The transfer matrix method has been used to calculate the Raman spectra of acoustic phonons in nearly periodic SiGe/Si superlattices,including the supperlattices with finite number of periods,the coupled supperlattices,and those with layer-thickness fluctuations.The properties of the above nearly periodic systems are found to be very different from those of periodic or disordered systems.In stead of the folded acoustic phonons appearing in the Raman spectra from ideal superlattices (main peak),the other satellite peaks are also observable in the finite system due to the existence of boundaries.For the coupled superlattices,the main peaks split into two due to the interference of the acoustic waves in different superlattices.For the superlattices with layer-thickness fluctuation,the main Raman peaks show asymmetric broadening and a tail towards high wave number are observable.The calculated results are compared to the measured spectra and good agreement between them is obtained.