  In this letter, we present the frequency dependence of the vortex surface
resistance of bulk niobium accelerating cavities as a function of different
state-of-the-art surface treatments. Higher flux surface resistance per amount
of trapped magnetic field - sensitivity - is observed for higher frequencies,
in agreement with our theoretical model. Higher sensitivity is observed for
N-doped cavities, which possess an intermediate value of electron
mean-free-path, compared to 120 C and EP/BCP cavities. Experimental results
from our study showed that the sensitivity has a non-monotonic trend as a
function of the mean-free-path, including at frequencies other than 1.3 GHz,
and that the vortex response to the rf field can be tuned from the pinning
regime to flux-flow regime by manipulating the frequency and/or the
mean-free-path of the resonator, as reported in our previous studies. The
frequency dependence of the trapped flux sensitivity to the amplitude of the
accelerating gradient is also highlighted.
