Why does FM sound better than AM?

I don’t buy this explanation. The FM modulation uses a much higher bandwidth than AM. The distance between channels on FM radio is 200 kHz compared to only 9 kHz on AM. That’s more than 20x more bandwidth for FM. On AM, no matter how deeply you modulate the carrier, the bandwidth will not exceed twice the bandwidth of the input signal. On FM, the deeper you modulate it, the wider the output spectrum will be, and it can easily exceed the bandwidth of the input signal.

In addition to that, the whole FM band is much higher frequency, while I guess quite a lot of noise, especially burst noise caused by eg thunderstorms is relatively low frequency. So it’s not picked up because it’s out of band.

Any noise that falls inside the channel does get picked up by the receiver regardless of modulation. However because the available bandwidth is so much higher than the real bandwidth of the useful signal, there is actually way more information redundancy in FM encoding, so this allows to remove random noise as it will likely cancel out.

If I encoded the same signal onto 20 separate AM channels and then averaged the output from all of them (or better - use median filter) that would cancel most of random noise just as well.

Also another thing with modulation might be that if there is any narrow-band non-white noise happening to fall inside the channel (eg a distant sender on colliding frequency), on AM it will be translated as-is to the audible band and you’ll hear it as a single tone. On FM demodulation it will be spread across the whole output signal spectrum, so it will be perceived quieter and nicer by human ear, even if its total energy is the same. That’s why AM does those funny sounds when tuning, but FM does not.

FM has 15kHz of bandwidth per stereo channel or an effective 30kHz sampling rate. The rest of the space is used for supplemental signals, including, the "pilot carrier" that is used to generate the "stereo image." There is space for three more full bandwidth mono channels on the end of an FM broadcast. One of them is often used for RBDS.

FM signals receive AM interference but heterodynes exclude them effectively. The cost is vulnerability to multipath reception in highly signal reflective environments and capture/wandering effects when two signals of similar strength are present.

AM _can_ sound pretty good. Most AM transmitter sites are poorly maintained, combined with other stations into one antenna system (something you can do on AM with a phasor), and are typically just simulcasts of FM content or satellite delivered content. There's no real care put into it. On a well maintained, tuned, and properly programmed station, mono content on AM sounds quite pleasant.

That's not even getting into "cost saving" measures that AM operators employ that completely compromise their signals. Or what Nielsen has convinced them to inject into their signals to register modern "ratings points" from the "portable people meter" system.

Guess where I used to work.