscope


  1. Find the frequency by the time taken between peaks

  2. This is equal to the time period

  3. Frequency = 1/Time Period
• Frequency = 1/T
• If T = 4 squares and 1 square = 0.25ms, then T = 1ms
• f = 1/T = 1000Hz
• Doppler Effect


  The change in frequency of a wave as noted by an observer when there is relative motion between the source and the observer
• If the source of the wave moves towards the observer

  The pitch becomes higher
• If the source of the wave moves away from the observer

  The pitch becomes lower