Long-range detection of acoustic vibrations by speckle tracking

When a rough surface is illuminated by a laser beam, it scatters the light, producing a random interference pattern also known as speckle pattern. By imaging the speckle pattern with a line-scan CCD, we are able to measure the acoustic vibrations of a scattering surface, which can be up to 300 m distant. We also show that our instrument can be used as a laser microphone capable of detecting ambient sound such as a human voice.