Aerofoil

An aerofoil having a leading edge and a trailing edge, the leading edge comprising a plurality of slits extending toward the trailing edge, each slit defining a slit height from a peak at the leading edge to a trough spaced from the leading edge, the troughs defining a repeating waveform pattern, such that the troughs define a plurality of mutually coherent noise sources to cause destructive interference of noise created by the troughs.