Why Your Basketball Shoes Squeak

As he watched the Boston Celtics play at TD Garden, one noise kept catching Adel Djellouli's ear. "This squeaking sound when players are sliding on the floor is omnipresent," he said. Returning home from the game, Djellouli wondered how that sound was produced—and as a materials scientist at Harvard, he had a way to find out. Djellouli and colleagues slid a sneaker against a smooth glass plate over and over, recording the squeaks with a mic and filming with a high-speed camera to see what was happening under the shoe. In a study published Wednesday in the journal Nature, they described what they found.

• Science behind the squeak: As the shoe works hard to keep its grip, tiny sections of the sole change shape as they momentarily lose, then regain, contact with the floor thousands of times per second—at a frequency that matches the pitch of the loud squeak we hear, per the AP. "That squeaking is basically your shoe rippling, or creating wrinkles that travel super fast," Djellouli said. "They repeat at a high frequency, and this is why you get that squeaky noise."

• Sole situation: The grip patterns on the soles may also play a role. When researchers slid blocks of flat, featureless rubber against the glass, they saw a series of chaotic, disorganized ripples but didn't hear squeaks. The ridgelike designs on the bottom of your shoes may organize the bursts to produce a clear, high-pitched sound.
• Deep dive into friction: Other researchers have studied these kinds of bursts before, but this sneaker study examines friction happening at much faster speeds. And for the first time, it links the speedy pulses with the squeaking sound they produce. "Friction is one of the oldest and most intricate problems in physics," wrote physicist Bart Weber in an editorial accompanying the research. Yet, despite its practical importance, he wrote, "it is difficult to predict and control."
• Other applications: Understanding friction better could help scientists better understand how the Earth's tectonic plates slide and grind during earthquakes, for example, or save energy by reducing friction and wear. It could also help eliminate moments off the court when squeaky shoes can be a little awkward or embarrassing, such as in a quiet office hallway.
• Quiet future shoes? Some of the insights from the study could help to design squeak-free shoes in the future, perhaps even designing our shoes to squeak in a pitch so high we can't even hear it. "We can now start designing for it," said Weber.

More here.