AudioScope
Why Does Water Lower a Wineglass’s Note?
Hear why adding water can lower a tapped wineglass’s pitch while raising the pitch of a blown bottle.
What is vibrating?
Tap a wineglass and the rim flexes. The moving glass carries some of the liquid with it. If stiffness stays the same, that added inertia lowers the frequency of the vibration. Blow across a bottle and the air in its neck moves against the air in the cavity. Adding water shrinks that cavity and can raise the note.
The 1,000 Hz and 800 Hz tones in the video illustrate the change in pitch. They are model tones. Actual results depend on the shape of the glass, how much water it holds and how the vibration dies away.