Combine Egs(19-1)and(19-6), we have Os△p(x,t △Pnsm(k x-ot s(x, t=Sm cos(kx-@t) (198) k kB I >u,(x, t)=2=um sin (kx-at)(19-9) v△ B u(r, t)is the velocity of oscillation of an element in fluids v=@/k is the velocity of sound wave(19-8) kB P k s m m m  =  =  0  s(x,t) s cos(k x t) = m − ( ) sin ( ) um k x ωt t s u x,t x = −   = B v P u m m  = Combine Eqs. (19-1) and (19-6), we have: sin( ) ( , ) k x t x t x s 0 m 0 −   = −   = −   (19-9) v= /k is the velocity of sound wave. μ (x,t) is the velocity of oscillation of an element in fluids. x 