The sounds of a basketball game don’t just include the smack of a bouncing ball, players’ shouts and the cheers of a crowd. They also feature lots of squeaking sneakers. Shoes make these sharp sounds as players abruptly stop or turn. Now, physicists understand why.

High-speed videos reveal that as a shoe slides on a floor, it goes through a stick-slip motion. Parts of the shoe’s sole stick in place as other parts slip forward. The shoe slips in bursts, as small regions of the sole wrinkle slightly and detach from the surface. The regular repetition of those pulses creates that squeak.

Adel Djellouli was part of a team that shared its findings February 26 in Nature. Djellouli is an applied physicist at Harvard University in Cambridge, Mass.

In their experiments, the researchers needed to see the sole of a shoe. So they scuffed it across a glass surface instead of a basketball court. Images from below captured the shoe’s sole. The images were bright where the sole touched the glass and dark where the shoe pulled away.

As a shoe goes through its stick-slip motion, they found, pulses of wrinkles travel along its sole. This is a bit like how a wrinkle might travel across a tablecloth as you snap it into place. But in the shoe, the pulses repeat about 4,800 times a second!

When a wrinkle meets the edge of the shoe, it smacks the surrounding air. That changes the air pressure. Repeated smacks make a regular change in air pressure, creating a sound wave.

The frequency of a sound wave determines its pitch. That is, how high or low it is. The pitch of a squeaking shoe matches the frequency of the bursts of slippage on its sole.

Squeaky details

The researchers also tried sliding blocks of silicone rubber across the glass. This revealed that a sneaker’s tread needs ridges to make the sound. As a flat piece of rubber moved across the glass, it made chaotic pulses at uneven intervals. That produced a muddled noise, not a clear-pitched squeak.

Ridges guide the pulses along a shoe’s sole and help organize them. That’s what makes the pulses regular. Djellouli’s team confirmed this by sliding a rubber block with ridges across the glass. It made a clear squeak, just as a shoe would.

The researchers discovered other fun facts about sneaker squeaks, too.

The thickness and stiffness of a rubber block or a shoe sole, for instance, is what sets a squeak’s pitch. That discovery suggests a way to make silent shoes: Tune that squeak to an ultrasonic pitch. Humans can’t hear sounds that high. Shoemakers could do this by making the soles of their shoes thin. Or they could tweak the material used to make the shoe’s sole.

Those changes might not improve athletes’ game, though. Plus, people may not be able to hear ultrasonic pitches, but dogs can. So your “silent” sneakers might be pretty annoying for your pet, Djellouli says.

The researchers also designed rubber blocks that squeaked at specific pitches. Then, they used the blocks to play “The Imperial March” from Star Wars — the song that plays whenever Darth Vader comes on screen. That revealed another scientific truth. Darth Vader would have seemed much less scary if he had squeaky shoes.