  We use dynamic coherent backscattering to study one of the Anderson mobility
gaps in the vibrational spectrum of strongly disordered three-dimensional
mesoglasses. Comparison of experimental results with the self-consistent theory
of localization allows us to estimate the localization (correlation) length as
a function of frequency in a wide spectral range covering bands of diffuse
transport and a mobility gap delimited by two mobility edges. The results are
corroborated by transmission measurements on one of our samples.
