You can pop open a chicken skull with a constant 5Mhz frequency. BTW, that's the frequency at which a chicken head resonates.
Lets put it this way, any solid object has a resonating frequency. This is how resonating works and how my old science teacher explained/demonstrated it.
Take a ball tied to a rope which is tied to a pole. Now tie the exact same thing on the other end of that pole. So you have 2 balls tied to opposite ends of a pole and dangling down. No start swinging 1 ball, it doesn't matter which. The energy of the swing will transfer into the pole as vibration. When that vibration frequency reaches the proper resonating frequency, the pole will vibrate exactly as the original ball does and transfer that vibration down it's entire length which will in turn transfer that vibration and energy to the other ball hanging down the opposite end of the pole. Eventually that ball will start swinging a little bit as well.
The only problem is that the first ball will run out of energy eventually from swinging and so the second ball will stop, but ifyou keep pumping more energy in, it will keep the second ball swinging too if you found the right frequency to swing the first ball at constantly.
Get it? Well sound is nothing more then air vibration and energy. EVERYTHING vibrates and finding the right frequency to cause that object to keep vibrating and building energy can eventually break that object. Using ones own voice to break glass in this manner can be and has been done. However, IT IS EXTREMELY DIFFICULT TO DO. Why? cause a different glass of a differently shape, size, consistancy, and what not is going to have a different resonating frequency. The resonating frequency is the frequency at which the glass conducts the energy vibration instead of trying to absorb the vibration.
BTW, if you know the resonating frequency at which a human skull pops.. you could do some really mean things to people with sound alone. And NO, I'm not going to give out that info.