QUESTION #232

# Can sounds actually break glass? If so, is there a special frequency or decibel level needed?

Of course! Yes! Sound can indeed break glass. This is a wonderful question. What make this a good question is that it indicates that, like many people, you do not realize how powerful unseen forces can be. It is good that you have understood enough to articulate this kind of question.

Try this little experiment: Fill a large balloon as fully as you can with air. Next, turn on your CD player and hold the balloon. What do you feel? What would happen if you turned the volume up higher; or down lower? Are there any changes to what you feel? Of course there are! What you are feeling is the power of sound.

A fine crystal wine glass will respond to the sound in the same way as the balloon and for the same reason. For that matter so will your least expensive ice tea glass! But with the tea glass, and the windows and the picture frames and the computer screen and every other piece of ordinary glass, the sound energy is never focused within the structure of the glass itself. In other words the glass is not made with ordered molecules of glass. Now, if you turned the volume way up I suppose you could break even these types of glass! But this would more likely be because the sound waves themselves broke the glass by sheer force of impact! Wow! That would be some sound! You could break other things this way too, like your own ear drums!

The wine glass is a different story. This glass has a structure within which the sound can become focused and as the volume is increased the vibration of the glass along these lines of focus become greater and greater until they break. The really neat thing is that they break all at once and the glass shatters!
Answered by: Tom Young, M.S., Science Teacher, Whitehouse High School, Texas

Yes, they can. What is needed is a sound at one of the natural (resonant) frequencies of the object you want to break. You could find the frequency needed to shatter a wineglass, for example, by tapping it sharply with a spoon (not hard enough to break it!) - the sound you hear is the one you require.

You need to put enough energy into the object to actually break it, so that the amplitude (size) of the resonant response is sufficient to propagate cracks in the material, and so shatter the glass.

Glass is great for this, since it is brittle (Which means that there are not many energy absorbing mechanisms in glass). The energy all goes into extending the length of cracks, which happens very quickly and so shatters the object.