New-coded signals, transmitted by high-sensitivity broadband transducers in the 40ââ?¬â??200 kHz range, allow subwavelength material\r\ndiscrimination and thickness determination of polypropylene, polyvinylchloride, and brass samples. Frequency domain spectra\r\nenable simultaneous measurement of material properties including longitudinal sound velocity and the attenuation constant as\r\nwell as thickness measurements. Laboratory test measurements agree well with model results, with sound velocity prediction\r\nerrors of less than 1%, and thickness discrimination of at least wavelength/15. The resolution of these measurements has only\r\nbeen matched in the past through methods that utilise higher frequencies. The ability to obtain the same resolution using low\r\nfrequencies has many advantages, particularly when dealing with highly attenuating materials. This approach differs significantly\r\nfrom past biomimetic approaches where actual or simulated animal signals have been used and consequently has the potential for\r\napplication in a range of fields where both improved penetration and high resolution are required, such as nondestructive testing\r\nand evaluation, geophysics, and medical physics.
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