Non-contact ultrasound characterization of bread crumb: Application of the Biot–Allard model

This paper presents the application of the Biot–Allard model for characterization of the porous structure of bread crumb. This model describes the wave propagation in porous materials and is commonly used for the evaluation of sound absorbing properties of soft poroelastic foams. Two different bread types, a fine grain bread (FGB) with small cells and a coarse grain bread (CGB) with larger gas cells, were analyzed using both image analysis (IA) and ultrasonic techniques. IA figures for mean cell area, cell to total area ratio and number of cells/cm2 were 0.25 and 0.47, 0.32 and 0.41, and 112.8 and 81.6, respectively for FGB and CGB. The ultrasonic technique used non-contact air coupled transducers at ultrasonic frequencies. The phase velocity and attenuation of ultrasonic waves were measured in transmission and reflection experiments. These measurements confirmed the structural differences between the two different bread types and the usefulness of the Biot–Allard model for description of wave propagation in bread crumb for a wide frequency range. In addition, the use of non-contact ultrasound allowed estimating flow resistivity, open porosity, a measure for the size of the intersections in the crumb cell walls, and tortuosity which can be considered as a structural form factor.