Airborne sound propagates from a sound source in a spherical form by compression and rarefaction of the air. The compression corresponds to an increased sound pressure, the dilution to a decreased one. In air, the speed of sound at freezing point is 331 m/s relative to an air pressure at sea level.
At high minus temperatures, high humidity and in higher air layers it decreases, even below 300 m/s. In water, the speed of sound is 1.4 km/s and in iron 5.1 km/s.