Air-coupled ferroelectret ultrasonic transducers for nondestructive testing of wood-based materials

Air-coupled ultrasound (ACU) is used in through transmission to detect delamination, rot, and cracks in wood without altering the structure permanently. Novel ferroelectret transducers with a high signal-to-noise ratio enable high-precision structure recognition. Transducers made of cellular polypropylene are quite suitable for ACU testing due to their extremely low Young’s modulus and low density resulting in a favorable acoustic impedance for the transmission of ultrasonic waves between the transducer and air. Thus, structures with great dimensions, with a thickness of up to 300 mm and material densities below 500 kg/m³, can be inspected. Promising results were obtained under laboratory conditions with frequencies ranging from 90 to 200 kHz. The advantage of ACU transducers is that they do not require contact to the sample; they are accurate and cost-effective. Ultrasonic quality assurance for wood is an important avenue to increase the acceptance of wooden structures and toward sustainability in civil engineering in general.