  The large frequency separation between modes of the same degree and
consecutive orders in a star is approximately proportional to the square root
of its mean density. To determine the large frequency separation as accurately
as possible a mean large frequency separation computed over several orders is
often used. It is, however, known that the large frequency separation varies
with frequency in a second order effect. From observations it has been shown
that this frequency dependence is more important for main-sequence stars than
it is for red-giant stars. Here we use YREC models to verify and explain this
observational result. We find that for stars with R > 8 Rsun the effect of the
Helium second ionisation zone is relatively small. For these stars the deep
location of the He II zone induces a frequency modulation covering only a few
large separations, while the amplitude of the modulation is low due to the
relatively weak and extended He II layer, causing a shallow wide depression in
the first adiabatic exponent. For less evolved stars the He II zone is located
closer to the surface, and it is more confined, i.e. a deep narrow depression
in the first adiabatic exponent. This causes frequency modulations with
relatively high amplitudes covering up to about 20 large separations, inducing
a relatively large frequency modulation. Additionally, we find that for less
evolved stars the He II zone is stronger and more localised for more massive
stars and for stars with low metallicities further increasing the amplitude of
the frequency modulation.
