FM Harmonic Amplitudes (Bessel Functions)FM bandwidth expands as the modulation-amplitude is increased in (G.2) above. The th harmonic amplitude is proportional to the th-order Bessel function of the first kind , evaluated at the FM modulation index . Figure G.7 illustrates the behavior of : As is increased, more power appears in the sidebands, at the expense of the fundamental. Thus, increasing the FM index brightens the tone.
Further Reading, Additive Synthesis