An acoustic radiometer

In contrast to Crookes’ radiometer, which rotates due to electromagnetic radiation, the analogous acoustic radiometer demonstrates radiation pressure. The apparatus consists of two panes attached to opposite ends of a horizontal arm that is pivoted at its center. One side of either pane is acoustically reflective and the other absorptive. The apparatus rotates when placed in an enclosure of high-intensity acoustic noise. Experimental values of the terminal angular velocity are in reasonable agreement with approximate theoretical predictions based on radiation pressure.