The spectrum is symmetric and shows three almost undamped modes (small negative real part), whereas the other modes are heavily damped. Their numerical values are ω ω 1 = − − 0.201 , ω ω 2 / 3 = − − 0.223 ± ± i 62.768 respectively. The real one corresponds to the mean of the field, whereas ω ω 2 / 3 corresponds to the imposed pattern with f = 10 Hz . Yielding a relative error of −1/1000. Increasing the noise to 10 times the signal value yields about the same error. The real and imaginary part of one of the latter two eigenmodes is depicted in the following figure.