When my boys were wee little lads, we had a fascinating discussion about personal weapons of all kinds. Being small boys, they were playing a little game of would you rather... with me. Asking, "Would you rather have an airsoft gun or a paintball gun?" and,"Would you rather have a pistol or a shotgun?" and other similar types of questions.
I was getting tired of the game, so on one of the Would you rather questions, I answered with something completely different. I described to them the perfect defensive weapon. A weapon that could destroy the molecular integrity of any gun or knife or even the projectile that the weapon fired. A weapon of directed sound waves, but a silent weapon that emitted a sound that could not be heard by humans.
I told them how this weapon had not yet been invented, but that if we could invent it, we would be wealthy beyond our wildest dreams.
Well, I was wrong. This weapon had been invented.

Dr. Vladimir Gavreau, a Russian born scientist working in France to develop mechanical drones for the military accidentally discovered just how such a weapon could work.
In the late 50’s and early 60’s Vladimir Gavreau, a robot scientist of Russian birth, and his assistant found that without warning they would become nauseous and have unbearable headaches when working in their lab. As soon as they left the lab the symptoms disappeared immediately.
They knew something in the lab was causing their sickness but had no idea what it was. Eventually, they noticed that a cup of coffee on the bench had strange ripples on it, at the same time as the ripples started they began to feel ill. When the ripples stopped they were fine again.
They found that the illness, and the coffee cup ripples stopped when certain windows were covered over. Extensive inquiries and dozens of tests lead to the discovery that a faulty motor driven ventilator had been installed in the building.
It’s movement had caused an infrasonic resonance, that when coupled with the concrete in the building, formed a huge infrasonic amplifier at a resonance impossible for humans to hear but able to make them ill.
SOURCE