We have proposed a method of simultaneously measuring aerodynamic sound and fluid ow using parallel phase- shifting interferometry (PPSI). PPSI can observe phase of light instantaneously and quantitatively. This method is useful for understanding the aerodynamic sound because PPSI can measure near the source of the aerodynamic sound. However, the components of sound and ow should be separated in order to observe detail near the source of sound inside a region of ow. Therefore, we consider a separation of the component of sound from simultaneously visualized images of sound and ow. In previous research, a spatio-temporal filter was used to extract a component satisfying the wave equation. The ow and the sound are different physical phenomena, and the ow cannot be expressed by the wave equation. Hence, we think that the spatio-temporal filter enables us to separate the component of sound from the simultaneously visualized images. In this paper, we propose a method for separation of ow and sound using spatio-temporal filter in order to visualize the component of the aerodynamic sound near its source. We conducted an experiment of the separation of data measured by PPSI. The results show that the spatio-temporal filter can extract the sound from air-ow except for the sound near objects and boundaries.
Optical methods have been applied to visualize sound waves, and these have received a considerable amount of attention in both optical and acoustical communities. We have researched optical methods for sound imaging including laser Doppler vibrometry and Schlieren method. More recently, parallel phase-shifting interferometry with a high-speed polarization camera has been used, and it can take a slow-motion video of sound waves in the audible range. This presentation briefly reviews the recent progress in optical imaging of sound in air and introduces the applications including acoustic transducer testing and investigation of acoustic phenomena.
Imaging of sound aids the understanding of the acoustical phenomena such as propagation, reflection, and diffraction, which is strongly required for various acoustical applications. The imaging of sound is commonly done by using a microphone array, whereas optical methods have recently been interested due to its contactless nature. The optical measurement of sound utilizes the phase modulation of light caused by sound. Since light propagated through a sound field changes its phase as proportional to the sound pressure, optical phase measurement technique can be used for the sound measurement. Several methods including laser Doppler vibrometry and Schlieren method have been proposed for that purpose. However, the sensitivities of the methods become lower as a frequency of sound decreases. In contrast, since the sensitivities of the phase-shifting technique do not depend on the frequencies of sounds, that technique is suitable for the imaging of sounds in the low-frequency range. The principle of imaging of sound using parallel phase-shifting interferometry was reported by the authors (K. Ishikawa et al., Optics Express, 2016). The measurement system consists of a high-speed polarization camera made by Photron Ltd., and a polarization interferometer. This paper reviews the principle briefly and demonstrates the high-speed imaging of acoustical phenomena. The results suggest that the proposed system can be applied to various industrial problems in acoustical engineering.