| MadSci Network: Physics |
If you tap a wineglass it produces a high-pitched tone. This is the "natural frequency" of the glass. A very steady, very loud sound with this frequency can break the glass because of "resonance".
To understand resonance, consider an adult pushing a child on a swing. If the adult stops pushing, the child will swing back and forth with a natural frequency of about one swing in 10 seconds. If the adult pushes at exactly this frequency the child will swing higher and higher.
Sound is a periodic wave of air pressure. If the peaks in pressure exactly match the natural frequency of the glass, the glass will vibrate more and more violently until it shatters.
The natural frequency of a glass depends on its shape, size, and chemical composition. It is typically between 1,000 and 10,000 vibrations per second (1-10 kHz). Because resonance is difficult to achieve, there is no one frequency that would break all glasses. Also, you can't break a square sheet of glass with resonance.
Very loud noises of any frequency can break glass if the sound wave packs enough energy. This is why an explosion can shatter windows.
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