Ultrasonic non-linear harmonic generation in air for the characterization of thin membranes over an ultra-wide frequency range

Air-coupled ultrasonic methods allow for the rapid and noninvasive characterization of porous materials extracting valuable information regarding their performance and structure directly from their ultrasonic response. Nevertheless, in some cases, both the accuracy and the amount of information can be increased if measurements are taken over a wide frequency range. This can be a significant limitation depending on the transduction technology to be employed. In particular, for the use of PMUTs and CMUTs that are better suited for scaling up industrial deployment of this technology, as these are, typically, narrowband and low frequency transducers when operated in air. To enable the use of low-frequency and narrowband transducers for the characterization of filtration membranes using air-coupled ultrasound, we propose to make use of large amplitude narrowband and low frequency signals and non-linear harmonic generation in the air. To validate the potential of this approach, we characterized several filter materials by applying the proposed technique using different excitation and setup configurations including different ultrasonic sensors: high-sensitivity and broadband piezoelectric self-made transducers (235 kHz, 650 kHz and 1000 kHz); narrowband, low-frequency and low-cost transducers (flexural 40 kHz and radial 120 kHz); and an ultra-wideband receiver self-fabricated based on PVDF film. The results obtained were compared with conventional linear wideband air-coupled ultrasonic measurements, which allowed us to validate this approach. This innovation is expected to have a positive impact on the applicability of ultrasonic techniques, as it will ease the measurement process being a less demanding alternative in terms of equipment needs and time required.