A sound wave is a pressure wave that is perceptible to the human ear. Sound waves can be produced by the human voice tract, machines, animals and other sources, and disseminate in a variety of mediums: solid, liquid or gaseous. Sound waves are subject to the usual physical effects of reflection, diffraction, breaking and absorption. The speed at which sound waves disseminate depends upon the transmission medium: 330 metres per second in the air, several thousand metres per second in some solid materials. The audible frequency range for the human ear is between 20 and 20,000 Hz. Sound outside this range is referred to as ultrasound or infrasound. Sound waves cannot disseminate in a vacuum, as they always require a transmission medium. In this respect, sound waves differ from electromagnetic waves, for example.