A modulated neutron source has been constructed by alternately spreading and converging the ion beam from a 2-MeV Van de Graaff accelerator striking a neutron-producing target. This spreading and converging is accomplished by applying a time-varying voltage to the electrodes of an electrostatic quadrupole lens located in the beam tube of the accelerator a few meters from the target. The system is capable of producing neutron waves of frequencies between a few Hz to over 100 000 Hz, with little harmonic distortion. No modification of the accelerator is required, and all necessary equipment is commercially available.