  A method of measuring the thickness of steel plates using through
transmission of an acoustic pulse is demonstrated. This study has been done on
a stainless steel plate with regions of thickness 10:0 mm, 9:8 mm, and 9:6 mm,
using broadband pulses with energy in 200 kHz to 600 kHz band. Ultimately the
goal is to perform similar air-coupled thickness measurements in a single sided
pitch-catch measurement setup.
  The spectra of the transmitted pulses show the first and second harmonics of
the compressional waves in the plate. When compared to a plane wave model of a
fluid layer embedded in air, the second harmonic of the plate resonance fits
well with the expected value. However, the first harmonic deviates such that
the plate appears thicker at this resonance. This is believed to be caused by
the finite aperture of the transmitting transducer, causing deviations from a
plane wave. Thickness differences of 0:2 mm between the different regions of
the plate were shown to be resolved.
  A third peak was found in the spectra. The origin of this peaks has not been
verified, but is believed to come from the third harmonic of the shear wave in
the steel plate.
