Breaks glass with sound. If you look carefully you can see a broken piece of glass.I built a copy of David Kardelis's glass breaker. <http://www.personal.psu.edu/ref7/apparatus/2006%20competition/kardelis.htm> It breaks pieces of window glass rather than wine glasses. It has some cool advantages over the traditional wine glass breaking with sound.
- The neatest thing is that it debunks the idea that only crystal wine glasses can be shattered with sound.
- You can see from a distance that the glass is moving and the mode of vibration is really obvious.
- The breaking frequency is about 34 Hz, nearly outside the range of human hearing. Much more comfortable.
The glass breaker has three holes. Two supports are added and a strip of window glass is placed on top them. Sound waves come out of the holes, with the two outside holes being totally out of phase with the center hole. The sound is generated by two speakers that face each other inside of the box.
The inside of David Kardelis's box. I forgot to take a picture before I sealed mine up.
I broke 3 inch by 24 inch strips of window glass at 33.5 Hz.
I've made a few changes to make glass breaking more reliable.
First, used window is full of micro-fine cracks almost invisible in the glass. These make the glass easy to break. New window glass is much, much harder to break and when it breaks it makes many small pieces. So, when I use new glass, I score it with a glass cutter around the center to help control the breaking.
Second, I bought a good subwoofer amplifier that cost about $100. Mine is a 12 V car amplifier that I run with a computer power supply. You could buy a plate amplifier too. The advantage over a normal receiver is that the output current and voltage have much lower distortion. Subwoofer amps also dissipate heat better. I have burned out a receiver running it for a long time.
Third, after some excellent advice from fellow physics demonstrators on the mailing list TAP-L, I bought an HP 204C frequency generator. It makes beautiful sine waves. Other frequency generators, especially digital ones, often produce sine waves with jaggeties that seem to interfere with motion. Many of the them make obvious high frequency sound. 
A picture from HP MemoryI bought mine on Ebay for a good price of about $50.
With these changes, it breaks glass more reliably than my wine glass breaker without blowing my eardrums out.