As the above data are sampled at 48 kHz and 512 points
were used to do the FFT analysis, the frequency granularity
in spectrum analysis (calculating ) is about 94 Hz.
In contradiction to the single-spring model, where in
the unsupported case k should be smaller and so the contact
time should be longer, we observe a shorter contact
time in the unsupported case. We speculate that this happens
because in the supported case the impact is coupled
to high frequency modes of the stiff under-structure.