AM and FM Radio Waves

While teams of frenzied German engineers are hard at work scaling an entire Bon Jovi cover band down to 3-Series console size for BMW, as of yet the radios in cars are just that—radio receivers. All that great music comes from nothing, plucked out of the ether by car antennas. If you've driven in a car since the early 1900s, chances are you probably know there are two kinds of radio that come over the air: AM and FM.

On the surface it may seem like the only difference between the two types are the content—AM is all local news and Bach concertos, while FM is vulgar talk radio and top 40 songs—but in reality, these two technologies are entirely different ways to move sound from a studio to your ears. Both use radio waves, which have long wavelengths and low energy, and live at the very bottom of the electromagnetic spectrum under visible light and microwaves. However, the big differences come into play when we look at frequency. You know 101.2 The Edge that your brother listens to? Ever wonder where the 101.2 comes from? You know you're curious now...

Each radio station has its own designated frequency to broadcast on. AM stations take up from 550 to 1700 kHz in 10 kHz increments (maybe you know an AM 680 or 1060 station?), while FM stations have much higher frequencies, ranging from 87.9 to 107.9 MHz in 0.2 MHz increments (here in the Bay Area Shmoop World Headquarters, we can listen to Live 105.3 or KLOS at 98.5). Any higher than that, and you move into microwave radiation—which runs the risk of cooking your car instead of feeding it great tunes.

The two types of radio broadcasting technologies are named for the way they store and send information from a station to your radio. "AM" stands for amplitude modulation, while "FM" stands for frequency modulation—hopefully these terms ring a wave-related bell. In AM radio broadcasts, data is sent by changing the amplitude of the waves and keeping the frequency constant:

(Source)

And in FM radio, data is sent by changing the frequency slightly around the central band that shows up next to the station name, but keeping the amplitude constant:

(Source)

AM radio has been around longer and the signals can travel further, but FM radio is much less susceptible to interference and has gradually surpassed AM as the most-listened-to radio format. That is, until BMW miniaturizes that Bon Jovi band successfully.