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, the library of fixed visual representations derived from a plurality of tests representative of the performance of the fan under a range of operating parameters, wherein the library of fixed visual representations comprises a plurality of visual representations each depicting only one of: normal flow conditions, incipient stall conditions, and stall conditions; selecting the fixed visual representation most closely matching the visual representation of the sound; determining if the fan is approaching incipient stall based on the selected visual representation; and generating a feedback control signal to control the operation and speed of the fan when the fan is determined to be approaching incipient stall.