We present results of simulation of time dependences of the Bi12SiO20 crystal transmission coefficient with harmonic wavelength modulation of probe radiation. The possibility for estimation of the spectral position of the Gaussian curves descriptive of the probability of intracenter transitions that contribute to the extrinsic absorption in sillenite-type crystals is analyzed. It has been shown that the amplitude of the second Fourier harmonic in the expansion of the time dependence of the transmission coefficient of the sample under investigation peaks near the central wavelengths of the intracenter transitions.