  The discrete tone radiated from a cavitating tip vortex, known as "vortex
singing", was first recognized in 1989, but its sound generation mechanism has
remained a mystery for over thirty years. In this letter, by means of the
correction for the cavitation bubble dynamics and the dispersion relation of
cavity interfacial waves, we found that after the far-end disturbances
propagate upstream, the whistling vortex should be triggered by near-end sound
sources, the breathing mode waves. Further utilizing the theoretical solutions
for singing lines and the potential singing cavitation number with frequency,
we accurately identified all available tests for seeking the vortex singing
over the past three decades, answering a long-standing perplexity: why such a
best whistler is able to appear only within a narrow range of the cavitation
number.
