A sound is a longitudinal pressure wave formed by compressions and rarefactions of the medium molecules in a direction parallel to that of the application of energy. In compression zones, the air molecules are forced, by applying energy, to a tighter configuration; in rarefactions zones, the molecules are less tightly packed. ![[20201029125724!GIF_ONDA_PRESION.gif]] Credits: [Wikimedia](https://commons.wikimedia.org/wiki/File:GIF_ONDA_PRESION.gif) The intensity of the sound, $I$, is the power carried by the wave per unit area, and it is measured in $W/m^2$: $ I = \frac{P}{A} $ Where $P$ is the power and $A$ is the area. The sound pressure is the deviation of the ambient air pressure caused by a sound wave. It is often measured on a logarithmic scale with a metric named [[sound pressure level (SPL)]]. The distance between two compression zones is the wavelength of the sound wave that depends on the frequency of the sound and the [[speed of sound]] in the medium. Humans can perceive acoustic waves with frequencies in the range from about 20 Hz to 20 kHz, called the audio frequency range. A sound in that frequency range can be represented as an [[audio signal]].