A loudspeaker is connected to an audio amplifier and used to play music. Explain how the loudspeaker converts the oscillating electrical signal from the amplifier into sound waves that can be heard.

Eduqas GCSE Physics — 8.4 Microphones, speakers, oscillating currents in detection · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The audio amplifier produces an oscillating electrical current that flows through the loudspeaker’s voice coil. The coil lies in a permanent magnetic field, so the changing current creates a changing magnetic force on the coil (Lorentz force). This force makes the coil, and the attached cone or diaphragm, move back and forth. The moving cone pushes on the surrounding air, creating alternating compressions and rarefactions that propagate as sound waves which can be heard.

Examiner tips

  • Use the term ‘voice coil’ and ‘permanent magnetic field’.
  • Explain the Lorentz force (changing current → changing magnetic force).
  • Show the chain: current → force → coil movement → cone vibration → air pressure waves.
  • Use the word ‘oscillating’ to link the electrical signal to the sound.

Mark scheme (4 marks)

  1. The oscillating electrical current passes through the coil (voice coil) in the loudspeaker
  2. The coil is situated in a magnetic field, so the changing current causes a force on the coil / the coil experiences a changing magnetic force
  3. The coil (and attached cone/diaphragm) vibrates / moves back and forth
  4. The vibrating cone creates compressions and rarefactions in the surrounding air, producing sound waves

Key terms in this question

oscillating electrical signal · sound waves

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