Explain how a moving coil loudspeaker produces sound
Trace a changing electrical signal through magnetic force and cone vibration into sound waves.
Core and Extended; Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Explain force from magnetic interaction.
- Distinguish sound frequency and amplitude.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What type of wave is sound in air?
What you will learn
- Trace a changing electrical signal through magnetic force and cone vibration into sound waves.
- Explain speaker sound through changing coil current and cone motion, with energy and response limits explicit.
A changing coil current produces changing force
A simple moving-coil loudspeaker has a coil attached to a light cone and placed in the field of a permanent magnet. Current in the coil interacts with the magnetic field and produces a force. Reversing the current reverses the force in the fixed arrangement, so an alternating signal drives forward and backward motion.
The attached cone makes nearby air vibrate, creating compressions and rarefactions that travel as sound. The magnet establishes a field; electrical energy supplied by the signal circuit drives the moving system and other losses. Air is not carried all the way from the speaker to the listener.
Relate the signal to a real response
Within a suitable operating range, signal frequency determines the repetition rate of cone vibration and hence the sound’s frequency. A larger suitable signal can increase vibration amplitude and perceived loudness, but mechanical resonance, response range, heating and distortion prevent an unlimited exact relationship.
An unchanged d.c. input gives a force in one direction and can produce an initial transient or displaced position; it does not sustain alternating cone vibration in this simple model. Examples include speech reproduction, music playback and audible alarms. This lesson describes magnetic action, not a particular speaker’s safe power rating.
Pause and explain
What does unchanged d.c. coil current do in the simple fixed-field speaker model?
Worked example
Explain why an alternating coil signal can produce sound but a steady d.c. force does not sustain the same vibration.
Show the worked solution
- Alternating current reverses force in the fixed field.
- The cone moves repeatedly and drives air vibrations.
- Steady current gives a fixed-direction force after its transient.
Answer A changing signal sustains vibration; steady d.c. does not sustain that alternating motion.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the energy and action chain from alternating signal to speaker sound.
Give me a hint
Connect electrical input, magnetic interaction and cone motion.
Compare my reasoning
- The signal drives changing coil current.
- Magnetic force drives cone vibration.
- The cone creates travelling air pressure disturbances.
Electrical input → magnetic driving force → cone vibration → sound.
Look for these in your work
- Name the driving field.
- Trace motion into the medium.
Compare increasing signal frequency with increasing suitable signal amplitude.
Give me a hint
Separate repetition rate from size of vibration.
Compare my reasoning
- Greater signal frequency gives faster vibration repetition within response range.
- Greater suitable amplitude can increase cone vibration size.
- Frequency relates to pitch; amplitude relates to loudness with response limits.
Frequency changes pitch; suitable amplitude changes loudness, within the model’s operating range.
Look for these in your work
- Separate the two properties.
- Retain response limits.
A report says steady d.c. creates continuous alternating speaker sound and the permanent magnet supplies all delivered energy. Correct it.
Give me a hint
Identify the changing force requirement and circuit input.
Compare my reasoning
- Steady current gives fixed-direction force after a transient.
- Repeated vibration needs a varying driving signal.
- The signal circuit supplies delivered electrical energy.
Steady d.c. does not sustain alternating vibration; the circuit, not a free permanent-magnet source, supplies energy.
Look for these in your work
- Correct motion reasoning.
- Correct the energy source.
Common mistakes
- The cone vibrates; it does not transport air to the listener.
- The circuit supplies energy.
- Amplitude and frequency have different roles.
What can you explain now?
What converts the coil motion into sound in this model?
Compare with the explanation
The attached cone drives air vibrations.
Repeated cone movement produces pressure disturbances that travel through air.
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Make it stick
Explain speaker sound through changing coil current and cone motion, with energy and response limits explicit.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.