Characteristics of vibration frequency measurement based on sound field imaging by digital holography

Abstract

We present the characteristics of indirect frequency measurement system based on off-axis digital holography (DH) for vibrating objects, which works on phase change characteristics of a medium where sound wave propagates. The sound field measurement method using off-axis DH already proposed by the author’s group is applied to the measurement of vibration frequency of arbitrary sound sources. In DH based sound imaging for frequency measurement, object wave passes near to vibrating object and the interference patterns are recorded in digital holograms as a function of time. After using inverse Fresnel calculation, acousto-optic data processing, and Fourier analysis, vibration frequency of an object of interest can be measured. In this paper, the measurement capability of vibration frequency for different objects and quantitative characteristics of the sound power are evaluated experimentally. The frequency range of the experimental results are from hundreds Hz to tens of kHz that covers audible range. The comparison with microphone recording and the analysis have also been carried out to check the strength and stability of the proposed scheme.