Is it possible to break a glass while singing?
In the history of opera, there are accounts of singers able to break a glass with their voice. According to physics, every object has a resonant frequency - i.e. the frequency at which it naturally vibrates. This means that at this frequency the object cannot dissipate its energy, which keeps building up. When a certain treshold is reached, the accumulated energy needs to do something and the object breaks. Therefore, a loud enough tone sung for enough time will result in the breaking of the resonating glass.
We can easily test this principle with our voice. First, we need to choose a suitable glass. Not all glasses are suitable. Those with a very elastic envelope will dissipate energy better than those with a rigid envelope. The easiest to break glasses are made with bohemian crystals, which contains an important part of lead. The added metal confers the typical rigidity of this glass and makes it very musical. When you hit it, you hear a very clear and loud sound.
The next step involves figuring out the exact natural frequency of the glass, which is also its resonant frequency. We can do this by running a wet finger around the rim and making it "sing". Or we can simply tap the glass, which will cause it to vibrate and push out waves of air pressure. Our ears and brain interpret these waves as sound, with the pitch of the sound determined by the frequency of the waves.
Unless we have a trained ear to recognize the exact pitch of the note, we need to have a microphone connected to a computer and an oscilloscope software. Through FFT, the software will tell us the pitch. An alternative is using an acoustic tuner.
Now, we are ready to break our glass. It is advised we wear some protection and glasses. We will sing the exact note with our mouth close to the glass, gradually increasing the volume. At a certain point, the glass will break.
To avoid unwanted security risks, we can also pre-record our voice and play it through a speaker in front of the glass. In this case, we can also use a strobo light and a video recorder, to capture what happens at resonance. Strobo lights generate rapid pulses of light, allowing us to capture the movement of the glass at different times in the movie. Therefore, we will be able to see the enormeous motion caused in the glass by resonance. The envelope will seem like a non rigid surface, collapsing upon itself, till it breaks.
In general, singers report that they need to sing really hard, when shattering occurs. Some find that the breaking point is not at the exact resonant frequency, but a little displaced further up. They find it easier to sweep a little upward around the resonant frequency. This should have the effect of inducing a turbolence during resonance, so that breakup is facilitated.