Why trainers really squeak

Michael Restin

5/3/2026

Translation: machine translated

Squeaky soles are annoying in everyday life, on the basketball court they are the background music and interesting for science: researchers from Harvard were surprised at what happens when rubber and floor rub together.

There's this noise. A high-pitched squeak. Over and over again. When Adel Djellouli attends a basketball game in the USA, the physicist takes a question back to the elite Harvard University: what is behind this sound? One research series, a Nature publication and a Star Wars theme tune squeaked with rubber blocks later, it is clear that this question was only a simple one at first glance.

Waves just under the speed of sound

The squeaking of the soles was previously attributed to the slip-stick effect. Friction caused by the rapid change between adhesion and sliding of two surfaces. That would mean: The sole sticks to the ground, while the leg pushes and stretches it further until it jerks loose and snaps forwards.

Instead, the researchers found that the rubber sole forms tiny detachment waves. These travel at almost the speed of sound through the material of the sole and sometimes even exceed it. They are around eighty times faster than the shoe moves as a whole. The air between the shoe and the ground is displaced and sucked back in as if by an extremely fast mini pump.

No squeaking without tread

Djellouli compares the effect to a carpet that you smooth out with a jerk. A small wave runs from one end to the other. With shoes, however, this wave movement occurs not just once, but up to 5,000 times per second.

Every time a rubber fold briefly loses and regains contact with the ground, a tiny shock is created and causes the air to vibrate - you hear the typical high-pitched squeak.

However, the sole needs a tread for this. If it doesn't have one, it makes friction noises but doesn't squeak. It is the geometry of the grooves that always produces the same sound, regardless of the speed of movement.

Music to the ears of the researchers

The researchers were able to vary the frequency in experiments with rubber blocks of different thicknesses. So precisely, in fact, that they came up with an entertaining idea: «The relationship between height and frequency was so precise that we could make music with the blocks», says Djellouli. They opted for the theme tune from Star Wars.

The musical interlude probably serves above all to present the results in an interesting way, even for laypeople. But it shows that the researchers have not only understood the physics of squeaking, but can also control it. These findings about friction are interesting far beyond the gymnasium.

What happens on a small scale under the sneaker is similar, for example, to the forces released during an earthquake when two earth plates slide past each other. Djellouli expresses the hope that the findings could help us to learn more about earthquakes and ultimately predict them better. However, industry also has a tangible interest: The study was supported by the German chemical company BASF, which supplies materials to sports brands such as Adidas.

Flashes of inspiration and light

As a result, the international research group was able to draw on a wealth of technical resources. To record what happens between the sole and the ground, they used a high-speed camera that delivers up to one million images per second. With its extremely high frame rate, the camera films how the contact of the shoe profile disturbs the light conduction in the illuminated glass. This in turn was visualised using a special microscopy process.

Djellouli describes the discovery that tiny dots of light flash under the sole during the image as a special wow moment. They appear where contact with the ground is briefly interrupted. However, these optical «mini thunderstorms» have nothing to do with the sound: after all, it doesn't thunder, it just squeaks.

Michael Restin

Senior Editor

Michael.Restin@digitecgalaxus.ch

Simple writer and dad of two who likes to be on the move, wading through everyday family life. Juggling several balls, I'll occasionally drop one. It could be a ball, or a remark. Or both.

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