How can glass be shattered with sound?

How can glass be shattered with sound? — JI, Rapid City, SD

When sound shatters glass, it breaks the glass in the usual way: by distorting the glass to its breaking point. Whenever glass is bent too far, a crack propagates into the glass from its surface (usually at a defect) and the glass tears. For sound to cause this tearing process, the sound must distort the glass substantially. An extremely loud sound can distort the glass to its breaking point in a single motion. For example, an explosion shatters windows when a surge in air pressure (which you hear as a very loud “pop” sound) exerts so much force on those windows that they bend and break.

However, a moderately loud tone can also break certain glass objects by pushing on those objects rhythmically until they distort beyond their breaking points. To understand how that’s possible, recall that you can get a child swinging strongly on a playground swing either by giving the child one hard push or by giving the child many carefully timed gentle pushes. The gentle pushes transfer energy to the child via a mechanism called resonant energy transfer—the child is exhibiting a natural resonance and you are using that resonance to transfer energy to the child a little bit at a time.

While most glass objects exhibit only very weak natural resonances and are therefore extremely difficult to break via resonant energy transfer, a good crystal wineglass is resonant enough to be broken by a loud tone. You can hear the appropriate tone by flicking the wineglass with your finger. If the wineglass emits a clear bell-like tone, you will be able to break that wineglass by exposing the wineglass to a loud version of that same tone. When the wineglass is exposed to this tone, it begins to vibrate in its natural resonance. Each rise and fall in air pressure associated with the tone adds energy to the vibrating wineglass until its surface is distorting wildly. If the tone is loud enough and its pitch is exactly right, the wineglass will distort a remarkable amount and it may shatter. I know from experience with this effect that the distortion a crystal wineglass can undergo without shattering is amazing—it usually won’t break until it’s upper lip is almost as oval-shaped as an egg. Finding the right tone and holding that tone accurately enough and loudly enough requires sophisticated equipment. Few humans have any chance of breaking a wineglass because the pitch accuracy and volume needed are beyond the abilities of all but the most remarkable opera singers. However, Enrico Caruso was apparently able to do this trick with a wineglass held directly in front of his mouth. Note also that normal window glass and normal drinking glasses are made from soft forms of glass that exhibit no strong resonances—if you tap them, you hear only a dull “thunk” sound, not a bell-like tone. As a result, you can’t break them with tones.

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