As the shoe strains to maintain grip, says Djellouli, tiny sections of the rubber sole rapidly change shape, momentarily losing and regaining contact with the floor thousands of times per second. These quick oscillations occur at a frequency that matches the high-pitched squeak heard on the court.
Djellouli's team repeatedly slid a sneaker against a smooth glass plate and recorded the squeaks with a microphone while capturing the motion with a high-speed camera. (Submitted by Adel Djellouli )
It was usually concrete, as that's what most outdoor courts are made of in the States. That's kinda the whole point, i.e. not wooden floors.
That's not correct about the NBA-level forces, as occasionally playing indoors instantly produced that range of squeaking sounds. Little kids produced those sounds as well. It's just the dynamic of sneakers upon wood, essentially.