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A theoretical model for calculating spontaneous and stimulated Brillouin seattering(SBS)spectra is described.An empirical formula for the Stokes output spectral linewidth,a function of spontaneous Brillouin linewidth and the exponential gain coefficient,is obtained by the calculated data fitting.The formula holds true for two cases involving pump undepletion and depletion.The lineshape change from spontaneous to highly pump-depleted SBS spectra is also investigated.The result shows that for the pump power below the SBS threshold,the Stokes output spectral lineshape evolves from Lorentzian to approximately Gauesian as the pump power increases.For the pump power near or beyond the threshold,the SBS spectrum is in the form of a steady Gauesian profile,and the spectral linewidth comes to a certain value about 7 times narrower than the spontaneous one.The theoretical results are experimentally demonstrated by using several common liquid media.