How does the shape of a glass affect how much you drink? A scientist set out to examine just this, and the results are rather astounding.
When the bottle vibrates, the water inside has to vibrate with it. Having to move all that water slows down the vibrations of the glass and that in turn slows down the frequency of the sound waves, producing a lower pitch. Adding more water slows down the bottle’s vibrations even more, creating an even lower pitch.