Inelastic neutron scattering, combined with Brillouin and elastic small-angle neutron scattering, were used to test universality in the spectral dimension d̃. The fracton density of states, obtained here on a new fractal silica aerogel and over a large frequency interval, now gives d̃≃1.8. This suggests that d̃ is a sensitive function of the fractal structure.

• Received 21 December 1988

DOI:https://doi.org/10.1103/PhysRevB.39.7384