Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Many of the proposed sound-source mechanisms involve vortex sound production, either by radial vibration processes (e.g., Abdullah 1966) or flow instabilities (Georges 1976), and corotating multiple vortices (e.g., Mitchell et al. 1992).
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