It is the unofficial soundtrack of basketball, a noise consistently heard but rarely considered – rubber-soled shoes squeaking on the hardwood.
Squeaks are the background rhythm to the game. But that sound is also one of the enduring mysteries of sports and presents a question that gets scientists talking: Why do basketball shoes squeak?
To understand, it may help to consider violins and the California spiny lobster.
Sheila Patek, a biologist at Duke University, is an expert in spiny lobsters, among other oddities of the animal kingdom, and several years ago discovered that some species of the clawless crustaceans do something utterly unusual. To scare away predators, they rub a smooth, rubbery protrusion at the base of each antenna against the smooth, hard part of their heads.
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The result is an audible squawk. The spiny lobster became the first known example among animals of the stick-slip phenomenon, a deeply studied principle of science and engineering. It is when two relatively smooth or flat surfaces become repeatedly stuck and unstuck by the forces of friction, creating a vibration that becomes a noise.
It is why brakes and doors squeak on dirty hinges and why wipers chatter on dry windshields. It is why a finger sings on the rim of a wine glass or screeches on the outside of an inflated balloon. And it explains how the bow of a violin, sticking and slipping almost imperceptibly as it crosses the string, creates sound.
“If you’ve ever played a violin, you have to push down and slide at just the right combination to get these two surfaces to stick and slip across each other,” Patek said. “And on each slip it makes a little burst of sound. And that’s the same thing for a spiny lobster.”
And, it now occurs to her, the same goes for something else, something more widely seen around Duke than a spiny lobster.
“A basketball shoe!” she said with a laugh.
In big arenas, like those of the NBA, the squeaking is mostly overpowered these days by the cacophonous, piped-in noise intended as entertainment and disguised as excitement. (The New York Knicks, deep into the experimental part of their losing season, recently played a first half against the Warriors with only ambient noise, telling fans to “enjoy the sounds of the game.” Steve Kerr, the Warriors’ coach, said the atmosphere was “weird,” and a player, Draymond Green, called it “pathetic.”)
But the squeaks will be heard, loud and clear, during the early parts of the NCAA men’s and women’s basketball tournaments. Some teams tip off on neutral courts far from home, where the crowds are reverent or unmoved. Pep bands play only during timeouts, and most sites do not add artificial noise during play. Perhaps unrealized at home, there are stretches when squeaking shoes and the occasional barking of coaches comprise most of the noise in the arena.
Few stop to consider just what it is that they are hearing when shoes squeak. Even fewer have researched the topic. Martyn Shorten, who has a biomechanics consulting firm in Portland, Oregon, and works mostly with athletic shoe manufacturers, is one of them. He presented his findings to the American Society of Biomechanics in 2006, and it remains a go-to resource in the industry.
“The herringbone structures of the shoe outsole are induced to vibrate at their low-order natural frequencies by stick-slip contact with the surface,” Shorten and his research partner Xia Xi concluded.
Those vibrations become sound – quick, high-pitched squeaks. To fans, they are noise. To players, they are necessities.
The squeak is reassurance to a lot of players. They listen for it. It gives them that audio sense of reassurance that they’re sticking.
Leo Chang, senior design director at Nike
“When we’ve tested shoes and they didn’t squeak, it comes up with our players and our testers,” Leo Chang, senior design director at Nike, said. “The squeak is reassurance to a lot of players. They listen for it. It gives them that audio sense of reassurance that they’re sticking.”
The squeak means that they are not just sticking, but sliding. It is a complex, wonky bit of science, deeply researched and best explained by a battery of experts – from shoe designers to rubber scientists, mechanical engineers to acoustic experts, even a biologist familiar with spiny lobsters.
They agree that at the core of a basketball shoe’s squeak is the stick-slip phenomenon – the same principle studied by engineers for its clattering effect on machinery and by seismologists for the way tectonic plates stick and slip to cause earthquakes. When you write on a blackboard and the chalk squeaks? Stick-slip. When you drive a car slowly around a corner in a slick-surfaced parking garage and the tires squeal? Stick-slip.
In basketball, Shorten said, it “tends to happen when the foot first contacts the ground and when the shoe is lightly loaded and moving quite quickly.”
On the foot’s way to stopping, or to twisting or springing to the next step, the sole’s intricate designs stick, then slip, then stick. It might feel like an instant stop, but the rubber sole is designed for flexibility. Too much grip in basketball is jarring on the body, Nike’s Chang said. Not enough traction is dangerous, too.
“I see the squeak as the perfect signal that you’ve got the right amount of slip and grip,” Chang said.
That makes sense to Judit Puskas, a chemical engineering professor at the University of Akron who recently won the Charles Goodyear Medal for her expertise in rubber. She described the “magic triangle” of rubber technology – rolling resistance (its ability to slip), traction (stick) and wear. Changing one can affect the others. The quest is to find balance, whether in designing tires or basketball shoes or anything else with rubber components.
“The sole makes sure you can stop, but also move quickly, which are two competing requirements,” Puskas said. “You want the rubber to stick enough that you can stop, but you don’t want to stick too much that you can’t move.”
The chirp signals balance, but it does not explain how the sound is produced – usually when a player makes a quick stop or a sideways cut.
Greg McDaniel is an assistant professor of mechanical engineering at Boston University, where he runs the sound and vibration laboratory. The basic rule for creating sound is compressing air, McDaniel said.
“If you wave your hand through the air, you don’t hear it,” he said. “Why? Because you haven’t compressed air. The air sloshes around like water sloshes, and the air has enough time to get out of the way before you compress it.”
But in the tiny, vibrating spaces under and within a rubber sole, the air gets compressed.
“That compressed air sucks in neighboring air, causing it to expand, and that expanding air compresses neighboring air, and you get this compression-expansion, compression-expansion, compression-expansion,” McDaniel said. “That’s an acoustic wave. The rubber’s moving, and as it moves, it is compressing air. And it compresses air at the same frequency of the vibrations.”
Shorten’s experiment tested two basketball shoes with herringbone designs, and they measured between 5 and 6 kilohertz – a high-pitched squeak at the high end of the spectrum for human hearing. The basketball hardwood creates not just the perfect frictional foil for rubber soles, but a surface that deflects sound so well that it can carry through an arena.
The squeaks are such an ingrained part of basketball that Nike once created a commercial around them. But Nike has also designed shoes that barely made a sound.
“It’s down to the geometry of the traction,” said Chang, who designs all the shoes for Kevin Durant, among others. “A lot of the herringbone and the more blade-type of geometry tend to give you a little more squeak, versus the more nub, lug-type of geometry, which tends not to.”
Midway through the NBA finals last year, Cleveland’s LeBron James changed shoes to Nike’s Soldier 10. Its nubby sole design was virtually silent.
Those discrepancies gave Patek, the Duke biologist, an idea. Could the squeaks be personalized, tuned so that each player had his or her own sound?
Chang laughed. He and others at Nike have joked about it, he said, but figure it would be difficult to create consistent, repeatable squeaking sounds, given all the various ways that players move.
McDaniel said that shoe sounds probably could be individualized (and probably already are, if unintentionally), but the frequency range of the squeaks is so narrow that they likely would sound the same to most listeners in the crowd.
Until then, fans will just have to appreciate the squeaks for what they are: a persistent reminder that, for as much as the game evolves, there is one part of its tradition that will not be silenced.