Lateral resolution beyond diffraction limit using 40 kHz air-coupled ultrasonic systems

A novel air-coupled ultrasonic imaging system introducing a conical acoustic probe (CAP) with a pinhole is constructed at the low frequency of 40 kHz. An ultrasonic wave creeping out of the pinhole with a diameter much smaller than the ultrasonic wavelength λ makes a sound field in front of the tip of the CAP. The -3 dB width of the lateral direction of the sound field coincides with the pinhole diameter (λ/27). Lateral resolution of λ/27 (0.3 mm) beyond the diffraction limit has been achieved using this system.