Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; We investigate experimentally and theoretically the acoustic phonon propagation in two-dimensional phononic crystal membranes. Solid-air and solid-solid phononic crystals were made of square lattices of holes and Au pillars in and on 250 nm thick single crystalline Si membrane, respectively. The hypersonic phonon dispersion was investigated using Brillouin light scattering. Volume reduction (holes) or mass loading (pillars) accompanied with second-order periodicity and local resonances are shown to significantly modify the propagation of thermally activated GHz phonons. We use numerical modeling based on the finite element method to analyze the experimental results and determine polarization, symmetry, or three-dimensional localization of observed modes. ; The authors acknowledge financial support from the European FP7 project MERGING (Grant No. 309150), the Spanish MINECO projects nanoTHERM (Grant No. CSD2010-0044) and TAPHOR (MAT2012-31392) and the program Severo Ochoa (Grant SEV-2013-0295). M.R.W. acknowledges support of the Marie Curie Fellowship HeatProNano (Grant No. 628197). ; Peer Reviewed