Why kettles whistle

Settled in 2013 by Ryan Henrywood, Anurag Agarwal (University of Cambridge).

Whistling kettles have been a household fixture for over a century, but the precise aeroacoustic mechanism was not rigorously studied until Cambridge engineers Ryan Henrywood and Anurag Agarwal combined flow-visualization experiments with acoustic measurements in 2013. They found the whistle arises from a 'hole-tone' system: steam jets through two closely spaced orifice plates in the spout, and vortices shed at the plates couple with the acoustic resonance of the cavity between them, with the coupling mechanism changing character as the flow rate rises through boiling. Their model let them predict how whistle pitch depends on kettle geometry, closing a mechanistic question about a familiar sound.

A mundane teatime sound turns out to be a textbook 'hole-tone' resonator, the same physics used to understand some wind-instrument and industrial-valve noise.

Explanations that circulated

• That the whistle is a single, simple resonance (like blowing across a bottle top) at all flow rates; Henrywood & Agarwal showed the kettle is a two-plate 'hole-tone' system whose behavior switches between two distinct regimes (Helmholtz-like at low flow, jet-tone/hole-tone at higher flow) as steam speeds up.

References

• The aeroacoustics of a steam kettle, R. H. Henrywood, A. Agarwal, Physics of Fluids, 2013