Visualization of equal phase lines of progressing flexural wave

This paper presents educational experiments, in which propagation and interference of elastic waves with a frequency of 25–30 kHz in thin plates are analysed. For obtaining these waves a magnetostriction radiator and an ultrasonic generator are used. The described devices are available for production in an educational laboratory. A standing flexural wave is visualized with dry fine powder. The powder accumulates in the nodes of the standing wave, and the distance between the nodes is equal to a half-wavelength. The unexpected phenomenon of visualization of a progressing flexural wave with powder is considered. The observed pattern consists of concentric circles, the smallest distance between which is equal to the wavelength. A simple theory of this phenomenon is offered. Measurements of the length and the velocity of the flexural wave in the plate are carried out. It is shown that the experimental results are well coordinated with the theory. The material of this paper is topical for students, graduate students and physics lecturers. (paper)