We study the response of a single-degree-of-freedom system with cubic
nonlinearities to an amplitude-modulated excitation whose carrier freq
uency is much higher than the natural frequency of the system. The onl
y restriction on the amplitude modulation is that it contain frequenci
es much lower than the carrier frequency of the excitation. We apply t
he theory to different types of amplitude modulation and find that res
onant excitation of the system may occur under some conditions.