Two-dimensional vibrations analysed by speckle photography

Speckle photographs of a laser-illuminated surface vibrating in its own plane give characteristic diffraction spectra. These have been investigated for a surface vibrating sinusoidally along two mutually orthogonal directions with equal amplitudes but differing frequencies. The diffraction patterns obtained are interpreted in terms of the Lissajous figures traced out by each speckle, taking into account the time-of-dwell of a speckle at any point on its trajectory. Results are given for frequency ratios of the two vibrations of 1:1, 2:1, 3:1. 4:1, and for non-integrally related frequencies.