The movements of a vibrating body, the operation of radio and television sets, the flow of gases, etc. give rise to disturbances in the surrounding atmosphere. These disturbances translate into contractions and dilations of elementary air volumes, corresponding to them respectively:
– a change in pressure, which at rest is the atmospheric pressure P0
– a vibrating movement of the air particles

Spreading out and reaching the perceiver, these disturbances impress the eardrum and consequently the human hearing  system. Therefore, and under certain conditions, you are in the presence of a sound.

For a certain point A, the resulting total pressure corresponds to the sum of the atmospheric pressure P and the pressure due to the mentioned disturbances, and is designated as P(t). In this circumstance we define sound pressure, P(t), which is a function of time, t, as the magnitude.