A nonscanning Fabry-Perot interferometer was used to examine the molecular weight dependence of the Brillouin spectra of glass-forming polypropylene glycol (PPG) and polypropylene glycol monomethyl ether (PPGME) along with their monomers. PPG series showed very similar acoustic properties except for the PG monomer, while those of PPGME were very sensitive to the molecular weight. These results correlate well with the temperature dependences of the dielectric relaxation frequency and the glass transition temperatures of these molecules obtained from dielectric and thermal measurements. All these results signified that the length scale of hydrogen bonded network of PPG may be so long compared to the length of one chain that the Brillouin scattering reflects the density fluctuations of only local segmental motions.