1. An ultrasonic gas leak detector configured to discriminate the ultrasound generated by a pressurized gas leak into the atmosphere from false alarm ultrasound generated by other mechanical, electrical, acoustic or biological sources in the vicinity, comprising: a sensor for detecting ultrasonic energy and providing sensor signals; an electronic controller responsive to digital sensor signals representative of the sensor signals, the electronic controller comprising: a threshold comparator to compare a sensor signal value representative of sensed ultrasonic energy to a gas detection threshold value to determine whether the sensor signal value exceeds the gas detection threshold value to provide a threshold comparator output indicating whether a gas leak may have been detected; an Artificial Neural Network (ANN) for processing signals derived from the digital sensor signals and applying ANN coefficients configured to discriminate false alarm sources from gas leaks, and providing an ANN output indicating a decision whether a gas leak has been detected; an output decision for generating detector outputs in dependence on the threshold comparator output and the ANN output.