FM side bands lie at Carrier ± n×Modulator, an in-principle infinite series set only by the Modulator's frequency

Where AM generates exactly two side bands, at Carrier+Modulator and Carrier-Modulator, FM generates a whole series at Carrier ± n×Modulator for every integer n (0,1,2,3,…). Because n can be any integer, the series is in theory infinite; in practice finite bandwidth and the modulation index make high-n side bands negligible. The key structural fact is that the Modulator’s frequency alone determines the positions of every spectral component — its amplitude changes their strengths, not their locations. Negative-frequency side bands fold back into the positive range, which can place components at frequencies you did not obviously expect.

Examples

Carrier 500 Hz, Modulator 300 Hz: components at 500, then 800 and 200, then 1100 and -100 (folds to 100), then 1400 and -400 (folds to 400), and so on at 300 Hz spacing.

Assessment

For a 400 Hz carrier and a 150 Hz modulator, list the frequencies of the first four side-band pairs (folding any negatives), and state what determines how many are audibly significant.