Stokes's law of sound attenuation applies to sound propagation in an isotropic and homogeneous Newtonian medium.  Consider a plane sinusoidal pressure wave that has amplitude  at some point. After traveling a distance  from that point, its amplitude  will be
The parameter  is a kind of attenuation constant, dimensionally the reciprocal of length.
In the International System of Units (SI), it is expressed in neper per meter or simply reciprocal of meter (m). That is, if  = 1 m, the wave's amplitude decreases by a factor of  for each meter traveled.