The size-dependence of the vibrational mode frequencies of octahedron-shaped gold nanocrystals has been measured by micro-Brillouin spectroscopy. A finite element analysis reveals that the nine well-resolved peaks observed are due to confined acoustic modes with each peak arising from more than one mode. The elastic constants of the nanocrystals are found to be comparable to those of bulk gold crystals. Findings suggest that the eigenfrequencies of any free regular-shaped homogeneous object always scale with its inverse linear dimension. Additionally, this universal relationship is valid for such objects of any size in the classical regime and is independent of elastic properties. © 2011 American Institute of Physics.