Figure 3: High-frequency, microscopic gap oscillations at the interface.

a, High-speed video stills of a single impact (H_i ≈ 3.5 cm) at high magnification and frame rate (15,625 fps) reveal that a minute gap below the sphere rapidly opens and closes many times during each impact (see Supplementary Movies 3 and 4). The timescale for one cycle is about 0.5 ms. b, Plotting the gap height (averaged over the central 100 pixels) versus time shows that the deformation of the sphere underbelly is on the order of 10² μm. c, The power spectra of the gap and the audio signal both have clear peaks around 2–3 kHz, which indicates that the gap oscillations are the source of the screeching.