Can people actually shatter glass by singing a particular note? I’ve seen it on television, but is that just Hollywood magic?

It’s no challenge to break a glass by dropping it. It’s trickier – but possible – to use the power of sound to shatter glass. But not just any loud sound will shatter a glass. Each individual glass is most vulnerable to vibrations of a specific rate – the so-called “resonant frequency.”

Every object has a resonant frequency – that’s the natural frequency at which an object vibrates. 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, the singer’s voice has to be able to match that frequency – or pitch.

Also, the singer has to sing very loudly. A jackhammer operates at a loudness of about 90 decibels. The human threshold for pain comes at about 120 decibels. To shatter a glass, a singer has to produce a note with an ear-splitting intensity of at least 135 decibels. No matter how loud the sound, if the pitch doesn’t match the glass’s resonant frequency, 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.
So there are physical conditions under which a human voice should be able to break a glass. But it’s unlikely that an unamplified human voice could actually do it.

Written by Marc Airhart

http://www.earthsky.org/faq/resonant-frequency