Sound is a pressure wave that travels through air or other media and can be detected by the human ear within a range of 20-20,000 cycles per second. Sound is produced by vibration and travels by pushing adjacent particles, propagating outward from the source. It can travel as longitudinal waves through solids, liquids, and gases, with particles vibrating parallel to the direction of travel. The Doppler effect describes how the observed frequency of sound waves is different for an observer moving relative to its source.