Very-Low-frequency evanescent liquid pressure waves
Keywords:Density of liquids, Elastic waves, Waves, Classroom demonstrations, Linear oscillations, Liquid particle, Liquid pressure, Spatial extension, Standing wave, Thermal motion, Very low frequency
AbstractEvanescent liquid sound-pressure waves are standing waves of limited spatial extension. These waves involve variable pressure and liquid-particle velocity but negligible liquid-density variation: they are predicted to occur also in strictly ideal liquids. The present note is intended to show that classroom demonstrations of these latter waves can provide a plausible introduction to the subject of spring-driven evanescent liquid sound-pressure waves. That inaccuracy occurs because the small- displacement approximation has been used. In both cases, standing waves with linear oscillations of liquid particles along time-independent streamlines are generated. Since thermal motion and diffusion of the molecules are neglected, in the no-wave state the liquid particle is concluded to keep its initial place and shape.
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