The graph shows a damped oscillation. What fraction of its starting energy does the oscillator still have at $t = 18\ \text{s}$?

Fig. 1: A graph of displacement x in centimetres against time t in seconds, from 0 to 30 seconds, with grid lines every 3 seconds and every 3 centimetres. A blue curve swings up and down, and its swings get smaller. A dashed red curve joins the tops of the swings. Red dots on it mark 12 centimetres at 0 seconds, 6 centimetres at 9 seconds and 3 centimetres at 18 seconds.