For the generation of second-order random waves in a flume the control signal for the wave board has to be correct up to second order. An expression for this control signal is derived with the perturbation method of multiple scales. It is much less complex and requires less computation time than the expressions obtained from the full second-order theory. A verification of the new method for second-order sub harmonics is provided for bichromatic and continuous first-order spectra. The data are analysed with the complexharmonic principal-component analysis to reduce the influence of noise. The validity of the new method is confirmed.