Why do sneakers squeak on the hall floor? Based on this question, researchers carried out extensive experiments and in the end not only recreated the theme song from Star Wars with squeaking rubber blocks, but also collected new insights for materials science and earthquake research. Accordingly, the squeaking noise of shoes depends, among other things, on the thickness and structure of their sole – and can be specifically influenced by varying these factors.
At a basketball game, squeaky shoes are just as much a part of the background noise as the ball bouncing. But how does this sometimes nerve-racking, high-frequency sound actually come about? Previous explanations assumed that the cause of the squeak was an alternation between the soft rubber sticking and sliding across the hard surface of the floor, scientifically known as stick-slip oscillation. However, this interaction has not yet been researched in detail.
Even frequencies
But a team led by Adel Djellouli from Harvard University in Boston wanted to know more. “Our project started with the simple question: Why do basketball shoes squeak?” says Djellouli. That’s why the researchers built an experimental device in which a commercially available sneaker with a grooved profile is automatically moved over a glass plate with a loud squeak at a fixed angle and at a constant speed. On the other side of the glass plate, the researchers mounted a high-speed camera that can record up to a million images per second. They simultaneously recorded the frequency of the squeak with a microphone.
This showed that the contacts between the shoe profile and the glass repeatedly broke off and re-formed in very rapid succession. The researchers refer to this as opening pulses. These pulses did not occur randomly, but spread in waves across the rubber profile of the sole – at a frequency that was almost identical to the frequency of the squeak recorded by the microphone. In the case of the sneaker tested, this frequency was around 4,800 Hertz. A smooth, unstructured rubber surface, on the other hand, did not produce uniform opening pulses, but instead detached itself from the glass surface in irregular patterns – audible more as a noise than a squeak.
Squeaky Star Wars
The researchers reproduced these results using different shaped and grooved rubber blocks. They discovered that the frequency of the opening pulses depends on the thickness, strength and structure of the rubber and can be specifically influenced. With this knowledge, Djellouli and his team created rubber blocks that produce a specific sound so reliably that they can even be used to make music. To demonstrate their findings, members of the research team recreated the “Imperial March” from Star Wars using squeaky rubber on glass.
(Video: Harvard John A. Paulson School of Engineering and Applied Sciences)
But the possible applications of the findings go far beyond squeaky shoes and musical gimmicks. “Spontaneous adjustment of friction behavior has long been a dream of engineering,” says Djellouli’s colleague Katia Bertoldi. “These new insights into how surface geometry influences sliding momenta pave the way for tunable friction metamaterials that can transition from a low-friction state to a high-adhesion state as needed.”
The study could even be relevant for earthquake research. “Our results show that the squeak of a sneaker can propagate as quickly or even faster than the rupture of a geological fault in an earthquake,” says Djellouli’s colleague Shmuel Rubinstein. “Their physics are strikingly similar.”
Source: Adel Djellouli (Harvard University, Boston, Massachusetts, USA) et al., Nature, doi: 10.1038/s41586-026-10132-3