Vibration detection by observation of speckle patterns.

When laser light illuminates a rough surface it is scattered into a speckle pattern that is strongly dependent on the surface geometry. Here, we show that it is possible to sense surface vibrations by measuring signal variations from a single pixel detector that collects a small portion of the scattered light. By carefully tuning the probing laser beam size and the detector's aperture it was possible to record a good quality signal in the acoustic band. This approach eliminates the need for an interferometer and thus opens the door to the possibility of detecting vibrations at distances of few hundreds of meters.