  Impinging compressible jets may cause deafness and material fatigue due to
immensely loud tonal noise. It is generally accepted that a feedback mechanism
similar to the screech feedback loop is responsible for impinging tones. The
close of the loop remained unclear. One hypothesis hold up in the literature
explains the emanated sound with the direct interaction of vortices and the
wall. Other explanations name the standoff shock oscillations as the origin of
the tones. Using direct numerical simulations (DNS) we were able to identify
the source mechanism for under-expanded impinging jets with a nozzle pressure
ratio (NPR) of 2.15 and a plate distance of 5 diameters. We found two different
types of interactions between vortices and shocks to be responsible for the
generation of the impinging tones. They are not related to screech.
