If you have ever watched a basketball game and noticed that familiar squeaking sound every time a player pivots or stops short on the court, you are far from alone in wondering what is actually causing it. Scientists have been curious about this for years too, and a brand new study published in the prestigious journal Nature has finally cracked the mystery wide open with some genuinely surprising results. Researchers at Harvard University used high-speed cameras to carefully film rubber shoe soles sliding against glass surfaces, and what they captured completely overturned the previous best explanation for the sound. Rather than the simple back-and-forth sticking and sliding motion between shoe and floor that most researchers had assumed, the squeak is actually caused by tiny rippling waves that race along the bottom of the sole at near supersonic speed, peeling away from the surface approximately 4,800 times every single second.
Those rapid, repeating pulses cause changes in the surrounding air pressure that our ears pick up as that unmistakable high-pitched squeal we all know so well from the sidelines. The researchers also discovered that the raised ridges on a shoe’s tread play a surprisingly big role in the whole process, since smooth rubber blocks made more of a broad swooshing noise, while ridged tread created clean and clearly defined musical notes. Even more remarkably, some of the pulses were found to be triggered by tiny sparks of static electricity produced by the intense friction between rubber and floor, a discovery that nobody in the field was expecting at all. The study now opens the door to engineering shoes with tunable squeak frequencies, including versions pitched so high that only dogs can hear them, which the lead researcher jokingly admitted might not go over very well with your pet at home.
