  The paper gives first quantitative estimates on the modulus of continuity of
the spectral measure for weak mixing suspension flows over substitution
automorphisms, which yield information about the "fractal" structure of these
measures. The main results are, first, a Hoelder estimate for the spectral
measure of almost all suspension flows with a piecewise constant roof function;
second, a log-Hoelder estimate for self-similar suspension flows; and, third, a
Hoelder asymptotic expansion of the spectral measure at zero for such flows.
Our second result implies log-Hoelder estimates for the spectral measures of
translation flows along stable foliations of pseudo-Anosov automorphisms. A key
technical tool in the proof of the second result is an "arithmetic-Diophantine"
proposition, which has other applications. In the appendix this proposition is
used to derive new decay estimates for the Fourier transforms of Bernoulli
convolutions.
