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Sound, usually in the band of audible frequencies, associated with wake-eddy, vortex- produced pressure fluctuations resulting from air flow around obstacles, such as wires and twigs.
Although many such sounds are irregular noises, other familiar sounds involve fairly clear musical notes or humming sounds. The latter sounds were called aeolian tones by Rayleigh. Their pitch is controlled by the frequency with which eddies are formed and detached in the wake region on the lee side of the obstacle. The tones produced by wind flowing over a cylinder, including stretched wire, were shown by Strouhal in 1878 to be of frequency (pitch) f given by
where u is the cross-axis wind velocity (m s-1) and d is the cylinder diameter (m).
Lord Rayleigh 1878. The Theory of Sound. Vol. II, . 412–413.
Humphreys, W. J. 1940. Physics of the Air. 3d ed., . 442–448.