SUBROUTINE acoustic3d_f77(N, niters, check)
      INTEGER N, niters, iter
      REAL check
      REAL P1(N,N,N), P2(N,N,N), P3(N,N,N), C(N,N,N)
      INTEGER i, j

      CALL acoust3d_f77_setup(P1, P2, P3, C, N)

      DO iter=1, niters
        DO k=2,N-1
          DO j=2,N-1
            DO i=2,N-1
              P3(i,j,k) = (2-6*C(i,j,k))*P2(i,j,k) + C(i,j,k)
     .          * (P2(i,j-1,k) + P2(i,j+1,k) + P2(i-1,j,k) 
     .           + P2(i+1,j,k) + P2(i,j,k-1) + P2(i,j,k+1)) 
     .           - P1(i,j,k)
            END DO
          END DO
        END DO

        DO k=1,N
          DO j=1,N
            DO i=1,N
              P1(i,j,k) = P2(i,j,k)
              P2(i,j,k) = P3(i,j,k)
            END DO
          END DO
        END DO
      END DO

      check = P1(N/2,N/2,N/2)

      RETURN
      END