Shoe soles crease in pulses that repeat thousands of times per second
The soundtrack of a basketball game is punctuated by the squeaking of sneakers. Now physicists understand why.
High-speed video of slipping shoe reveals stick-slip movementa stop-and-go situation in which parts of the sole stay in place while others slide forward. The shoe slides in impulses, like small areas of the sole curl lightly and detach from the surfaceHarvard applied physicist Adel Djellouli and colleagues report in the February 26 Nature. Researchers have found that regularly repeating these pulses produces a squealing sound.
The pulses move along the sole, much like the way a tablecloth can be put into place by sending a ripple of motion onto it. But in the shoe, the pulses repeat about 4,800 times per second, producing a kick that changes the surrounding air pressure to create a sound. The pulsation rate is the frequency of the sound emitted by the shoe, which determines its pitch.
In the experiments, a glass surface served as a substitute for the hard, smooth floor of a basketball court, allowing the researchers to image the shoe from below. Based on a concept called total internal reflectionimages of the shoe sole were light where it touched the glass and dark where the shoe had moved away from the surface.
Further investigation using blocks of silicone rubber revealed that ridges on a sneaker’s tread are essential to sound. A flat piece of rubber, moved along the glass plate, produced chaotic pulses at unequal intervals, producing a garbled noise rather than a clear tone. The ridges help organize the impulses by guiding them, so that the rubber blocks with tread squeak vigorously.
The thickness and stiffness of the block determine the pitch of the sound, the researchers found. And that suggests one way to make quiet shoes: set the squeak to the ultrasonic range, inaudible to humans. This could be accomplished by making the sole thinner – although this may not be ideal for sporting activities – or by changing its composition, Djellouli says. “As long as you don’t mind annoying your dog.”
The researchers also designed rubber blocks that squeaked at specific heights, then used them to play “The Imperial March” from Star Warsrevealing another scientific truth: Darth Vader would have seemed much less intimidating if squeaky shoes had marked his entrance.