Does anyone else ever hear a sound like a metal ball or pin or marbles hitting the floor which disturbed you way more than it should? This is called a piezoelectric transducer. Its the same principal as an ultrasound probe but en large and crude. If you put current through a quartz crystal or piezoelectric element you produce a sound, and likewise if you strike a crystal or element it produces current and, with the current, an electromagnetic field. The thickness of the crystal or element determines the frequency of the emf field. A large thick crystal when struck will produce a very low frequency emf which, like infrasound (sound waves below 20hz) travels easily through solid material and fills you with a haunted, dreadful, uncomfortable feeling that makes you miserable. This is the same reason that pipe organs in cathedrals are so awesome. The military is developing man-portable weapons based on this principal called BARRACUDA but thats all classified for now.