s=NDSolve[{D[u[x,t],{t,2}] == D[u[x,t],{x,2}], 
         u[x,0] == Sin[4*Pi*x], Derivative[0,1][u][x,0]==0,
         u[0,t] == u[1,t]}, u, {x,0,1}, {t,0,1}];
Plot3D[Evaluate[u[x,t]/. s[[1]]],{t,0,1},{x,0,1}]
