Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone. The SI unit of sound pressure is the pascal (Pa).

A sound wave in a transmission medium causes a deviation (sound pressure, a dynamic pressure) in the local ambient pressure, a static pressure.

Sound pressure, denoted p, is defined by $${\displaystyle p_{\text{total}}=p_{\text{stat}}+p,}$$ where

In a sound wave, the complementary variable to sound pressure is the particle velocity. Together, they determine the sound intensity of the wave.

Sound intensity, denoted I and measured in W·m⁻² in SI units, is defined by $${\displaystyle \mathbf {I} =p\mathbf {v} ,}$$ where

Acoustic impedance, denoted Z and measured in Pa·m⁻³·s in SI units, is defined by $${\displaystyle Z(s)={\frac {{\hat {p}}(s)}{{\hat {Q}}(s)}},}$$ where

Specific acoustic impedance, denoted z and measured in Pa·m⁻¹·s in SI units, is defined by $${\displaystyle z(s)={\frac {{\hat {p}}(s)}{{\hat {v}}(s)}},}$$ where

The particle displacement of a progressive sine wave is given by $${\displaystyle \delta (\mathbf {r} ,t)=\delta _{\text{m}}\cos(\mathbf {k} \cdot \mathbf {r} -\omega t+\varphi _{\delta ,0}),}$$ where