Multi-spectral acoustic imaging on object surface in air

Abstract

Acoustical imaging has been performed using mono-frequency or a limited number of frequencies in the previous studies. The frequency dependence, however, may provide rich information on surface profiles, structures hidden under surfaces, and material properties of objects. In this study, acoustic imaging on object surfaces was conducted over a wide frequency range with a fine frequency step. A rigid surface with different profiles and a boundary between two objects composed of different materials were illuminated by sound wave swept over the frequency range from 1 kHz to 20 kHz with a 30-Hz step. The scattered sound field was recorded two-dimensionally using a scanning microphone, and the holographic method was used to reconstruct the sound pressure distribution on the surface from the recorded data. From the experimental results, the characteristics of the surfaces with respect to the shapes and material properties were demonstrated experimentally. The depth of the holes was identified by its own resonance frequency, and the two different materials were successfully distinguished by multiple images obtained at different frequencies.