Loudspeakers and Microphones

This lesson covers how loudspeakers and microphones work, linking the motor effect and electromagnetic induction — as required by the Edexcel GCSE Physics specification (1PH0), Topic 8: Magnetism and Electromagnetism. You need to understand that these two devices are essentially the same device operating in reverse.

---

How a Loudspeaker Works

A loudspeaker converts an electrical signal (varying current) into sound. It uses the motor effect.

Components of a Loudspeaker

Component	Function
Permanent magnet	Provides a constant magnetic field
Coil of wire (voice coil)	Carries the varying current; placed in the magnetic field
Paper cone (diaphragm)	Attached to the coil; vibrates to produce sound waves

How It Works — Step by Step

1. An alternating current (AC) signal from an amplifier flows through the voice coil.
2. The coil is in the magnetic field of the permanent magnet.
3. By the motor effect, the coil experiences a force (Fleming's left-hand rule gives the direction).
4. Because the current is alternating, the direction of the force reverses with each half cycle.
5. The coil moves back and forth rapidly.
6. The paper cone attached to the coil also moves back and forth.
7. This pushes and pulls the air, creating compressions and rarefactions — producing a sound wave.

Key Relationships

Feature of the Electrical Signal	Effect on the Sound
Frequency of the AC signal	Determines the frequency (pitch) of the sound
Amplitude of the AC signal	Determines the amplitude (volume/loudness) of the sound

• A higher frequency AC signal → the coil vibrates faster → higher pitched sound.
• A larger amplitude AC signal → the coil moves further → louder sound.

Exam Tip: The loudspeaker uses the motor effect — a current in a magnetic field produces a force. The alternating current makes the force alternate direction, so the coil (and cone) vibrate back and forth, producing sound waves. Make sure you can explain each step clearly for a 6-mark question.

---

How a Microphone Works

A microphone converts sound into an electrical signal (varying voltage). It uses electromagnetic induction.

Components of a Microphone

Component	Function
Diaphragm	A thin membrane that vibrates when sound waves hit it
Coil of wire	Attached to the diaphragm; moves back and forth in the magnetic field
Permanent magnet	Provides a constant magnetic field around the coil

How It Works — Step by Step

1. Sound waves hit the diaphragm, causing it to vibrate back and forth.
2. The coil attached to the diaphragm moves back and forth inside the magnetic field.
3. By electromagnetic induction, the movement of the coil in the magnetic field induces a varying voltage across the coil.
4. The pattern of the induced voltage matches the pattern of the original sound wave.
5. This varying voltage is the electrical signal that can be amplified, recorded or transmitted.

Key Relationships

Feature of the Sound	Effect on the Electrical Signal
Frequency of the sound	Determines the frequency of the induced voltage
Amplitude (loudness) of the sound	Determines the amplitude of the induced voltage

• A louder sound → diaphragm vibrates more → coil moves further → larger induced voltage.
• A higher pitched sound → diaphragm vibrates faster → voltage changes faster → higher frequency signal.

Exam Tip: The microphone uses electromagnetic induction — a conductor moving in a magnetic field induces a voltage. Do not confuse this with the motor effect. The motor effect is electricity → movement; electromagnetic induction is movement → electricity.

---

Loudspeakers and Microphones — The Same Device in Reverse

One of the most important concepts in this topic is that loudspeakers and microphones are fundamentally the same device — they just operate in reverse:

Feature	Loudspeaker	Microphone
Input	Electrical signal (AC)	Sound waves
Output	Sound waves	Electrical signal (varying voltage)
Principle	Motor effect	Electromagnetic induction
Energy conversion	Electrical → kinetic → sound	Sound → kinetic → electrical
What happens to the coil	Current causes coil to move	Coil movement induces a voltage
What happens to the cone/diaphragm	Moves to produce sound	Sound causes it to vibrate

The Reversibility Principle

You can actually use a loudspeaker as a (crude) microphone:

• If you speak into a loudspeaker, the sound waves vibrate the cone, which moves the coil in the magnetic field, inducing a small voltage.
• Similarly, if you connect a microphone to an amplifier and play a signal, the coil will vibrate slightly (though it is not designed to produce loud sound).

This reversibility demonstrates the deep connection between the motor effect and electromagnetic induction — they are the same physical phenomenon working in opposite directions.

graph LR
    A["Electrical Signal<br/>(AC current)"] -- "Motor Effect" --> B["Coil vibrates<br/>in magnetic field"]
    B -- "Cone moves air" --> C["Sound Waves"]
    C -- "Sound hits diaphragm" --> D["Coil vibrates<br/>in magnetic field"]
    D -- "Electromagnetic Induction" --> E["Electrical Signal<br/>(induced voltage)"]

    style A fill:#2980b9,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#2980b9,color:#fff

---

Practical Considerations

Loudspeaker Design

• The voice coil must be light so it can respond quickly to changes in the current.
• The permanent magnet must be strong to produce a large force on the coil.
• The cone must be light and stiff to vibrate efficiently and produce clear sound.
• Larger cones are better for low-frequency sounds (bass); smaller cones are better for high-frequency sounds (treble).

Microphone Design