How a Loudspeaker Works

From a wire to a wall of sound

Plug in your headphones or turn up a speaker and music fills the air. But a speaker receives nothing but a wiggling electric current down a wire. How does an invisible electrical signal become sound you can feel in your chest? The answer combines magnetism, motion and waves.

What sound actually is

First, recall what we are trying to create. Sound is a pressure wave travelling through the air, made when something vibrates and pushes the air molecules back and forth — exactly the picture in sound and how we hear. To make a sound, a speaker simply has to make the air vibrate in the right pattern. The challenge is doing that on demand, fast, and precisely.

The motor effect: the trick at the heart of a speaker

A loudspeaker uses one of the most useful rules in physics: a wire carrying a current, placed in a magnetic field, feels a force. This is the motor effect, and the direction of the force depends on the directions of the current and the field.

Inside a speaker there is:

• A permanent magnet, creating a steady magnetic field.
• A coil of wire (the voice coil) sitting in that field, free to slide in and out.
• A paper or plastic cone attached to the coil.

When current flows through the coil, the motor effect pushes the coil — and the cone it is glued to — either forward or backward. Reverse the current direction and the force reverses too, pulling the cone the other way.

Turning a signal into vibration

Now comes the clever part. The electrical signal coming from your phone or amplifier is not steady — it is the music itself, encoded as a current that constantly changes in size and direction, hundreds or thousands of times a second.

Because the force on the coil follows the current, the cone is pushed and pulled in exactly the same pattern as the original sound. A loud passage means a big current and a big cone movement; a quiet, high note means a small, rapid wiggle. The cone faithfully copies the shape of the sound wave that was recorded.

As the cone moves out, it compresses the air in front of it; as it moves back, it leaves the air rarefied (spread out). These pressure changes ripple outward as a sound wave, travel to your ear, and your brain hears the music.

Loudness and pitch

Two features of the signal control what you hear:

• Loudness depends on how far the cone moves — the amplitude of its vibration. A bigger current swings the cone further, pushing more air, and the sound is louder.
• Pitch depends on how fast the cone vibrates — the frequency. A high-pitched note makes the cone shake quickly; a deep bass note makes it move slowly but over a larger distance.

This is exactly the relationship described in wavelength, frequency and amplitude.

A microphone is a speaker in reverse

Here is a satisfying symmetry. A loudspeaker turns a current into movement and then sound. A microphone does the opposite: incoming sound waves push a small cone and coil, and as that coil moves through a magnetic field it generates a current by electromagnetic induction. So the same parts — a coil, a magnet and a diaphragm — can either make sound or capture it, depending on which way the energy flows.

Try it yourself! 🧪 (safe version)

You can feel the motor effect without taking anything apart.

1. Find a small portable speaker playing music and gently rest your fingertips on its front grille (do not press hard). You will feel it buzzing — that is the cone vibrating the air.
2. Turn the volume up a little: the buzzing gets stronger because the cone is swinging further (bigger amplitude = louder).
3. Play a song with deep bass and you will feel slow, powerful pulses; play something with high notes and the vibration feels fast and light.

You are feeling the cone copy the music's pattern in real time. Every wobble is the motor effect pushing the coil, turning electricity into the sound waves reaching your ears.