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

Abstract

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 (' 9 mm), and forms a unique sound field. The lateral À3 dB widths of both theoretical and experimental sound fields approximately coincide with the pinhole diameter. A lateral resolution of =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 mm (=18) drilled hole in an aluminum plate was successfully imaged. The surface topographical profile of a step difference of 1 mm (=9000) was also measured.

Analysis of sound field profile formed by conical acoustic probe with pinhole

“…In this paper, we revealed the pinhole diameter dependence of lateral −3 dB widths by calculating the Rayleigh integral and the finite element method (FEM) that allows modeling of the shape of the CAP. Both calculated results verified the lateral resolution that has been achieved in our previous paper [6]. Figure 1 shows the principle and schematic of the CAP with a pinhole much smaller than λ.…”

“…The aim of the CAP is to realize a lateral width equal to the pinhole diameter in the sound field near the pinhole. A low-frequency 40 kHz air-coupled ultrasonic system with our CAP has only recently been able to realize a lateral resolution of λ/30 depending on the pinhole diameter [6]. This effort overcame the present situation where a limitation of a working frequency range of air-coupled ultrasonic waves makes a higher resolution difficult.…”

“…The present study investigated a method to realize a lateral resolution much smaller than λ without higher frequency systems involving coupling fluids. We have proposed a conical acoustic probe (CAP) [4,5,6] with a pinhole much smaller than λ. The aim of the CAP is to realize a lateral width equal to the pinhole diameter in the sound field near the pinhole.…”

“…This effort overcame the present situation where a limitation of a working frequency range of air-coupled ultrasonic waves makes a higher resolution difficult. To verify the achieved lateral resolution, a sound pressure field profile of the pinhole was analyzed using the Rayleigh integral [6]. This analysis, however, cannot model the shape of the CAP.…”

Measurement of ultrasonic transmission attenuation characteristics of canvas fabric

“…Acoustic sensing in air has potential applications for obtaining various types of information about the surrounding object such as its position, shape, material and movement [1][2][3][4][5][6][7][8][9][10]. We reported results of the position detection of small objects with a size comparable to the signal wavelength, using an M-sequence-modulated signal [11,12].…”