Loudspeakers & Headphones (Edexcel GCSE Physics)

Higher Tier Only

Microphone

• A moving coil microphone also work using the principles of electromagnetic induction
• The convert the pressure variations in sound waves into variations in current in electrical circuits

A moving coil microphone

• When sound waves reach the microphone, the pressure variations cause the diaphragm to vibrate
  • This in turn causes the coil to move back and forth, through the magnetic field

• As it does so, the coil cuts through the field lines, inducing an alternating potential difference in the coil and therefore an alternating current

Loudspeakers & Headphones

• Loudspeakers and headphones convert electrical signals into sound
  • They work due to the motor effect

• They work in the opposite way to microphones
• A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet

Diagram showing a cross-section of a loudspeaker

• An alternating current passes through the coil of the loudspeaker
  • This creates a changing magnetic field around the coil

• As the current is constantly changing direction, the direction of the magnetic field will be constantly changing
• The magnetic field produced around the coil interacts with the field from the permanent magnet
• The interacting magnetic fields will exert a force on the coil
  • The direction of the force at any instant can be determined using Fleming’s left-hand rule

• As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction
  • This makes the coil oscillate

• The oscillating coil causes the speaker cone to oscillate
  • This makes the air oscillate, creating sound waves