Suppose I apply a pair of equal and opposite harmonically varying forces perpendicular to an infinite drum membrane. Consider the following forcing functions at two nearby points,(x=0,y=a) and (x=0,y=-a), separated by a distance 2a, F(t,0,a) = Acos(ωt), F(t,0,-a) = -Acos(ωt) Let the forcing functions act not at a point but over some very small area so we don't puncture the membrane. Let ω be small enough so that 2a<<c/ω I suspect that far from the origin, (0,0), that the waves will not propagate equally in all directions? Is there a simple argument one can make to show that my suspicion is correct or incorrect? Thanks!