Both micromachined air-coupled capacitance transducers and piezoelectr
ic air-coupled devices have been used to form images of artificial gra
phite foil defects in a fibre-reinforced thermoplastic composite plate
. This has been achieved using toneburst excitation at the fundamental
through-thickness resonant frequency of the plate. Images are present
ed for both types of transducer system, and the results discussed. It
is found that capacitance transducers have distinct advantages in mate
rials characterization due to their wide bandwidth. These advantages i
nclude the ability to vary the excitation frequency in order to match
to higher order resonant modes of the plate.