The surprising new physics of squeaky basketball shoes

Creak! The surprising new physics of why basketball games are so noisy

By Joseph Howlett edited by Lee Billings

It’s officially squeaker season.

The National Collegiate Athletic Association’s March Madness is right around the corner. The National Basketball Association (NBA) is fresh off its All-Star break, with the playoffs on the horizon. The Women’s Three-on-Three League Playoffs Unmatched kick off this weekend – and Angel Reese is back!

So turn on the TV and turn up the volume. Try tuning out the color commentary, the pulsating music and the “defense” chants, and what you’ll hear is the true soundtrack of basketball: a symphony of squeaks.

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Today, in a study published in the journal Nature, a team of researchers has taken a “Wemby”-sized step into the timeless scientific mystery of why basketball sneakers make those happy sounds.

“We didn’t expect to find so much richness and depth, from a physical point of view, under the sole of a shoe,” says Adel Djellouli, a researcher at Harvard University and co-author of the study.

Most researchers who had considered the problem believed that shoe squeaking was a simple example of the common “stick-slip phenomenon”.

It’s easy to see stick-slip in action. Just place a heavy book on a table and try to gently push it across. Instead of a smooth slide, you get a jerky, stop-and-start motion.

Basketball squeaks, the theory went, were an example of the same phenomenon. When a player stopped on a dime, the shoe’s rubbery sole would slip slightly—many times a second in the same stop-and-start pattern—producing a squeak. This is how violins work and why a squeaky door hinge rings with a lower pitch when you open it slowly.

But with the power of high-speed cameras and acoustic analysis, Djellouli and his co-authors have shown that basketball shoes are special.

It’s all about unevenness. The long, raised patterns of ridges that line the bottom of a sneaker are truly the championship of basketball. Watch the bottom of a shoe rub against the hardwood in slow motion and you’ll see.

The ridges of the sole do not lift and stick at once. Rather, only a small portion of each ridge separates from the ground at any given time. That separation pocket slides down the back until it reaches the end of the sole, at which point the air outside the shoe gets a little kick. These separation waves ripple down the ridges thousands of times per second, rhythmically kicking the air. The kick frequency is exactly the frequency of the squeak – the faster the kicks, the higher the pitch.

This frequency depends on the shape of each ridge, which guides the waves down at a characteristic speed. “The idea of ​​a waveguide for friction was not known,” says Gabriele Albertini, a structural engineer at the University of Nottingham in England and Djellouli’s lead author. To demonstrate their findings, the researchers reversed rectangular blocks of synthetic sole with distinct pitches. They were even able to play Darth Vader’s theme Star Wars on a piece of glass. “It took us three days to rehearse,” says Djellouli. “We could have just shown it in a graph, but where’s the fun in that?”

The sneaker study falls under the larger umbrella of “bimaterial friction,” the special physics of two different materials rubbing together. The phenomenon of two different faults sliding against each other to produce an earthquake, for example, is much like a sneaker rubbing against hardwood. Instead of the entire fault stopping and starting, ripples of separation move along it in the same way as what happens with the sneaker. The team believes the rubber setup could become an easy way to study earthquake physics in a laboratory.

“This is a more advanced and technically sophisticated analysis of a problem I dipped my toe into 20 years ago,” says Martyn Shorten, a stick-slip expert at BioMechanica, an Oregon consulting firm. “I love it!”

So the next time you see NBA player Shai Gilgeous-Alexander grab someone’s ankles, remember that the play’s creaking score is also something to behold. And when you wear your favorite player’s new signature shoe, you buy a finely tuned musical instrument that simulates an earthquake with every step. Who knows – maybe we’re just a few years away from “signature squeak!”

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