A voice is a slow wiggle in the air, and on its own it's far too lazy to travel across a whole city. So radio gives the voice a ride on a much faster wave called the carrier. There are two ways to load the voice onto that ride. The first, called AM, makes the fast wave taller where the voice is loud and shorter where it's quiet — it changes the wave's height. The second, called FM, keeps the wave the same height but squeezes its wiggles closer together where the voice is loud — it changes the spacing. Your radio listens to the fast carrier, reads those changes back off it, and rebuilds the original voice for your ears. Flip between AM and FM in the simulator and watch the carrier change shape as the message rides along.
Most people think a radio sends your actual voice out through the air. In fact the voice is far too slow and weak to travel, so it rides on a fast carrier wave that the receiver reads back off.
What's actually happening
The problem radio has to solve is that sound and radio waves live on completely different timescales. A human voice wiggles a few hundred to a few thousand times a second; a radio wave wiggles hundreds of thousands or millions of times a second. You can't just send the voice out of an antenna — it's far too slow and weak to radiate and travel. So radio borrows a trick from everyday life: if something can't make the journey itself, give it a ride on something that can.
That something is the carrier, a fast, steady radio wave whose only job is to travel. By itself it carries no information at all; it's a blank wave humming along. To put the voice on it, you let the slow voice signal reshape the fast carrier as it goes. AM does this by changing the carrier's height: where the voice swings high, the carrier grows tall; where the voice dips, the carrier shrinks. FM leaves the height alone and changes the spacing instead, bunching the carrier's wiggles tighter when the voice is strong and loosening them when it's weak. Either way, the carrier now wears the shape of the voice, and a receiver tuned to that carrier can peel the shape back off and turn it into sound.
The reason both methods survive is that they fail differently. Most electrical noise (lightning, sparks, a passing motor) messes with a wave's height, which is exactly the thing AM uses to carry its message, so AM stations crackle and hiss in a storm. FM hides its message in the spacing instead, which that same noise barely touches, so FM comes through clean. That's why music, which needs fidelity, lives on the FM band (88 to 108 MHz), while AM (around 540 to 1600 kHz) carries talk and news — and travels much further at night, when its waves bounce off the ionosphere and skip clear across a continent.
Radio loads a slow voice onto a fast carrier, and because AM rides the wave's height while FM rides its spacing, FM shrugs off the noise that makes AM crackle.
- 1Tune a cheap AM radio to a quiet spot between stations, with the volume up so you hear the hiss.
- 2Now strike a piezo lighter (the click kind) or flick a light switch nearby a few times.
- 3You will hear a sharp click or buzz from the radio with each spark — the tiny spark is radiating radio noise straight into AM's amplitude, exactly the weakness FM was invented to dodge.
Common questions
Most electrical noise, like lightning and sparks, disturbs a wave's height, the very thing AM uses to carry its message. FM hides its message in the spacing instead, which that noise barely touches, so FM comes through clean.
After dark, AM signals bounce off the ionosphere and skip hundreds of miles, so a distant city station can appear on your dial. The same waves are absorbed by the daytime atmosphere.
The carrier is a fast, steady radio wave whose only job is to travel. By itself it carries no information; the slow voice signal reshapes it so a receiver tuned to it can peel the shape back off.