As he watched the Boston Celtics play from the stands of TD Garden, one noise kept catching Adel Djellouli’s ear.
“This squeaking sound when players are sliding on the floor is omnipresent,” he said. “It’s always there, right?”
Squeaky shoes are part of the symphony of a basketball game, when rubber soles rasp against the hardwood floors as players jab step, cut and pivot and defenders move their feet to stay in front of their assignment.
Returning home from the game, Djellouli wondered how that sound was produced. And as a materials scientist at Harvard University, he had a way to find out.
Djellouli and colleagues slid a sneaker against a smooth glass plate over and over. They recorded the squeaks with a microphone and filmed the whole thing 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. 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.
“That squeaking is basically your shoe rippling, or creating wrinkles that travel super fast. They repeat at a high frequency, and this is why you get that squeaky noise,” Djellouli said.
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 ridge-like designs on the bottom of your shoes may organize the bursts to produce a clear, high-pitched sound.
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.
These insights don’t just serve to satisfy the curiosity of a basketball fan. They could also help answer important practical questions. “Friction is one of the oldest and most intricate problems in physics,” wrote physicist Bart Weber in an editorial accompanying the new research. Yet, despite its practical importance, he wrote, “it is difficult to predict and control.”
Understanding friction better could help scientists better understand how the Earth’s tectonic plates slide and grind during earthquakes, for example, or to 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.
This research doesn’t offer a fix, though the internet has plenty of advice that may be risky, including rubbing soap or a dryer sheet on the soles. But some of the insights from the study could help to design squeak-free shoes in the future.
For example, one additional experiment found that changing the thickness of the rubber could make the squeak sound lower or higher in pitch. In the future, could we fine-tune our shoes to squeak in a pitch so high we can’t even hear it?
“We can now start designing for it,” said Weber, who is with the Advanced Research Center for Nanolithography and the University of Amsterdam, in an interview. “We can start making interfaces that either do it if we want to hear this sound, or don’t do it if we don’t want to hear it.”
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