Why does FM sound better than AM?

I always assumed it was because FM station bandwidths (200kHz) are much wider than AM (10kHz). AM's 10 kHz chops off a lot of human-hearable frequencies.

AM doesn't use the frequency for modulation though so it shouldn't matter.

AM does use the frequency, it just doesn't need as much and uses it differently than FM. If it was all at a single frequency, there just be a single tone getting louder and softer.

When you amplitude-modulate a carrier wave with an audio signal, you spread it out into a bunch of sum and difference frequencies, as you can see if you use the trigonometric angle-sum formula to factor cos(85000·2πt) · (2 + cos(440·2πt)), a 440-hertz flute being transmitted on 85-kilohertz AM. These so-called "sidebands" mean that the bandwidth of AM does matter, and consequently, using a too-narrow bandpass filter on your AM radio station will result in low-pass filtering your demodulated audio signal.

Thanks. I just learned that doing a rabbit hole about sidebands! Still getting my head around it.

Once you change the amplitude of a sine wave (modulate it) it's no longer a side wave. It spreads in the frequency domain. Take the fourier transform of that and you can see the frequency components.

Other comments gave a nice explanation of why AM does need a bandwidth, but here's the information theory explanation: https://en.m.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theo...

TL;DR: the information one can reliably send through a noisy channel (C) is proportional to the bandwidth of that channel.