  The aim of this paper is to model the propagation of slow magnetohydrodynamic
(MHD) sausage waves in a thick expanding magnetic flux tube in the context of
the quiescent (VAL C) solar atmosphere. The propagation of these waves is found
to be described by the Klein-Gordon equation. Using the governing MHD equations
and the VAL C atmosphere model we study the variation of the cut-off frequency
along and across the magnetic tube guiding the waves. Due to the radial
variation of the cut-off frequency the flux tubes act as low frequency filters
for waves.
