IGCSE Physics / Electric motors and electromagnetic induction
The loudspeaker revision notes
Edexcel International GCSE Physics (4PH1), chapter 21: Electric motors and electromagnetic induction, specification point 6.12. Free lesson.
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A moving-coil loudspeaker uses the motor effect to turn an alternating current into vibrations in the air. The current’s frequency sets the pitch, while its size sets the loudness.
What parts make up a loudspeaker
A loudspeaker uses the motor effect to create back-and-forth movement. A coil of wire sits in the magnetic field of a permanent magnet and is fixed to a paper or thin plastic cone. Wires connect the coil to an alternating current supply. The permanent magnet provides a steady magnetic field. When the coil moves, the attached cone moves with it.
How current becomes sound
An alternating current repeatedly changes direction. Since the coil is in a magnetic field, the current produces a force on it. When the current reverses, the force reverses too. The coil and cone therefore move in and out, making the air vibrate as a sound wave. A steady direct current would push the cone out once and hold it there, rather than making it vibrate.
How the current controls pitch and loudness
The frequency of the current sets the pitch. A higher frequency makes the cone vibrate more times each second, producing a higher-frequency sound. The size of the current sets the loudness: a bigger current produces a bigger force, so the cone moves further in and out. This creates a sound wave with a bigger amplitude. If the current’s frequency doubles, the force reverses twice as often. The cone vibrates twice as many times each second, so the pitch gets higher.
How to explain loudspeaker operation
For a full explanation, link the current to the force, then to the reversing movement and the sound. Each part explains what causes the next.