Additional investigations have shown that at high frequencies, parameters such as porosity, tortuosity and viscous and thermal characteristic lengths play a key role, while at low frequencies, porosity, air flow resistivity and thermal permeability defined by Lafarge et al.  are the key parameters . Experimental studies have proven that selleck chem inhibitor the acoustic behaviour of porous granular media can be basically described in terms of porosity, air-flow resistivity and tortuosity [36,37]. Furthermore, Guo et al. Inhibitors,Modulators,Libraries  found that in heaped seeds, the mechanism of sound absorption significantly depends on particle size and shape, which are parameters related with the previous ones.1.3.
Aim of the StudyThe aim Inhibitors,Modulators,Libraries of this study is to develop a novel non-destructive and in situ sensor approach based on Inhibitors,Modulators,Libraries sound absorption phenomena for identifying and classifying different seed types by means of multivariate Inhibitors,Modulators,Libraries statistical analysis. The proposed method is based on the already shown positive potentials of using multivariate statistical classification methods for analysing acoustical spectra of different agricultural materials .2.?Materials and MethodsThe experimental assessment was based on the calculation of the absorption coefficient (��) (ratio of absorbed sound energy to the incident sound energy) spectrum of different types of seeds and its respective multivariate statistical analysis for developing a seed classification model.2.1. Absorption Measurement PrincipleThe absorption coefficient (��) was determined by means of the impedance tube method.
The impedance tube consists of a hollow metallic cylinder where the test sample is placed at one end of the tube, while at the other end plane sound waves are generated by a loudspeaker (see Figure 1).Figure 1.Impedance tube. Where 1 is a rigid termination, Entinostat 2 the test sample, 3 and 4 the microphones and 5 the loudspeaker.The sound pressure is measured by two microphones at two different locations near to the sample. Then, the calculation of �� is performed by invoking a transfer function based on the pressure differences between the two microphone positions . These pressure differences depend on the degree of reflection and absorption of the standing waves that are influenced by the sample under test.2.2. Equipment SpecificationsThe impedance tube set used in this study (SCS9020 Kundt tube, S.C.S.
Controlli e Sistemi, selleckchem Tofacitinib Italy) was manufactured according to the ISO 10534-2 and ASTM E1050-98 standards. This set consisted of two impedance tubes with different internal diameters (28 and 100 mm) and different microphone locations. The two different tubes were developed for working at two different sound frequency ranges in order to ensure the creation of plane waves and subsequently the execution of an accurate phase detection by the microphones.