Acoustic water dance / Parametric waves

--== Video featured in one of episodes of "JoeGenius" (see here - https://www.youtube.com/watch?v=0OWZWMg6xkQ ) ==-- Simple sound experiment - I filled plastic cup with water and placed directly on speaker cone. Then used PC and software SineGen to generate sine waves from 300Hz down to 15Hz. Result is similar to cornstarch videos in YouTube, only everything is happening much faster. Interesting note: sometimes vibrations are in sync with light bulb 50hz frequency, sometimes with cameras 30fps and sometimes it simply resonate. All this variety gives interesting wave dance. Upd.#1 - tried natural oil, nothing interesting. Upd.#2 - tried triangle/square waves. very similar to sine but at 1:05-1:25 waves were stationary instead of slowly moving around.