Our last paper describing the use air-coupled ultrasound to test facemasks is already available on-line

The paper is accepted in “Ultrasonics” and pre-proof available on-line since 19th August 2021

Using two different approaches based on air-coupled ultrasound in the frequency range 0.15–1.6 MHz with equivalent results, this work shows that it is possible to transmitt air-coupled ultrasonic waves through facemasks and ther layer components. 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.

https://www.sciencedirect.com/science/article/pii/S0041624X21001839