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If a scream resonates with the natural frequency of the glass for long enough, it's possible that the glass would shatter, but a variety of factors are involved. First, the type of glass matters. As Louis Bloomfield of How Things Work points out, the glass usually found in windowpanes and cups is relatively soft, so it vibrates poorly and has no strong natural frequencies. If you tap a glass of this sort, all you hear is a dull "thunk" sound. There's nothing with which a high-pitched sound can resonate. Crystal is better suited because it vibrates well and emits a clear tone when you tap it. Lead oxide is added to the glass, making the
resulting crystal stronger than ordinary glass. Crystal wine glasses work well for this experiment because, in addition to being crystal, they are thin and delicate, and the tubular shape enhances the sound frequency. The real trick to breaking glass with sound is to match the sound's frequency to that of the glass. You might be able to do this with a scream, but it's easier for a singer with perfect pitch to create the right note, especially if that singer's voice is amplified. Each glass will have a slightly different natural frequency due to minute variations in shape and composition. When the high-pitched sound and the glass resonate, it causes the glass to vibrate. If the singer keeps singing the same note at high volume, eventually the glass will vibrate itself into pieces.
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