This paper presents an analysis of data generated by means of Large Eddy Simulation for a single-stream, isothermal Mach 0.9 jet. The acoustic field is decomposed in Fourier modes in the azimuthal direction, and filtered by means of a continuous wavelet transform in the temporal direction. This allows the identification of temporally localised, high-amplitude events in the radiated sound field for each of the azimuthal modes. Once these events have been localised, the flow field is analysed so as to determine their cause. Results show high-amplitude, intermittent sound radiation for azimuthal modes 0 and 1. The mode-0 radiation is found to be an indirect result of the transition from axisymmetric to antisymmetric organisation which occurs towards the end of the potential core: energy is transferred from the axisymmetric mode at a temporal scale corresponding to a frequency f0 to the antisymmetric and higher order modes at a scale corresponding to f0/2. The result is a time-varying modulation of both the amplitude and spatial extent of the axisymmetric wavepacket; the strongest axisymmetric propagative disturbances are produced when the wave envelope is truncated. The observed behaviour is modelled using a line-source wave-packet ansatz which includes parameters that account for the said modulation. Inclusion of these parameters, which allow the wavepacket to 'jitter' in a manner similar to that observed, leads to good quantitative agreement, at low emission angles, with the acoustic field of the LES. This result shows that the said modulations are the salient source features for the low-angle sound emission of the jet considered.
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