Fast and non-destructive ultrasonic test for face masks.

As a consequence of the large demand of face masks due to the COVID19 pandemic, cheap, fast and non-destructive tests that can verify in-line the variability of the filtration capacities, prove the potential disinfection and/or evaluate the performance of new filtering materials are needed. Using two different approaches based on air-coupled ultrasounds (0.15-1.6 MHz) with equivalent results, this work shows that each face mask presents a distinctive ultrasonic signature that enables the classification and the evaluation of their performance. Moreover, it is shown that the ultrasonic propagation through the face masks and the main filter layers takes place through the pore space and that low frequency response of the attenuation and the velocity is highly dispersive and is dominated by the interaction between the air in the pores and the fibers in the filters. Hence, the parameters that describe ultrasonic velocity, attenuation and dispersion can be related with their filtration efficiency and breathability. These techniques are fully contactless, non-invasive and fast.