Excellent question! It actually depends on
the glass. Crystal works because the way the
atoms line up in crystal makes the wine glass
vibrate in just the right way. In glass wine
glasses, the atoms line up differently, and they
don't vibrate the right way. That's why crystal
wine glasses "hum" when you run your wet finger
along the rim, and glass wine glasses don't.
Singing really loudly (or making any noise
that's loud enough) causes the crystal glass to
break only when the song (or other noise) is loud
enough to cause the crystal glass to vibrate so
hard that it can no longer hold itself together.
The pitch and loudness of the sound necessary to
do this will vary depending on the size and
thickness of the crystal glass. Larger or thicker
glasses are stronger, so it will take a louder
volume and probably a higher pitch (higher
pitched sounds carry more energy) to break the
glass. Smaller and thinner glasses will break at
lower volume levels, and probably at lower
pitches. The design of the glass probably affects
its structural strength,too.
Either way, the sound has to be
very, very loud. Don't try this at home, unless
your parents like really loud noises and don't
mind losing their crystal wine glasses!
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