Loudspeakers & Headphones

Principle of Loudspeakers & Headphones

Loudspeakers and headphones use the motor effect to convert electrical signals into sound waves. Inside a loudspeaker or headphone, there is a coil of wire, often called the voice coil, placed within a magnetic field created by a permanent magnet.

When an electric current flows through the coil, it experiences a force due to the motor effect. This force causes the coil to move back and forth. The coil is attached to a cone (or diaphragm), so when the coil vibrates, the cone vibrates too. These vibrations push and pull the air particles around the cone, creating sound waves that we hear.

The frequency of the sound produced depends on how the current changes over time, causing the cone to vibrate at different speeds.

Example: If the current in the coil changes direction 440 times per second, the cone vibrates at 440 Hz, producing the musical note A above middle C.

Role of the Motor Effect

The motor effect is the key principle behind how loudspeakers and headphones work. When an electric current passes through the coil in the magnetic field, a magnetic force is produced. This force acts on the coil and causes it to move.

The direction of the force changes when the direction of the current changes. This means the coil moves back and forth as the current alternates, making the cone vibrate and produce sound waves.

The frequency of the sound depends on how quickly the current changes direction. For example, a high-frequency alternating current makes the cone vibrate faster, producing a higher-pitched sound.

Fleming's Left-Hand Rule

Fleming's left-hand rule helps predict the direction of the force on the coil in a loudspeaker or headphone. It uses the orientation of three factors:

• Thumb – direction of the force (motion) on the coil
• First finger – direction of the magnetic field (from north to south)
• Second finger – direction of the current in the coil

By arranging your left hand so the first finger points in the magnetic field direction and the second finger points in the current direction, your thumb will point in the direction the coil moves.

This rule is useful for understanding how the coil moves to create vibrations and sound.

For example, if the magnetic field points from left to right and the current flows upwards through the coil, Fleming's left-hand rule shows the force will push the coil either forwards or backwards, causing the cone to vibrate.