Why Do Shoes Squeak?

On a basketball court, the squeak of sneakers is part of the atmosphere. That sharp, high-pitched screech accompanies every change of direction, every sudden stop, every burst of acceleration. But where exactly does this distinctive sound come from? Physicists at Harvard University, in collaboration with the University of Nottingham and France's CNRS, have just published a study that upends our understanding of this phenomenon.

A classic theory called into question

Until now, the scientific community explained shoe squeaks through a mechanism called stick-slip. According to this model, the sole rapidly alternates between phases of sticking and slipping, creating vibrations that produce the sound. This principle works well for explaining the creak of a poorly oiled door or the friction between two rigid surfaces. But soft interfaces, like a rubber sole on hardwood, obey more complex laws.

Cameras recording one million frames per second

Adel Djellouli, a postdoctoral researcher at Harvard, came up with the idea for this study while attending a Boston Celtics game. Intrigued by the constant squeaking of sneakers on the court, he decided to observe what was actually happening beneath the sole. Along with colleagues Gabriele Albertini and Katia Bertoldi, he designed an original experimental setup: a transparent glass plate replacing the floor, illuminated by LEDs, allowing the contact interface to be filmed using cameras capable of recording one million frames per second.

The researchers slid Nike basketball shoes and rubber blocks across this glass surface while simultaneously recording the sound produced. The results, published in the journal Nature (Djellouli et al., Nature 650, 891-897, 2026), reveal an unexpected mechanism.

Invisible supersonic waves

Contrary to what stick-slip theory suggested, the sole does not repeatedly detach entirely from the floor. Instead, tiny regions of the surface locally pull away, forming "wrinkles" or "ripples" that propagate across the interface at speeds approaching 190 mph—nearly the speed of sound in the material. These detachment waves, which the researchers call "opening slip pulses," repeat approximately 4,800 times per second in a standard sneaker. It is this repetition rate that determines the pitch of the squeak we hear.

The crucial role of geometry

The study reveals that the shape of the sole plays a decisive role. When the researchers slid perfectly smooth rubber blocks across the glass, the detachment waves were chaotic and disorganized, producing a dull noise resembling peeling adhesive tape rather than a sharp squeak. In contrast, the treads and ridges found on athletic shoe soles act as waveguides: they channel the detachment pulses and organize them into a regular pattern, producing a clear, recognizable note.

The thickness and stiffness of the rubber also influence the sound's frequency. The thinner the sole, the higher the pitch. This relationship is so precise that the Harvard team was able to design rubber blocks of varying heights to play the Imperial March from Star Wars by rubbing them on a glass plate. The experiment required three days of rehearsal.

Tiny lightning bolts beneath our feet

The high-speed footage also revealed a surprising phenomenon: triboelectric discharges—genuine micro-lightning bolts caused by the friction of the rubber. These discharges sometimes appear to trigger the detachment waves. Although they are not the main cause of the squeaking, they show that electrical phenomena accompany friction, opening new avenues for research.

Applications beyond sports

These discoveries are relevant far beyond the world of basketball. The detachment waves observed in sneakers share characteristics with ruptures that propagate along tectonic faults during earthquakes. As physicist Shmuel Rubinstein, a co-author of the study, explains, "the squeak of a sneaker can propagate as fast as, or even faster than, the rupture of a geological fault, and their physics is strikingly similar." Laboratory experiments on rubber blocks could therefore serve as scale models for studying seismology.

On the engineering side, these findings pave the way for designing materials with adjustable friction properties. Bart Weber, a physicist at the Advanced Research Center for Nanolithography in Amsterdam, believes we could now design interfaces that squeak or not depending on our needs. Perfectly silent sneakers? It is technically feasible by modifying the sole's thickness to produce an ultrasonic sound, inaudible to the human ear. Applications also extend to bicycle brakes, tires, and hip replacements, where polymer liners slide against metal or ceramic surfaces.

A still-emerging science of friction

Djellouli and his colleagues' study falls within the field of tribology—the science of friction, wear, and lubrication. While its foundations were laid by Leonardo da Vinci over five hundred years ago, understanding soft interfaces remains a challenge. The empirical laws inherited from the past are insufficient to describe what happens when a soft material like rubber slides on a rigid surface at high speed. This study shows that simplified one-dimensional models fail to capture the richness of the phenomena at play.

For Martyn Shorten, a biomechanics expert who explored this topic twenty years ago, this research represents "a more advanced and technically sophisticated analysis of a problem I dipped my toe into." The next step will be translating these laboratory observations into practical applications.

What about you—does the squeak of sneakers on hardwood annoy you, or is it part of what makes a basketball game special?