The technique of processing data in the wavenumber domain based on the Spatial Fourier Transform (SFT), is a powerful tool to compute higher-order partial derivatives occurred in the expressions of the structural intensity and its divergence. However, performing directly the SFT usually results in great distortions if a discontinuity occurs in spatial periodicity (leakage effect). The worst thing is that the divergence of a free plate cannot correctly be estimated by existing wavenumber processing such as the STF and zero padding method. In this paper, a new algorithm -- mirror processing, is developed. By the use of vibrating velocity measured from the technique of laser scanning vibrometry, the structural intensity, its divergence and the force distribution are evaluated by different techniques of wavenumber processing. It is shown that the distortions caused by leakage effects can be removed by using advanced algorithms.