How does sound break glass?

We’ve all heard about opera singers breaking a glass using voice but how does sound break glass and is it really possible at all?

The answer is that an empty wine glass can be broken by sound alone if the sound is at the right frequency and volume.

The following is a brief explanation of how sound breaks glass. Don't try this at home! 

A glass has a natural resonance. Resonance is the natural frequency at which the glass will readily vibrate. To find the resonance of the glass, ping the glass and listen to the sound. That is the correct frequency (or tone) for the glass to start to vibrate.

The glass itself must not be affected by damping. Damping is interference with a force which will act to prevent the glass from vibrating. Embossed glasses should be avoided. Lead in lead crystal may provide damping although otherwise the glass is very pure. The glass should be empty.

A wine glass is a good shape because it stands on a stem. This reduces the amount of damping that could be caused by a glass sitting directly on a surface such as a pint glass. The walls of the glass should be as thin as possible.

Making the same tone as the natural frequency of the glass will induce vibration in the glass. However, the note alone is not the only factor - volume is also important. The louder the sound, the more violent the vibrations will be. When they reach a level that the glass cannot withstand it will shatter.

The volume required is more than 100 db, a level that is difficult but not impossible to reach with just the voice. Normal speech is around 50 db. In experiments, often an amplifier and speaker are used so achieve the required volume.

This works even better if the sound is concentrated on one area by putting the glass behind a wooden screen containing a hole. The sound is directed through this hole and on to the glass. Research has been carried out which shows that the glass needs to contain a small flaw. The shattering starts with this small crack. The blemish can be too small to be seen by the naked eye. Good quality glasses with no flaws are unlikely to break.

Opera singers have been rumoured to break glass with just their singing note. They have been trained to produce more than 100 db of volume and would have good pitch to match the resonance of the glass.

More recently, rock singer Jamie Vendera has been filmed successfully breaking a wine glass using just his voice. To do this, he directs the sound on a concentrated area of the glass very close to his mouth. To help him check he has the right frequency, he places a straw in the glass. When it starts to vibrate he knows he is close.

Hope this answers the question "How does sound break glass?".

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