Experiments on the flow acoustics of human whistling

Human pucker whistling is a poorly understood natural phenomena involving a strong interplay between fluid mechanics and acoustics. Humans whistle by raising the tongue against the roof of the mouth, forming an orifice, and then form another orifice with their lips. The oral cavity between these two orifices forms a resonant chamber that controls the sound frequency. We show through visualization by flow seeding over axisymmetric toruses that human whistling involves periodic vortex shedding starting at the lips, and quantify it using hot wire anemometer measurements. We found that the frequency of vortex shedding is identical to that of acoustic pressure fluctuations. Furthermore, the curvature of the divergent portion of the outer orifice, representing the lips, has a significant influence on boundary layer separation and amplifies or attenuates the sound produced. We also found a strong correlation between velocity fluctuations inside the resonant chamber and sound pressure levels.