The secret of squeaky basketball shoes

Basketball players often sense the movements of teammates and opponents not only by sight but also by the sharp squeaks that their shoes make on the polished court. These familiar sounds arise from rapid, subtle dynamics at the shoe–floor interface that are invisible to the naked eye. Writing in Nature, Djellouli et al.1 report high-speed optical-imaging experiments showing that the squeaking originates from wave-like deformations of the shoe sole. These waves sweep across its interface with the floor at velocities approaching 300 kilometres per hour. They lift the shoe out of contact at spatially localized points as they sweep past, enabling it to slide across the floor. The repetition rate of the waves moving across the shoe is set by the stiffness and thickness of the shoe sole and directly matches the frequency of the emitted sound.