A LOUDSPEAKER converts electrical energy (alternating current) into sound energy using the MOTOR EFFECT.
COMPONENTS:
PERMANENT MAGNET: provides a strong, constant magnetic field.
VOICE COIL: a coil of wire attached to the cone.
CONE (DIAPHRAGM): large paper or plastic surface that moves air to create sound.
OPERATION:
1. Alternating current (from amplifier) flows through the voice coil.
2. Voice coil is in the field of the permanent magnet → motor effect.
3. AC reverses direction → force reverses direction → coil moves back and forth.
4. Coil attached to cone → cone vibrates in and out.
5. Vibrating cone pushes and pulls air → creates compressions and rarefactions → SOUND WAVES.
FREQUENCY OF SOUND = frequency of the alternating current.
AMPLITUDE (loudness) ∝ amplitude of current (larger current → larger force → bigger vibrations → louder sound).
Headphones
HEADPHONES work on the same principle as loudspeakers but are miniaturised.
Similar construction: permanent magnet + voice coil + small diaphragm.
The smaller diaphragm moves the smaller volume of air needed for in-ear listening.
ELECTROSTATIC HEADPHONES (high-end):
Use electrostatic force instead of motor effect.
A thin membrane between two charged plates.
Alternating voltage changes force → membrane vibrates → sound.
Very low distortion — expensive.
BALANCED ARMATURE (in-ear monitors):
A small armature (iron bar) is balanced between magnets.
Alternating current through a coil unbalances the armature → vibrates → moves diaphragm.
Very efficient — used in hearing aids and professional in-ear monitors.
IMPEDANCE MATCHING:
Different headphones have different electrical resistance (impedance).
High-impedance: require amplifier, better for home use.
Low-impedance: work directly from phones, more common.
Energy Transfers in Loudspeakers
ENERGY TRANSFER IN A LOUDSPEAKER:
Electrical energy (from amplifier) → kinetic energy (voice coil moving) → sound energy (sound waves).
Inefficiencies: some electrical energy → thermal energy (resistance heating of coil).
Typical efficiency: 1–5% for most loudspeakers (most energy wasted as heat).
FACTORS AFFECTING SOUND QUALITY:
Frequency response: good loudspeakers reproduce a wide range of frequencies equally well.
Distortion: non-linear movement of cone → distorted sound.
Basic loudspeakers struggle with very low (bass) and very high (treble) frequencies.
DESIGN FOR DIFFERENT FREQUENCIES:
WOOFER: large cone → moves large volume of air → good for low frequencies (bass).
TWEETER: small cone → moves quickly → good for high frequencies (treble).
FULL-RANGE SPEAKER: compromise design.
SUBWOOFER: very large driver specifically for very low frequencies (<200 Hz).
⚠️ Common Mistake
The cone of a loudspeaker is driven by the MOTOR EFFECT — the force on a current-carrying conductor in a magnetic field. The alternating current changes direction, so the force changes direction, making the cone move in and out. The frequency of sound produced equals the frequency of the AC signal.
📌 Key Note
Loudspeaker: AC through voice coil in permanent magnet field → motor effect → cone vibrates → sound. AC frequency = sound frequency. Amplitude of current → loudness. Headphones: same principle, miniaturised. Energy: electrical → kinetic → sound (with some thermal losses).
🎯 Matching Activity — Loudspeaker Operation
Match each component to its role in a loudspeaker. — drag the symbols on the right to match the component names on the left.
Permanent magnet
Drop here
Voice coil
Drop here
Cone (diaphragm)
Drop here
Frequency of AC
Drop here
Provides constant magnetic field in which the voice coil experiences a force
Attached to coil — vibrates to push and pull air, creating sound waves
Carries the AC signal — force from motor effect drives it back and forth
Determines the frequency (pitch) of the sound produced
⭐ Higher Tier Only
HT only — describe how the motor effect drives a loudspeaker. Explain why alternating current is needed. Relate the frequency and amplitude of the AC signal to the properties of the sound produced.
🔬 Triple Science Only
Loudspeakers and headphones (HT only, physics only) — not in Combined Science.
🎯 Test Yourself
Question 1 of 2
1. Why must the electrical signal driving a loudspeaker be alternating current (AC)?
2. A loudspeaker produces a loud, low-pitched sound. What can you say about the AC signal driving it?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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