AM vs. FM Radio: What’s the Difference?

Do kids these days even know what this is?
Do kids these days even know what this is? / Tuckraider/iStock via Getty Images

Thanks to the digital revolution, members of Generation Z don’t spend much time twiddling a radio dial for a chance to hear their favorite song. But Millennials and their predecessors no doubt remember the experience—as well as the initialisms, AM and FM, that went along with it.

You probably know that AM and FM describe different kinds of radio stations. But how exactly are they different?

In order to process that answer, it helps to have at least a cursory understanding of how radios produce noise in the first place. Each radio station emits what’s known as a carrier wave or carrier signal, which is basically a steady electromagnetic wave whose variables—like frequency and amplitude—don’t change. That is, until a DJ starts playing a song at the station (or some other sound is broadcast). Those sound waves get converted into electrical signals that travel to your radio antenna via the carrier wave, and your radio converts them back into sound waves for your speaker to emit. When the carrier wave is transmitting those signals, it changes in one of two ways.

If it’s an AM radio station, the carrier wave’s amplitude—the height of the wave—gets manipulated based on the signals. As Soundfly’s Flypaper reports, this is known as amplitude modulation, hence the AM. For FM radio stations, the amplitude remains the same, while the frequency—the number of completed wave cycles in a given time—changes.

frequency modulation vs amplitude modulation
FM vs. AM, illustrated. / petrroudny/iStock via Getty Images

FM stands for frequency modulation, and is generally considered better than AM. When random electrical signals interfere with a carrier wave, they can affect its amplitude. Since FM carrier waves don’t transmit signals via amplitude, as PBS explains, this has no bearing on the quality of an FM radio broadcast. But altering the amplitude of an AM carrier messes with the data it’s trying to transmit, which is why AM radio typically has more static than FM does. FM radio also broadcasts across a much larger range of frequencies—known as bandwidth—so it’s able to handle more nuance and variation in sound. If you want to broadcast music, you’ll get better quality by using FM radio.

That said, AM radio isn’t without its advantages. As explained in the video above, its wavelengths—the distance between one wave crest to the next—are significantly longer than FM wavelengths, which helps the signals travel farther and also stay intact when encountering barriers like buildings or mountains. If you want your broadcast to reach as many people as possible, AM radio may be your best bet.

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