Abstract: An electrical signal generator comprises an electric wave oscillator for producing oscillations at one or other of two predetermined frequencies, an attenuator device for attenuating oscillations produced by the oscillator in a manner which varies over a period of time, and control means for controlling the oscillator to produce a burst of oscillations at one said predetermined frequency and a subsequent burst of oscillations at the other said predetermined frequency. The control means controls the attenuator device increasingly to attenuate the oscillations over the period of each burst.