Holographic visualization of low-frequency acoustic fields

A novel optical holographic method has been developed which uses conventional films for the visualization and measurement of low‐frequency sound fields. The method uses two nearly coincident optic beams, instead of the single beam used in schlieren optical methods, to produce a gratinglike image plane hologram. Two holograms are superposed and then optically processed to reconstruct an image of the visualization area. The reconstructed image contains a pattern of optical interference fringes which characterize the sound field. If the acoustic pressure is constant in one direction, the interference fringes can be interpreted in a particularly simple way: as contours of equal instantaneous acoustic pressure. Sensitivities of the order of 196 dB re 1 μPa are achieved. Results obtained with the new holographic visualization method are compared with those of classical schlieren visualization methods. The new holographic visualization method is found to have better sensitivity, dynamic range, and spatial resolu...