I hestitate to contribute to this trite little query, but alex.tan's w/u is plain out wrong. (Sorry, alex.tan.) These high-frequency vibrations in the air which fall into the spectrum of human auditory perception are called sound-waves. Right now, my stereo is playing in the living room, but I am unable to hear it. It still produces sound. As to the metaphysical question, our sanity and maybe existance is based on the assumption of a physical world.

"Deep", you say? Deep is a very shallow word.