NSF Award
2332390
Whistling with our lips is a skill that a majority of us humans master at an early age. In many societies, whistling is an expected stage in the process of growing up. But as many as one-third of the population do not master the skill, and this is not for lack of musical talent since professional musicians are also counted among non-whistlers. Whistling involves the coordination of many muscles at the front and back of the oral cavity and requires making a very precise shape with one’s lips. Most fundamentally, and despite this being a common human function, we still do not know how the sound is produced. Musical instruments of similar timbre, like the flute, contain a sharp edge where an air jet can be diverted to either side. But a person’s lips do not have any sharp edges, and there are no alternative spaces where the air might go. Unlike flutelike instruments, whistling is also reversible, so that the sound can be produced both when the air is flowing out of or into the mouth. Fundamental research on the fluid dynamics of whistling has been scant, with the last serious effort having taken place more than fifty years ago. In this proposal, we plan to update and go beyond that research using modern experimental techniques, with five specific aims: (1) validation of a hypothesized mechanism for sound generation, (2) determining whether the process is essentially axisymmetric, (3) finding the most relevant dimensionless parameters controlling the process, (4) optimizing those parameters toward practical use, and (5) designing musical instruments based on the physics and optimized parameters, and testing them in a music instruction environment.<br/><br/>This will be the first time in over fifty years that this human function is studied systematically as a Fluid Mechanics problem, primarily through measurements of acoustic power amplification over a multidimensional parameter space, aided by flow visualization and solid structure characterization. Cross-sectional diameter and eccentricity, profile diameter and eccentricity will be added to the parameters studied by others, which will allow us to find the dimensionless groupings that best collapse the measurements into simple correlations. A mechanism for sound generation in human whistling is hypothesized, to be validated through systematic measurements involving those geometric parameters, plus average flow velocity, density, and sound speed, controlled via temperature. Visualization will reveal whether periodic vortex shedding occurs, and how important it is for the physics.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
