There are essentially four techniques in the family of
local methods. In addition to the coin-tap test, these are the
mechanical impedance method, the membrane resonance
method and the velocimetric method. The mechanical
impedance method measures the resistance to motion of
the local structure given the applied force; the membrane
resonance method measures the fundamental resonance
frequency under wide-band excitation; the velocimetric
method is similar to the membrane method but the exciter
and the transducer (receiver) are set farther apart making it
sensitive primarily to the structure flexing (bending)
modes.