Loudspeakers and Microphones

• Loudspeakers


  Devices that convert electrical signals into sound waves
• Microphones


  Devices that convert sound waves into electrical signals
• Components inside headphones


  • Tiny crinkly paper cone
• Loudspeakers work because of the motor effect
• How loudspeakers work


  1. An alternating current is sent through a coil of wire attached to the base of a paper cone

  2. The coil surrounds one pole of a permanent magnet, and is surrounded by the other pole, so the current causes a force on the coil (which causes the cone to move)

  3. When the current reverses, the force acts in the opposite direction, which causes the cone to move in the opposite direction too

  4. Variations in the current make the cone vibrate, which makes the air around the cone vibrate and creates the variations in pressure that cause a sound wave

  5. The frequency of the sound wave is the same as the frequency of the ac, so by controlling the frequency of the ac you can alter the sound wave produced
• Loudspeakers and headphones


  • Both use electromagnets
• How microphones work


  1. Sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet

  2. This causes the coil of wire to move in the magnetic field, which generates a current

  3. The movement of the coil (and so the generated current) depends on the properties of the sound wave (louder sounds make the diaphragm move further)

  4. This is how microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit