The ratio of the complex amplitude of spatially averaged acoustic pressure over a surface to the complex amplitude of volume velocity on that surface. The unit is the newton-second/meter⁵, or the mks acoustic ohm. In the cgs system the unit is the dyne-second/centimeter⁵. The volume velocity is the integral over the surface of the particle velocity normal to the surface. Acoustic impedance measures the degree to which an acoustic medium impedes the fluid motion created by an imposed pressure. It was originally defined by analogy to electrical systems.