That's a interesting test. Roughly speaking, , this is a 2-dim "standing wave" system with boundary conditions that satisfies the wave equation. In the image, the positions where sand accumulates are "nodes", which are the locations where the amplitude of the wave is zero. Therefore, the sand remains stationary. The "specific frequencies" tested by 胡子 are the eigenvalues (eigenmodes) of this system, which depend on the size of the box (boundary). Only at these specific frequencies, the drumhead forms a standing wave pattern as shown in the image. The frequency and wavelength are inversely proportional. The smallest eigenmode corresponds to the longest wavelength and lowest frequency vibration, which is called the fundamental frequency (85 Hz in the video). As the frequency increases (1300 Hz in the video), higher eigenmodes are observed with shorter wavelengths and more intricate patterns in the standing wave (referred to as wave patterns in the video). This makes sense.