P Lucas of Palmerston North asks :-

How can glass be broken by sound?

John Campbell, a physicist at the University of Canterbury, responded.

By the boom of a supersonic aircraft or by using a powerful sound wave to set the glass object resonating to destruction.

The standard example is a wine glass. If you tap a thin walled wine glass it will ring for a short time. A crystal (lead) glass will ring for longer which means it is less lossy when vibrating. This is the simple way to select an appropriate glass.

A method of continuously resonating the glass is to run a wet finger around the rim at the right speed which comes with experiment and practice. The catching and slipping of the finger with the glass rim imparts energy to the glass and if the timing is correct the glass will ring continuously. The frequency of oscillation will depend on the glass; its size (diameter), type of glass, and wall thickness. A resonating frequency of a particular glass can be altered by partly filling the glass so a glass can be tuned. Some people play music by resonating an array of glasses tuned to the same notes as a piano. Benjamin Franlin was so impressed he invented the glass harmonica in which glass bowls are rotated on a common axle whilst the player uses moist fingers to play the bowls. Mozart composed music for this instrument.

Like all resonating systems (eg swing bridges and violin strings) if the energy is put in at the same rate as it is lost by the vibration (ie in heating the glass) the system resonates in equilibrium. If energy is put in at a greater rate then the amplitude of the vibration can drive the object to destruction. The forces between the atoms exceed the strength of the material.

It is commonly stated that opera singers can resonate a wine glass to destruction by singing the same note at which the glass resonates, but I dont think this has ever been demonstrated using the unaided human voice. Many years ago I asked John Amis, of My Music fame, and who had good contact with many famous opera singers, if he had ever heard of this being done and he hadn't. It has been done for TV adverts whereby the glass has the speaker from a powerful amplifier near it.

So the way to break glass using sound is to determine the main resonance frequency of the glass object and to use an oscillator, a power amplifier and a loudspeaker near the glass object. There may be other ways, for example using a powerful ultrasound device to heat up a spot on the glass until the thermal strains cause the glass to crack. If it is tempered safety glass it will shatter.