  This work aims to understand the behavior of non-linear waves in the vicinity
of a coronal null point. In previous works we have showed that high frequency
waves are generated in such magnetic configuration. This paper studies those
waves in detail in order to provide a plausible explanation of their
generation. We demonstrate that slow magneto-acoustic shock waves generated in
the chromosphere propagate through the null point and produce a train of
secondary shocks that escape along the field lines. A particular combination of
the shock wave speeds generates waves at a frequency of 80 mHz. We speculate
that this frequency may be sensitive to the atmospheric parameters in the
corona and therefore can be used to probe the structure of this solar layer.
