📖 In-Depth Theory
How Loudspeakers Work
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.