Stationary waves

• • a stationary wave does not appear to travel.
  • The energy is confined to the medium, usually because it reflects back. This means there is no net energy transfer
  • When two waves travelling in the opposite direction meet in the same medium, a new ‘stationary wave’ is temporarily formed
  • The meeting of these two waves is called a superposition.
  • When waves are in superposition, their amplitudes add together to produce a single wave with a larger displacement.
• Interference patterns

  • Constructive interference: the amplitudes of two waves in phase add together to produce a resultant wave of double the amplitude. e.g. when a peak meets a peak
  • Destructive interference: if two waves are 180o out of phase they will interfere ‘destructively’ to produce zero wave displacement. e.g. when a peak meets a trough 
• musical instruments

  • stationary waves form when two progressive waves - which are travelling in opposite directions
  • we can identify nodes and antinodes. The distance between two nodes or two antinodes is equal to half a wavelength.
• • Stationary waves produced on instruments trigger longitudinal sound waves
  • The antinodes on the stationary wave cause the surrounding air particles to vibrate and triggers a longitudinal wave that is detected by our ear drums.
• Resonance

  • Stationary wave patterns most often occur in resonators (e.g seashell), where the wave motion is confined in a fixed space.
  • has boundaries that prevent the wave progressing further and reflect its energy back. This reflection allows the stationary wave to form
  • will interact with and absorb travelling wave energy from outside itself. Small amounts of energy collected over a period of time can be stored up in the stationary wave and build up a much larger amplitude oscillation
• Resonance in wind instruments

  • Blowing into wind instruments like a flute forces the air inside the pipe into stationary waves with harmonics. Players can change the pressure inside the pipe by playing different notes and therefore creating stationary waves with different frequencies inside the pipe of the instrument.  
  • The higher harmonic, the higher the frequency, the higher pitch.
• wave speed under tension

  = mass (kg) /length (m)