In this paper, we report the systematic results of folded LA phonon modes in the a-Si: H/a-SiN:H and a-Si: H/a-SiC:H periodic superlattices. We use the elastic continuum model to obtain the theoretical results, which are in good agreement with the experimental values. Finally, the problem of the existence of frequency-gap at the center of the Brillouin zone is briefly discussed.