Bioinspired low-frequency material characterisation

New carefully created signals, transmitted by high sensitivity broadband transducers in the 40-200 kHz range, allow material discrimination and thickness determination of polypropylene, polyvinylchloride and brass samples to sub-wavelength resolution. Analysis of frequency domain spectra, acquired experimentally using a water coupled through transmission experiment, enable material and thickness measurements that agree well with modelled data, with sound velocity prediction errors of less than 1 %, and thickness discrimination of at least wavelength/15. The accuracies found using our technique were similar to those obtained with higher frequencies; therefore, the lower frequency experiment demonstrates comparable performance to datasets obtained using devices possessing higher frequencies. This is advantageous when dealing with highly attenuating materials. The proposed signals and experimental approach will have an impact in the fields of non-destructive evaluation and imaging related to geophysics, and medical physics, where improving penetration whilst maintaining resolution is highly desired.