Sketch of the normal modes associated with P_L (top) and P_H (bottom) of an infinite array of aligned strands along the horizontal axis. The strands are equally spaced with a separation d between the strands. Arrows indicate the direction of the motion. In the top panel, the strands move along the horizontal axis and in the bottom panel the strands move perpendicular to this axis. The three dots at both ends mean that the system continues to the infinity in both directions. The numbers indicate how the strands are labelled.