1. A method of detecting and controlling stall in an axial fan through acoustical measurements in the air flow, comprising: measuring the sound emanating from the flow, preparing a visual representation of the sound, comparing the visual representation with a library of fixed visual representations derived from a plurality of tests representative of the performance of the fan under a range of operating parameters, and selecting the fixed visual representation most closely matching the visual representation of the said sound, wherein the selected visual representation is used to generate a feed back control signal to control the operation and speed of the fan.