Low-Frequency Air-Coupled Ultrasonic System beyond Diffraction Limit Using Pinhole

A novel noncontact air-coupled ultrasonic system introducing a conical acoustic probe (CAP) with a pinhole is constructed at low frequency. A 40 kHz ultrasonic wave in air goes through the pinhole, which has a diameter much smaller than the ultrasonic wavelength λ (≃9 mm), and forms a unique sound field. The lateral -3 dB widths of both theoretical and experimental sound fields approximately coincide with the pinhole diameter. A lateral resolution of λ/30 (0.3 mm) beyond the diffraction limit has been achieved using this system. As an application of the system, the surface profile for a 0.5 mmφ (λ/18) drilled hole in an aluminum plate was successfully imaged. The surface topographical profile of a step difference of 1 µm (λ/9000) was also measured.