The incremental variation in the static pressure of a medium when a sound wave is propagated through it. Sound refers to small-amplitude, propagating pressure perturbations in a compressible medium. These pressure disturbances are related to the corresponding density perturbation via the material equation of state, and the manner in which these disturbances propagate is governed by a wave equation. Since a pressure variation with time is easily observed, the science of sound is concerned with small fluctuating pressures and their spectral characteristics. The unit of pressure commonly used in acoustics is the micropascal (1 μPa = 1 μN/m² = 10⁻⁵ dyne/cm² = 10⁻⁵ μbar). One micropascal is approximately 10⁻¹¹ times the normal atmospheric pressure.