Aside from the function accomplished with an audio compressor, compression is an area of increased air pressure caused by a sound wave. Sound waves, which are caused by a vibrating source in the atmosphere (such as a speaker), propagate as waves of compressed and uncompressed air pressures. The changes in pressure are very, very minute in comparison to meteorological pressure differences, but our ears are quite sensitive to the vibrations, which we pick up as sound. In a graphical depiction of a cyclical waveform, compression occurs when the wave is in the top segment (approaching what is known as the node).