Air-coupled ultrasound spectroscopy and its potential application in agro-industry

Ultrasonic Spectroscopy (US) is a new alternative technique to extract parameters of vegetal tissues and properties of agro-industrial materials. The US technique involves a set-up in transmission of air-coupled ultrasonic transducers to make inspections contactless and in a non-invasive way. This is an advantage for some samples which cannot be in contact with the sensorial system. In this work we have tested Helliconia leaves and paper samples, focusing in the extraction of mechanical properties as density, acoustic velocity, acoustic attenuation and thickness of samples. Special interest is the relationship between the natural vibration mode and the water content in Helliconia leaves. The extraction of properties involves the solution of an inverse problem associated with the transmission of acoustic waves in three media (air-sample-air). Results show that estimation of these characteristics can be obtained with errors less than 12%. Also, a sensitivity analysis was performed to study how well the numerical method works in presence of noise, obtaining minimum errors in computations at signals to noise ratios as low as 6dB.