Holographic visualization of acoustic fields

Abstract

Optical holographic visualization of sound is demonstrated. Instantaneous pressure distributions are determined from patterns of optical interference fringes which appear in reconstructed images of sound fields. 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. A series of reconstructed images recorded at 20 microsecond intervals are presented which show the propagation of a 16 kHz acoustic toneburst in water. 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 resolution than schlieren methods at frequencies of most practical interest in underwater and environmental acoustics problems (below 50 kHz). Sensitivities of the order of 78 dB re 10 −12 watts/m 2 appear possible in air. The new method could find important applications to experimental investigations of the near fields of objects radiating or scattering sound at audio (or sonar) frequencies.