The acoustic "eld of rotating sources such as aeroplane propellers and helicopter rotors is known to develop a marked asymmetry when the incoming #ow is not parallel to the axis of rotation. This e!ect has been modelled using a "eld composed of &&wobbling modes'', azimuthal modes whose amplitude is a function of azimuth. A computationally e$cient method is developed for the calculation of these wobbling modes and of the acoustic "eld at incidence. Results are presented for operating conditions representative of a range of aircraft from a high-speed propeller at low incidence to a helicopter rotor. Detailed contour plots of the acoustic pressure are presented and discussed in the context of the geometry of the acoustic "eld. The "eld structure is interpreted in terms of the tunnelling of acoustic radiation across a transition region around the sonic radius (where the source has a Mach number of unity) and the asymmetry of the "eld is shown to arise from variations in the thickness of this transition region as the sonic radius varies during a revolution of the rotor.