P5.1 - Wave Behaviour

• Every time you speak, vibrations in your vocal chords produce sound waves.
• A wave is an oscillation that transfers energy.
• Electromagnetic waves are transverse waves that travel through a vacuum of air, transferring energy from a source to an absorber.
• Mechanical waves are a disturbance that travels through a medium, causing the particles of the medium to vibrate.
• Mechanical waves are longitudinal.
• Electromagnetic waves do not need a medium to travel through.
• A longitudinal wave is a wave where the particles of the medium vibrate parallel to the direction of wave propagation.
• Longitudinal waves consist of rarefactions and compressions.

  
• A transverse wave is a wave where the direction of vibration is at right angles to the direction the wave is travelling.

  
• If you make a transverse wave on a spring, the individual coils move up and down but the energy is transferred horizontally.
• Amplitude - difference from middle to the top (crest) or bottom (trough) of a wave.

  • Symbol - A
  • Unit - depends on the wave (metres or volts)
• Wavelength - distance from one point of a wave wave to the same point of another wave.

  • Symbol - (lambda)
  • Unit - metres.
• Frequency - number of waves (oscillations) per second.

  • Symbol - F
  • Unit - Hertz
• Time period - time taken for one wave to pass a given point.

  • Symbol - T
  • Unit - seconds
• A time trace shows how displacement varies with time at a particular position.

  
• Waves have a wave velocity - speed at which a wave travels through a medium (distance covered per unit of time).
• wave velocity = frequency * wavelength
• To measure velocity of ripples:

  1. Set up the ripple tank with 5 cm of water.
  2. Adjust the height of the wooden rod so that it touches the surface of the water.
  3. Switch on the lamp and motor and adjust until low frequency waves can be observed.
  4. Measure the length of a number of waves then divide by the number of waves to record wavelength. It may be more practical to take a photo of the card with the ruler and take measurements from the still picture.
  5. Count the number of waves passing a point in 10 seconds, then divide by 10 to find frequency.
  6. Calculate speed of waves with: wave speed = f*w

  
• To measure velocity of sound:

  1. Stand a known distance away from a wall.
  2. Clap and time how long it takes until you hear an echo (reflection).
  3. Find velocity of sound using distance velocity and time equation.

  Alternatively, connect a pair of microphones a certain distance apart to an oscilloscope.

  • This method is used to measure velocity of sound in a liquid or a solid.
  • Sound is a longitudinal wave, the trace on the oscilloscope's screen shows the variation in pressure, not the sound wave itself.
• The velocity of sound can vary with temperature and pressure.
• Refraction is when a wave's velocity and direction changes when it travels from one medium to another.
• Suppose a sound wave travels into a region where it travels faster.

  • Direction of sound wave changes to move away from the normal (line at 90 degrees to the surface).
  • The wavelength and velocity increases, frequency stays the same.
• A sound wave that its the boundary at 0 degrees to the normal will speed up but not change direction.
• There are three things that can happen when a wave hits the boundary between two media:

  • reflected (echo).
  • transmitted (possibly refracted).
  • absorbed.

  What happens to the sound depends on the density of the regions either side of the boundary. If the densities are very different, then more of the sound will be reflected.

  
• Ultrasound has a very small wavelength, so it can be focused into a beam.
• How to use ultrasound:

  1. Transmitter beams ultrasound into the mother.
  2. Waves reflect from the different boundaries.
  3. Machine calculates distance using time and velocity, and uses those distances to produce an image.

  • Used to see kidney stones, babies, and monitor bloodflow.
  • The same method can be used to find other distances, like water depth:
  • Transmitter sends out a pulse and calculates distance based on when the echo is picked up by the reciever.
  • Echo-sounding and sonar use this method. (sonar in submarines to find distances using echo and speed of sound in water).
• When sound is absorbed by a wall, it makes the particles vibrate and the wall gets a bit hotter.
• The ear can detect, amplify, and convert sound to an electrical signal:

  1. The outer ear (pinna and auditory canal) gathers the sound wave and directs it to the ear drum, which vibrates.
  2. As the ear drum vibrates, it makes the ossicles vibrate. They act like small levers to amplify the vibration and pass it to the inner ear through the oval window.
  3. The cochlea contains fluid which transmits the movement of the oval window to small hairs inside it.
  4. These hairs are attached to sound-detecting cells that release chemical substances, which make nerves send down the auditory nerve to your brain. Your brain processes the signal and you hear a sound.

  
• The natural frequency is the specific frequency at which a system or object tends to vibrate when disturbed.
• Resonance is when a vibration is applied at the natural frequency of a mass, and it leads to a vibration with a very big amplitude.
• The hairs inside the cochlea have different lengths and resonate at different frequencies of sound, this is how your ear puts together an electrical signal that contains all the different frequencies in the sound wave.
• The range of frequencies that you can hear depend on the range of lengths of hairs in your cochlea.
• As you get older, you lose shorter hairs, this means you find it harder to hear higher frequencies.