Ultrasonic Non-Linear Harmonic Generation In Air For Ultra-Wide Band Characterization Of Thin Membranes

The characterization and test of porous and microporous thin layers using air-coupled ultrasound is extremely attractive for quality control in different industries, including air-filtration, Li-ion batteries, venting materials, etc. This work explores the possibility of using low frequency and narrowband transducers to generate harmonics in the air and use them to characterize this type of materials. This can help to avoid transducers limitations in terms of both center frequency and frequency bandwidth. The proposed set up consist of low frequency and narrowband excitation, harmonic generation in the air and the use of a wide band receiver. The setup configuration requirements are analyzed and described in detailed in this study. A comparison of the proposed technique results with linear conventional wideband air-coupled measurements demonstrates the potential of the presented method as a less demanding alternative in terms of equipment needs and time required. This can be of special interest for the deployment of solutions based on MEMs transducers, mainly PMUTs, that show some limitations in the maximum frequency and bandwidth when operating in air.