Optical sound field measurement and imaging using laser and high-speed camera

Optical sound measurement, which acquires acoustical quantities by means of optical techniques, is of growing interest as an alternative method for the sound field imaging. There are two remarkable aspects of the optical sound measurement. The first is non-intrusiveness. Since the measurement is achieved by observing the light passed through the sound field, the instruments can be arranged outside the measurement field; non-contact and non-destructive measurement can be achieved. The second one is spatial resolution. Instead of building an array, expanding or scanning of light are often used for the optical imaging. Therefore, the optical imaging does not have the limitation of interval of measurement points due to the size of the instruments as microphone arrays. These two aspects make the optical method possible to image sound field with high spatial resolution and without any disturbance to the original field. In this paper, we show several methods for the optical sound imaging. Laser Doppler vibrometer can be developed as the imaging methods by scanning a narrow light beam. The two dimensional of transient field measurement can be achieved by using a high-speed camera because of single-shot. In addition some signal processing techniques introduced to optical measurement are also described.