This term is not as scientifically grounded as it is descriptive, but we do hear it used to describe sound propagation quite a bit. When a sound first occurs there is always an initial wavefront or pressure that is generated in the air. Changing air pressure is how sound is heard by the ear and also how sound is able to move through the air. There are waves of high and low pressure that correspond to the frequency(s) and volume of the sound. The phrase “pressure wave” is usually used to describe the “initial” high pressure zone created by the onset of some sound. For example: If a drummer hits a drum, the movement of the drum head when first struck creates an area of high pressure around the drum that then moves the surrounding air molecules, and so on until it reaches the ear. This is the initial pressure wave. It is followed by other waves of higher and lower pressure that correspond to the sound of the drum.