  We present an improved model for an asteroseismic diagnostic contained in the
frequency spacing of low-degree acoustic modes. By modelling in a realistic
manner regions of rapid variation of dynamically relevant quantities, which we
call acoustic glitches, we can derive signatures of the gross properties of
those glitches. In particular, we are interested in measuring properties that
are related to the helium ionization zones and to the rapid variation in the
background state associated with the lower boundary of the convective envelope.
The formula for the seismic diagnostic is tested against a sequence of
theoretical models of the Sun, and is compared with seismic diagnostics
published previously by Monteiro & Thompson (1998, 2005) and by Basu et al.
(2004).
