Sound is a wave that moves through matter. Sound cannot exist in a vacuum. The speed of sound increases with the density of the material its passing through. In fact, the denser the material the more conductive it is to sound waves. Sound is just the vibration of matter, that is all... a rhythmic movement of matter at a frequency that we dont feel on our skin, but that we "feel" with the nerves in our ears... our brains interpret it as a sound. Sound, therefore, is really just an illusion of the brain, a perception of what is nothing more than a vibration of matter, a wave, that is at a frequency that is "audible", or detectable within our ears.
