  The structure of the surface layers of the Sun is changed by magnetic
activity which, in turn, changes the eigenfrequencies of the acoustic modes.
These frequency shifts have been observed both in low- and high-degree data,
and are found to be correlated with the solar cycle. Time-distance
helioseismology has shown that waves travelling through sunspots suffer time
delays. Also it is known that sunspots induce phase shifts between inwardly and
outwardly propagating waves.
  In the present work we show how these phase shifts are related to the
observed time delays by treating the scattering of acoustic waves off sunspots
as a diffraction problem.
  These time delays are then related to the frequency shifts of the modes. From
this relation we obtain a lower limit for the contribution of sunspots to the
observed frequency variations with the solar cycle. Moreover, we predict the
frequency dependence of the time delays suffered by rays travelling through a
sunspot.
