Loudspeakers and Microphones
Understand Loudspeakers and Microphones for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: loudspeakers use the motor effect to turn electrical signals into sound, while microphones use the generator effect to do the reverse.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
Loudspeakers and headphones use the motor effect to convert variations in current into the pressure variations of sound waves. This is a Triple-only and Higher-only topic.
An alternating current passes through a coil attached to a cone or diaphragm, which sits in a magnetic field. The force on the coil changes direction as the current alternates, so the cone vibrates back and forth. The vibrating cone produces pressure variations in the air, which are sound waves.
The frequency of the sound matches the frequency of the alternating current, and the amplitude of the sound depends on the size of the current. A microphone works in reverse, using the generator effect to convert sound into an electrical signal.
Revision notes
How a loudspeaker works
An alternating current passes through a coil attached to a cone, which sits in a magnetic field.
The motor effect produces a force on the coil, and because the current alternates the force reverses direction repeatedly. The cone therefore vibrates back and forth.
Producing sound
The vibrating cone causes pressure variations in the surrounding air.
These pressure variations are sound waves. The frequency of the sound is the same as the frequency of the alternating current, and the amplitude depends on the size of the current.
Microphones work in reverse
In a microphone, sound waves cause a diaphragm attached to a coil to vibrate in a magnetic field.
This induces a potential difference in the coil by the generator effect, producing an electrical signal that varies with the sound. It is the loudspeaker process reversed.
Key points
- Loudspeakers use the motor effect.
- An alternating current passes through a coil in a field.
- The force reverses as the current alternates.
- The cone vibrates and produces pressure variations.
- Sound frequency matches the current frequency.
- Microphones work in reverse using the generator effect.
Worked examples
Example 1
Explain how a loudspeaker produces sound. [4 marks]
Model answer
An alternating current passes through a coil placed in a magnetic field (1 mark). The motor effect produces a force on the coil, which reverses direction as the current alternates (1 mark). The coil and the attached cone therefore vibrate back and forth (1 mark), causing pressure variations in the air which are heard as sound (1 mark).
Example 2
State what determines the frequency of the sound produced by a loudspeaker. [1 mark]
Model answer
The frequency of the alternating current passing through the coil (1 mark).
Example 3
Explain how a microphone differs from a loudspeaker. [2 marks]
Model answer
A microphone uses the generator effect rather than the motor effect (1 mark): sound waves make a diaphragm and coil vibrate in a magnetic field, inducing a potential difference that varies with the sound (1 mark).
Common mistakes
Saying a direct current is used.
An alternating current is needed so the force reverses and the cone vibrates.
Confusing loudspeaker and microphone.
The loudspeaker uses the motor effect; the microphone uses the generator effect.
Not linking the cone to pressure variations.
The vibrating cone creates the pressure variations that are sound.
Saying amplitude depends on frequency.
Amplitude depends on the size of the current; frequency depends on its frequency.
Exam tips
- Describe the chain: current, force, vibration, pressure variations.
- Say alternating current explicitly.
- Keep loudspeaker and microphone the right way round.
- Remember this is Triple-only AND Higher-only.
Key terms
- Loudspeaker
- A device converting an electrical signal into sound.
- Diaphragm
- The vibrating surface in a microphone or loudspeaker.
- Pressure variation
- The changes in air pressure that constitute sound.
- Generator effect
- Inducing a potential difference by movement in a field.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.