Noncontact, high-resolution ultrasonic imaging of wood samples using coded chirp waveforms

A noncontact ultrasonic inspection technique has been developed to study the properties of wood samples in air. The system makes use of two broad bandwidth capacitive transducers, combined with signal processing techniques. A coded chirp signal was used in the current application to provide a waveform that could be postprocessed to provide sufficient sensitivity for transmission across samples of wood. It will be shown in this paper that the signal-to-noise ratio (SNR) can be greatly improved using two signal recovery techniques, namely pulse compression and swept frequency multiplication (SFM). A simulation of both techniques is presented and compared to experimental data. As will be seen from the experimental results, it is possible to perform noncontact ultrasonic experiments to extract a range of useful information such as ring density and the presence of microcracks.