Holographic real-time imaging of standing waves in gases

Stroboscopic holographic interferometry has been developed at an advanced undergraduate level allowing real-time imaging of standing sound waves in gas filled, closed tube resonators. A heterodyne Mach-Zehnder interferometer was first built by students to show the feasibility of interferometric detection of sound waves in a small cell, In the subsequent holographic study, the laser irradiance is modulated by an acousto-optic cell at frequencies near that of the standing wave, and a video camera records the fringe motion due to sound pressure changes. Fractional fringe shifts are observed for an air filled cell, and multiple fringe shifts are imaged for the case of freon. Sound reflections from the cell ends are easily observed, with nonsinusoidal waveforms dominating at high intensities due to superposition of resonator harmonics. (C) 1996 American Association of Physics Teachers.