Posted by jay on Tuesday, July 20, 2010 at 1:30am.
The pouring of liquid into the glass can excite weak "organ pipe" vibrations of the air column above the liquid. As the column of air gets shorter, the frequency that you hear gets higher.
It has nothing to do with tapping the glass, but a similar thing can happen with the vibrations of the glass itself if you do; in that case the frequency gets lower as the glass is filled.
Plunger Jihads for the win!
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