Abstract: A method of detecting stall in an axial fan through acoustical measurements in air the air flow, includes measuring the sound emanating from the flow adjacent the fan, 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, selecting the fixed visual representation most closely matching the visual representation of the said sound, and deriving a control signal from the selected fixed visual representation. The visual representations are formed by a symmetrised dot pattern (SDP) technique.