In acoustics, Stokess law of sound attenuation is a formula for the attenuation of sound in a Newtonian fluid, such as water or air, due to the fluids viscosity.  It states that the amplitude of a plane wave decreases exponentially with distance traveled, at a rate  given by
where  is the dynamic viscosity coefficient of the fluid,  is the sound's angular frequency,  is the fluid density, and  is the speed of sound in the medium.
The law and its derivation were published in 1845 by the Anglo-Irish physicist G. G. Stokes, who also developed Stokes's law for the friction force in fluid motion. A generalisation of Stokes attenuation taking into account the effect of thermal conductivity was proposed by the German physicist Gustav Kirchhoff in 1868.
Sound attenuation in fluids is also accompanied by acoustic dispersion, meaning that the different frequencies are propagating at different sound speeds.