If you have ever heard me sing, I’m sorry. If you have, you’re lucky. That being said, I am an awful singer. When I was little I would always try and sing to break glass, I always wanted to witness that happen. I thought I was bad enough to make it happen, but clearly not. How high do you have to sing to make that happen??
The note that a wine glass makes when you hit it is its resonant frequency https://en.wikipedia.org/wiki/Resonance – the frequency at which the glass vibrates most efficiently. If you can match this tone, which I cannot you can break the glass. The note is around one octave above middle C, https://www.youtube.com/watch?v=URkQGLBseAM depending on how big or small the glass sits. The hardest part is singing loudly enough, but that isn’t hard for me, just ask my friends. Even with expensive, glasses, which vibrate with a clear tone that doesn’t break quickly, the amplitude required is at the limit for the human voice. An untrained singer can do it relatively easily with good amplification so there is hope for me, but achieving the same thing without a speaker has only been done once on TV, by singer and voice coach Jaime Vendera for the Discovery Channel’s Mythbusters show.
You know when you were out for dinner and you would sit in the restaurant with your class cup and you would rub your finger around the top to make that humming noise? Well that process uses the same physics as making the glass shatter from singing. Breaking glass with sound is hard; it really takes a lot to get it to work. The glass in question needs to be crystal. Crystal resonates all at one particular harmonic frequency. If you can match the pitch of your voice to the resonant frequency of the glass the vibrating air will start the glass vibrating too. If this can be done with sufficient volume, the glass will try to move in, “its vibration farther and faster than the material in the glass is able to move, and the glass will break under the strain.” Check out this funny video of a little kid breaking the glass with his voice. https://www.youtube.com/watch?v=sH7XSX10QkM