A human voice can shatter a glass. Every object has a resonant frequency – the natural frequency at which something vibrates. Wine glasses, because of their hollow shape, are particularly resonant. If you run a damp finger along the rim of a glass, you might hear a faint, ghostly hum – the resonant frequency of the glass. Or you can simply tap the glass and hear the same frequency. To shatter the glass, a singer’s voice has to match that frequency, or pitch, and the glass must have microscopic defects.
As you heard in the video above, the singer has to be very loud – an ear-splitting intensity of at least 105 decibels, according to Jaime Vendera, a vocal coach who demonstrated his technique for the Discovery Channel in 2005. Conversational speech is normally around 50 decibels. A jackhammer operates at a loudness of about 90 decibels. The human threshold for pain comes at about 120 decibels. Highly trained opera singers can sustain notes above 100 decibels. But no matter how loud the sound, if the pitch doesn’t match the resonant frequency of the glass, the glass will reflect most of the energy and won’t break.
And the singer also needs to hold that note for at least two to three seconds for the vibration to build up enough to cause the glass to shatter. It helps to have the right kind of glass – a large one – with thin, nearly vertical sides. In the late 1970s, laboratory experiments with a professional soprano and a trumpet player showed that neither could shatter glass. The famed tenor singer Enrico Caruso was said to be able to do it, but his wife denied it.
If you try this at home, be sure to wear eye protection!
The Discovery Channel MythBusters show recruited Jaime Vendera for a demonstration and produced the first documented proof that an unassisted voice can shatter glass.