  The spoke cavities mounted in the China Accelerator Driven sub-critical
System (C-ADS) have high quality factor(Q) and very small bandwidth, making
them very sensitive to mechanical perturbations whether external or
self-induced. The transfer function is used to characterize the response of the
cavity eigen frequency to the perturbations. This paper describes a method to
measure the transfer function of a spoke cavity. The measured Lorentz transfer
function shows there are 206 Hz and 311 Hz mechanical eigenmodes excited by
Lorentz force in the cavity of C-ADS, and the measured piezo fast tuner
transfer function shows there are 12 mechanical eigenmodes from 0 to 500 Hz.
According to these results, some effective measures have been taken to weaken
the influence from helium pressure fluctuation, avoid mechanical resonances and
improve the reliability of RF system.
