A periodic wave is a series of regular timed disturbances in a medium. The mediums that carry such waves varies, but for periodic sound waves, air is a common medium.
Periodic waves are usually characterized by their amplitude, frequency and wavelength. The amplitude is the vertical distance between one of the wave’s troughs and the next crest. The wavelength of a periodic wave refers to the distance between sequential troughs or crests. The frequency of a wave is the number of troughs or crests that occur over time. The wavelength of a periodic wave is inversely proportional to its frequency.